OD Retrofitting on a Vintage Volvo   
first published:  Jun 2007 R. Kwas, Updates on-going  [Comments Added]


My 122 BW35 Automatic to M40/M41OD Conversion Notes.  I have also started collecting all OD and transmission related info on this page. 

Splined Output Shaft
Wiring of the OD
    OD Wiring 1
    OD Wiring 2
    OD Wiring 3
Unwanted Reactivation 
    OD Wiring 4

OD Solenoid Notes

Additional Discussion of OD Control Wiring and the OD Indicator

Reference Information
    Excerpt for OD, of 1984 240 Wiring Diagram
    OD Sequencing Relay Info
    122 Wiring Diagram with Separately Fused Power for OD
    Transmission Support from BW35 to Manual

    Differences in M40 to M41/OD
    Preventing a broken Interlock Plate on Gearbox Cover


Terminology and Clarification:  An M40 is a 4 speed manual transmission with a sealed output bearing housing followed by an output flange to which the mating flange of the front driveshaft, with its universal joint, bolts.  An M41 is the version of the same 4-speed trans which has instead an open Rear Seal Flange (Item 13, below right)  which allows sharing the lubricating fluid with the OD which is mounted to it by means of what Volvo calls an "Intermediary Flange", and more importantly, has an extended Splined Output Shaft (part 30 in the right diagram  following) which serves as the OD input shaft.  This output shaft cannot be installed without a complete transmission disassembly!  Note therefore:  It is not possible to simply "bolt an OD onto an M40" because of this difference, that is why it is most assuredly better to just get the two as a set or at least know the providence and year of the vehicle a unit came from.  Both have number plates from which this information can be decoded.       

A comparison od M40 and M41 Gearboxes.  Highlighted in Red are the output differences, in Orange, differences in the Clustershaft, showing the extension in the later, which drives the OD.    

Splined Output Shaft (which is also the extended, main Clustershaft of the gearbox) of the M41, intended for the D-Type OD, has completely different splining, and so is very different from the Clustershaft of the M41 intended for the J-Type OD.  D-type and J type ODs therefore cannot simply interchange. 

Picture:  Rick Watson and used with his kind permission. 

Comparison of early and later Clustershafts from M41 gearboxes. 

See also:  Differences in M40 to M41/OD

Americans want their automatics!...and Volvo of the sixties naturally offered one to them...but because of the sluggishness and power-robbing and non-sporty nature of the Borg Warner BW35 automatic transmissions which were fitted to Amazons, conversion to a manual transmission is often undertaken today.  The standard differential ratio for the autos was 4.30, so most conversions are are done to the M41/OD combination, because without the OD, the higher revs and associated passenger compartment noise are a factor which most owners like to reduce (not that a B18 or B20 wouldn't be happy running like that all day), but it does become a bit much at prolonged highway speeds (especially with a compromised exhaust system!...see:  History of J5R).  There is also an associated speedometer error of at least (4.30/4.11= , not to mention tire circumference error).  The conversion to a Volvo standard transmission of the time is the most straightforward, because bell housings are available and little fabrication needs to be done...converting to any other trans will certainly involve more custom fabrication work which I have no hands-on experience with...here, I recommend posting to the Fora out there, for specifics...there are a few brave and industrious individuals who can help here...
Once you've decided to perform the conversion, you basically procure the manual trans plus ancillary components...getting them while still on a donor car helps because the car is like a checklist (it's simple to look at a component and quickly decide if you need it, but I've also included a list below)...don't forget the pedal works including clutch (hydraulics OR cable, and this also determines which bell housing, since they are different for hydraulic or cable type).  Under the vehicle, the M40 crossmember can be used, but needs to be relocated forward (by means of some custom plates which need to be made is the simplest).

List of components required:
- Pedal pivot, plus brake and Clutch pedals from a 122 (clutch may be hydraulic if the parts come from a 122 OR later cable type if they come from a late 122 or 140, but associated bell housing must be used...hydraulics push, cable pulls to apply force to pressure plate and disengage clutch friction plate...and they can't be mixed and matched...cause you can't push a rope...or...well, I'm still trying to figure out a good analogy for not being able to suck on a pipe and get much work done on the other end!

- Flywheel/friction plate/pressure plate (I prefer the Fichtel-Sachs type) / throw-out bearing (suitable for Fichtel-Sachs, height differences exist with Borg & Beck type...again getting everything from one donor vehicle has the distinct advantage that you are certain the components all played nice together, and will again...there are enough details to worry about when doing a conversion...installing the incorrect throwout bearing and then wondering why the &#(^$*&% clutch wont disengage when everything is together shouldn't be one of them!)

- M40/Crossmember/trans-mount from a 122, recommend six cylinder motor mount type PN 1206612 / (long) front driveshaft (with similar spline-count as rear shaft...there ARE differences).

- OR M41/OD/Crossmember/trans-mount from a 122, recommend six cylinder motor mount type, (short) front driveshaft (with similar spline-count as rear shaft).

Notes on installation:  Crossmember location must be moved forward to accommodate new mounting location of shorter manual trans. This may simply be done with two 1/4" plates (about 4 X 8") which bolt to the original auto-trans mount, and bring new mounting holes, which accept the crossmember, forward.

Throttle pedal will be somewhat "crowded" by the wider transmission tunnel, but one can get used to it.

Electrical considerations:  There are two electrical contact functions which are performed by the BW35 auto.  The "Start-Inhibit-While-In-Gear", and the "Activate-Backing-Lights-When-In-Reverse" (Item 11.  Refer to wiring diagram:  122S Wiring Diagram ).  The former, and its associated relay (located in the relay cluster on the driver side inner fender) falls away with the M40/M41OD conversion (remove relay, tie connector from relay terminal 87 together with connector which was on terminal 85, insulate with sleaving, tuck into harness, and forget!), and the later, should get transferred over to be activated by the M40 or M41 (by either the aluminum can switch located on the transmission case end, which monitors the reverse actuator rod, or the lid mounted switch which does the same).  If an OD is being installed as a part of the conversion, IGN power to the OD activating solenoid must be supplied to engage the OD.  There are a couple of versions of this...by way of a momentary switch contact which is latched by a bystable relay, which toggles its mechanical state once for every momentary activation, (like in the headlight circuit) see also:  Headlight control upgrade , OR by means of a simple toggle switch located on the dashboard which applies IGN power from Fuse 1.  In BOTH cases, the wire running to the solenoid should be interrupted, and therefore enabled by, the "Fourth-Gear-Sensor" switch, also located on the M41 lid, which allows the OD to be engaged ONLY while in the top gear, sparing it excessive torque of the lower gears.

Wiring of the OD:  A number of different manners of controlling the OD by way of energizing the control solenoid from fused IGNition power, were employed by the factory.  These include latching toggle switches on the dashboard (with and without a relay), or a momentary non-latching switch (stalk behind steering wheel). 

OD Wiring 1 shows a simple mechanically latching toggle switch on the dashboard, plus a control relay, extracted from an early 1800 wiring diagram.

OD Wiring 1 

Reference:  Comparing 3, 4, and 5 terminal relays

OD Wiring 2 shows the momentary switch style in the form of a stalk behind the steering wheel....since the switch only has a momentary closed contact action, the relay must also perform the latching function...a bistable latching relay, similar to the Bistable Control Relay, with mechanical toggling function, is used.  

OD Wiring 2 
Note that only terminals on relay which are used in the OD control circuit, are shown here...
for full details on the Bistable Latching Relay, see: 
Bistable Control Relay


OD Wiring 3 shows a third wiring technique which eliminates the relays altogether, extracted from a 123GT diagram.  Here the switch must control all of the solenoid current instead of just a small relay control current...not a problem with a suitable switch.  The observant reader will note that in this configuration, the “hot” wire must now run to the dashboard instead of just a grounding relay control wire.  Of course, the "hot" indicator power wire runs to the dash in all OD control wiring variations! 

OD Wiring 3 


The sequence of gears and shifting from third gear to fourth/OD:  In all of the control circuits above, the issue still remains, that if you are in fourth gear/OD and you shift out of fourth gear without first deactivating the OD, as soon as you shift back into fourth (and close the fourth gear enable switch) you will immediately shift back into fourth gear plus OD.  To prevent this Unwanted Reactivation (and the RPMs falling way out of the engine powerband which comes along with it), takes some getting used to...but the first few times it happens will probably bring home the point.  The dashboard indicator does help as a reminder, in that it is wired to indicate when the control circuit is switched ON, and not the OD itself.  As it is wired upstream of the fourth gear enable switch, it will remain illuminated when the transmission is shifted out of fourth gear without deselecting the OD, reminding the driver that the OD selection is active and will take place immediately upon shifting into fourth.  ...sooo:  The OD Indicator is actually more of an indicator of the control instead of the OD itself!

Given the above, the correct sequence of downshifting from fourth/OD is therefore to first deactivate the OD (by toggle or momentary), the OD Indicator extinguishes at that time, and gearing drops to fourth (only), then shifting into third, using the clutch each time to decrease shock loads on the drivetrain.

Which circuit should be used?  None of the above circuits are technically better than the others, so if one is retrofitting a vehicle with an OD, the driver interface (switch style), and location of the OD Indicator should first be decided upon and selected, this in-turn dictates the rest of the hardware and circuitry required.  If you place value on simplicity, the single heavy duty toggle switch, rated to handle the full Solenoid current) on the dashboard (OD Wiring 3) is hard to beat...it certainly doesn't get any simpler than that!

Finally, presented here also is OD Wiring 4 which uses a more modern electronically controlled Sequencing Relay to make switching into and out of the OD foolproof and not subject to "Unwanted Reactivation".

OD Wiring 4:  Use of Sequencing Relay Prevents "Unwanted Reactivation":  This design uses the OD control (PN1259750) relay from a later 240 model.  The relay uses electronics to do the logic and sequencing required to prevent what the author calls "Unwanted Reactivation" condition.  Note also that the Indicator is in parallel with Solenoid, so Indicator truly shows state of power to Solenoid and therefore OD.  

See also:  OD Sequencing Relay Info

Two options are shown.  Option 1 shows a momentary switch closure from power to the control input of the relay.  Since the more common momentary switch closure is to chassis, another simple relay can be used to perform this "inversion".

OD Wiring 4 Based on wiring as shown in Reference Wiring Diagram below. 
OPTION 1 shown with a Momentary Contact Switch where both contacts are isolated and wired to supply Seq. Relay with a 12V input, as in the original 240 circuit where Switch is located in Shifter Knob.   OPTION 2 is added and shows the more common Momentary Contact Switch with one side tied to chassis.   


OD Solenoid Notes:  The OD solenoid is one of those British automotive electrical products with questionable reliability...because on top of the manufacturer, Lucas, which is the kiss of death in itself, there is the additional complexity, of rather than being a simple coil-making-magnetism-which-pulls-in-an-armature like a starter solenoid, designers had to take into account the fact that the OD solenoid would have to endure continuous duty, unlike the Starter Solenoid, which gets to rest (and more specifically: COOL) most of the time.  The way sharp sixties designers did this is by taking advantage of the fact that it takes a lot more magnetic force to initially pull in the armature, than it does to hold it once it gets to the end position.  By placing two coils in the same case, a strong  "pulling coil" (of heavy wire gauge, which results in a higher current and generates a stronger magnetic field) and a weaker "holding coil" (of a smaller wire gauge which allows a lower current to flow thereby generating a weaker, but adequate for holding magnetic field), and equipping the assembly with a switch which stops the (high) current-flow to the "pulling-coil" once the armature has reached the end-stop, the solenoid uses a clever pulling-current, dropping to holding-current, strategy to prevent the solenoid from frying itself to death.  An OD solenoid would never survive the long-term power dissipation of the high pulling-current, but it can run continuously with just holding-current! 

As can be seen in the following graphic, both coils are actually energized when applying power to the solenoid, and as the armature mechanically opens the internal switch, the current drops to a much lower level, and this in-turn drops the heat generated to a tolerable level.  

OD Solenoid inner workings.  "...there's more going on than meets the ear!" (Clouseau). 

OD Failure Modes:  To go along with any of the hydraulic failures which can occur within the OD, any electrical failure which keeps the solenoid from being pulled into the activated position will keep the OD from engaging.  This includes the typical range of external problems, from poor connections anywhere along the current path, including the at the bystable relay, to opens at the Fourth-Gear-Sensor switch on the transmission cover to mechanical looseness of the housing (which supplies the chassis return path).  The typical electrical failure mode internal to the OD solenoids is that the internal switch contact intended to break the pulling-current fails to do so...the solenoid then does its best impression of a or burrito in a micro-wave...it cooks from the inside! 


Additional Discussion of OD Control Wiring and the OD Indicator: 

Three terminal relay circuit from 1800 Wiring Diagram.  Note that in this arrangement Relay and (high) Solenoid current are supplied by Fuse 1, which is the Ignition Power Fuse in the 1800.  This means that high Solenoid current is being routed through Ignition Switch.  In order to spare the vintage Ignition Switch this duty, installing an Ign Slave Relay and routing the high Load Current through it is highly recommended!  See:  http://www.sw-em.com/Ignition_Slave_Relay.htm


The early scheme.  In effect from Chassis 1 to 12500.  Source:  '67 Factory manual:  http://volvo1800pictures.com/document/Service_manual_jack/part_3_electrical_system_p1800/part_3_electrical_system_p1800.pdf

Note in the previous Wiring Diagram OD Indicator and Solenoid are actually connected in parallel.  Indicator Lights when OD is being powered.  This makes more sense to me, considering I've always seen the control system with the 4th gear lockout in series with solenoid.  Driver control switch 29 is a latching type, and switch 22, which supplies the whole OD control system is 4th gear lockout...if you're not in 4th gear you can't even activate relay!...opposed to later schemes, (see below), where 4th gear lockout switch is in series with solenoid, sure, you can activate relay anytime your heart desires, but ONLY while in 4th gear does anything happen in terms of activating OD.  Indicator scheme is intuitively correct above.  It indicates OD function!  Below, Indicator function takes some interpretation!

OD Control using Latching Relay (and Momentary Switch).  Note:  Relay is shown as a four terminal relay, but this is because only four terminals are used for OD control...there are actually more terminals on the relay!  (When this same relay is used in the Headlight control circuit, those terminals are used!  See:  http://www.sw-em.com/headlight_control_upgrade.htm  ) 

Later OD Control with latching relay.   In effect from Chassis 12501.   Source:  (same) '67 Factory manual:  Driver control switch 23 is momentary type, switch 12 is 4th gear lockout switch on tranny. 

In this circuit, you can activate OD control circuit any time your little heart desires(!)...in any gear, and Indicator lamp indicates state of the control relay immediately, (not Solenoid)....of course nothing happens in terms of powering Solenoid, until you shift into 4th gear, and close the 4th gear Interlock Switch, but it's a point of philosophical discussion. 

As a rule, an Indicator is supposed to indicate the end function, [so should be wired in parallel with Load] and in this scheme, it indicates state of control system (relay) and not the OD...it's intuitively deceiving...when Indicator is ON, one still needs to think about if the transmission is in fourth...I don't prefer it, but I suppose one would get used to either scheme.  One would certainly feel/hear the drop in RPMs when OD engaged.  

Of course, the 4th gear lockout switch could be wired in a latching relay scheme in the earlier manner, supplying the entire circuit.  That way one would have the best of both.

Note also that in the 1800ES, the OD Indicator is also wired to the Indicator Brightness Control which allows driver to dim indicator (as well as Hi-Beam Indicator to prevent glare.



OD manuals:

http://www.vclassics.com/archive/laycock.htm  Good INFO

also:  http://hem.bredband.net/b215872/amazon/downloads/Overdrive%20in%20Volvo%20Amazon.pdf

Brickboard Thread:   Seek circuit info on OD control relay with self-cancelling. 200

Brickboard Thread about driveshaft lengths:  http://www.brickboard.com/RWD/volvo/1455904/120-130/driveline_comparison.html

My response to Brickboard Thread:  Over drive not engaging due to low voltage/current.  https://www.brickboard.com/RWD/volvo/1638752/120-130/drive_engaging_due_low_voltagecurrent.html  [Info and comments not part of the original posting added.]

I advise monitoring Elec System Voltage in general and maybe even specifically at OD Sol in support of your assertion that Low Voltage is preventing it from engaging...without some concrete proof of low Voltage, its presumptive and really only unfounded conjecture, because it could be electrical but it could also be inadequate OD oil pressure (hows OD Oil level?) which is causing symptoms! This is possibly confirmed by temperature dependency, which is not/(less) consistent with an elec issue

I agree that OD Sol is a heavy load, but 30A is way too high (that would be the entire rated output of Bosch Gen!...and the resulting 360Watts of dissipation would be enough to make the Sol visible from Space on IR satellites! ...where did you get that number?)...I believe it is more like 10A inrush (Pulling Coil AND Holding Coil together), dropping to under 5A Holding Coil only). See also: http://www.sw-em.com/OD_Retrofitting.htm#OD_Solenoid_Notes

If this is a 123GT (doubtful, as they had factory Alts) OD is on Fuse1 circuit. If this is an OD retrofit, assure OD is Fused!...if Lucas Sol fails, without in-line protection, your asking for big trouble!  [Failure of Lucas components often results in plasma or other effects also visible by IR from space!  It's best to fuse solenoid individually at 20A as shown below!]

Upgrading to an Alt is a good Idea (anyway!), if low Elec Sys voltage is indeed the issue because of OD Sol load, it wont be after installing Alt! If it's one of my Alt Kit Brackets, contact me off line for installation info/help.

Good Hunting!

Derek; Water and bucket analogies are OK, but I have to gently disagree, and point out that when engine is running, Charging System is supposed to supply all Loads, AND have sufficient OOMPH left over to keep sys voltage up which in-turn charges Battery...if it does not, sys voltage is down and Battery needs to add power, that's a bad, undesirable (net Discharge!) situation that's unsustainable in the long-run!


Snubber of Flyback Diode:  https://en.wikipedia.org/wiki/Flyback_diode


Reference Information:

Excerpt for OD, of 1984 240 Wiring Diagram

Notice Snubbing Diode (see:  Reference Info below) internal to Sequencing Relay (from Pins 87 to 31).  This component is intended to clamp the reverse voltage Spike occurring when Solenoid control contact opens, to protect sensitive internal electronics (also decreases arcing and resulting contact erosion).  Notice also that OD Indicator is wired in parallel with, so indicates power applied to Solenoid, so it much more indicates the actual condition of OD, unlike the earlier control circuits.  When Indicator is ON, the OD better also be engaged or something is wrong!  Here, the 4th Gear Lockout Sw is simply an input to the Seq Rel control circuit.



OD Sequencing Relay Info:

Stribel (Kaehler?) version of 1 259 750 relay. 


Art Benstein took the time to reverse engineer the internals of the Stribel relay.  Schematic posted with his kind permission.

Source: Thread on Turbobricks forum:  M46 Overdrive Relay Pinout/Wiring Diagram?  http://forums.turbobricks.com/showthread.php?t=202693


My response to a FB thread on OD Sol current:

"I haven't measured it but as OPs say...initially (and only momentarily), both Pulling and Holding Coils will draw an (estimated) 10A+...then it drops to (also estimated 2-3A) see also: http://www.sw-em.com/OD_Retrofitting.htm#OD_Solenoid_Notes
On a 122, power for OD control circuit and Solenoid are simply added to Fuse 1 (25A) circuit, but given the fact that the Sol is a Lucas (EGAD!) manufactured part, I recommend adding a separated 10 or 15 A Fuse just for that circuit. Good Hunting! " [Correction:  To avoid possible "nuisance blows" of fuse, use 20A Fuse, taking Ign Pwr from input side of Fuse 1!]

Here is the recommended wiring: 

122 Wiring Diagram with Separately Fused Power for OD: 


Transmission Support from BW35, to Manual: 

Guest Contribution by Russ Prior's showing his implementation of the bracketry necessary to relocate the  (both M40 or M41/OD).

Russ Prior material reposted with his kind permission.


Dimensioned Sketches: 






Differences in M40 to M41/OD: 

Alfred Watermann pictures, used with his kind permission. 

M41 with J OD.  Notice the original (lower) Fluid fill port has been sealed, and the newer Fill Port is in the upper location.  The 4th gear Enable Switch is clearly apparent on the Gearbox Cover. 

3rd and 4th Gear Selector Rod with actuator tab and the associated notch for its setscrew.

Rear Seal Flange M41

Rear Seal Flange with a hole to allow oil exchange between Gearbox and OD. 


Preventing a broken Interlock Plate on Gearbox Cover:

[Until there is a SW-EM Page on the Gearbox itself, I figured this was the best/most logical place to add the following information, since in the course of adding an OD, the Gearbox Cover might/will likely be removed and need to be replaced.] 

Friendly Tip:  DON'T force installation of the Gearbox Cover! 

BEFORE torquing Cover Bolts, assure Gearbox is in Neutral, and dogs on Interlock Plate (shown broken below!), as well as selector dog on Gearshift Lever are in clearance pocket of Shift Levers all aligned!  No force should be necessary during Cover assembly!  Temporarily inserting the Shiftlever and checking that it is not jammed, and will wiggling freely, is a good way to check for the proper alignment.  Shiftlever can then be removed again so that is is not present during Gearbox installation in chassis.  

Underside of a Gearbox Cover with broken Interlock Plate.

The Interlock Plate (82) prevents engaging more than one gear at a time in the Gearbox...(that would be baaad and break gears!). 

Of course if the Gear Selector Rods and Shift Lever are not properly aligned while installing the Cover (74), and applying unnecessary force by wrenching down on the Cover Screws, Interlock Plate will break instead...! 

Please remember what Arnie says!

With apologies to Mr. Schwarzenegger! 


External material sources are attributed where possible.  Otherwise, this article is Copyright © 2007-2022.  Ronald Kwas.   The terms Volvo and Lucas (gag!) are used for reference only.  I am not affiliated with Volvo…although I do also Roll…and to try to keep their products working for me, and I also try to keep Lucas products from doing their well-known self-disassembling tricks (often accompanied by copious amounts of "Smoke in the Cockpit"), and help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor, celebrity advice, and probably wise-a** comment.


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