"Starter Cutout and Reversing Light Contact" on (BW35) Gearbox ...including disabling / bypassing this function as an Emergency Repair.   R. Kwas Oct 2022 [Comments Added]

Bypassed "Starter Relay"
Evaluation / Rebuild of "Starter Cutout and Reversing Light Contact on Gearbox"

Reference Information


The BW35 equipped cars have "Starter Cutout" switch on the Gearbox, which prevents cranking the Starter when in gear.  This is pretty important safety feature to assure starting the engine is only possible when it is not in gear.  The Switch also has a secondary circuit which provides a contact for the Reversing Lights.  As the BW35 Automatic Gearbox was made in Britain, the switch may have been supplied by you-know-who...which, in itself, is enough of an explanation of why they are known to be less than reliable. 


Starter Cutout and Reversing Light Contact Assembly.  Current replacement switches seem to have two terminals at a bit of an angle to identify them...no manufacturer's identification is apparent...

...but an original spare has the terminals unmistakably identified.  Chris Horn picture used with his kind permission.


Wiring Diagram excerpt showing Gearbox combination contact and associated circuitry.  As can be clearly seen, both contacts supply a chassis return path to their respective independent circuits, thereby enabling their respective control relays. [...and making both Low-Side Switched circuits.]

Reversing Lights are an accessory, not super-critical, and something a driver can live without, but if the "Starter Relay" is not enabled when it should be, powering the Solenoid and Starter will be impossible. 


First thing which should be checked if either function fails, is that all connections at the Switch Assembly down at the Transmission are securely in-place, and have not become dislodged possibly due to road debris kicking up.  

Refer to Wiring Diagram excerpt above.

Reversing Lights:  With Ignition ON (engine does not need to be running), momentarily connect Term 85 (Blk wire) of "Relay for Reversing Light" located at inner fender, to chassis and listen for Relay to click and watch for Reversing/Backing Lights to come ON.  If no click is heard, check Fuse 2, if a click is heard, but no Reversing Lights result, check for 12V at Gray wire (Term 30/51) of Reversing Light Relay, OR (Pin 5) of 6 Pin Junction Unit/Connector in Truck at the rear Lighting fixture.      

Starter Cutout (Enable) Contact:  Have a helper try to Start using Ign Key (or much better yet(!), Momentary Start Switch, so that Helper does not break the Ign Key Off) and listen or feel for the energizing click at the Starter Relay, at which time the Solenoid and Starter should also engage...but if no click is detected, check for 12V at Green wire (Term 30/51) of Starter Relay, being supplied by Ign Sw, and being sent to (in other words:  enabling) Starter Solenoid by way of Term 87 of Relay.  If chassis return path is not being provided by Switch contact at transmission, have helper wiggle and vary the Gear Selector position to check for intermittency or possibly an adjustment issue.  As an emergency Repair, the Starter Relay can be bypassed, in order to allow Starting, BUT, this involves bypassing a safety and brings with it the need for manually observing a safe Starting Procedure...see below! 

Location for Relays:  Both Reverse Light and Starter Relays are located in the circled area.

Location of Reversing and Starting Relays.


My answer to a where a poster noted:  "...I recall you can fire the car up on any gear in selector." [That is NOT a normal condition!...and given the propensity of overflowing SUs to result in the dreaded ECE B18 Syndrome, please refrain from using the term "fire the car up"...Thank You!]

Reference: http://www.sw-em.com/Wiring_Diagrams_and_Related.htm... (See: BW35 Highlighted area) "Starter Cutout and Reversing Light Contact on Gearbox" enables "Starter Relay" when Gear Selector ONLY has Park [or Neutral] selected...to ALLOW Starting ONLY in Park, but since these switches are known for getting intermittent, the "St Rel" is often bypassed (like also on mine!) to allow Starting at all...but then you can Start in ANY gear(!) which means you have to be carefully aware of what you're doing (it's no different that Starting in gear on a Man Gearbox, really, but there's no Clutch Pedal to push when you realize it!)...I suspected that is condition of a car you had, which exhibited that condition also...



Bypassed "Starter Relay":

On my automatic Amazon, the Starter Cutout function also became intermittent, so until a root cause could be determined, and repairs made, I bypassed the "Starter Relay" to enable starting at all, but also in ANY gear!  CAUTION:  Bypassing the Statrer Relay is not safest thing, as it bypasses a "safety-by-design-feature", bringing along with it the need to be EXTRA-CAREFUL not to engage Starter when anything but Park or Neutral are selected!), but at least one is able to Start the engine and get the car home, without having to climb underneath!  This bypass is simply accomplished by removing the wires from engine compartment Relay Terminal 87 and Terminal 30/51, and connecting them together, also keeping them insulated, and from making inadvertent connection to chassis.  (Refer to Wiring Diagram excerpt above!)  Notice:  This information is presented here strictly as an Emergency Repair!  Repeating...if you make this emergency wiring and function modification, YOU will be bypassing an important safety function, so YOU are responsible for SPECIAL CARE called for, when Starting!


Evaluation / Rebuild of "Starter Cutout and Reversing Light Contact on Gearbox"

I am arranging for a used switch, and will subject it to the typical disassembly/evaluation/documentation...watch this space!

UPDATE:  Chris Horn has once again supplied a victim switch to be subjected to a SW-EM technical evaluation. 

Upon receipt and initial inspection, the used switch displays several immediate issues...the actuating Plunger, although somewhat free to move, does not protrude the switch case very much (not nearly as much as in the pictures above!), one of the spade terminals is missing from its rivet, and another is quite loose on its...but that wont stop an up-close-and-personal internal critical evaluation...

As received...


After clean-up, terminal end doesn't look so good...  I've never heard of EGEN is a component manufacturer.  It does look like the Housing crimps have been worked on/attacked before, but the effort was not successful.  Not counting original assembly, the switch has not been open before.


The securing crimps are bent open a bit more to allow disassembly.


A wooden drift [chopstick!...what else did you think!], is used to gently push the insert from the metal shell (a rubber cushion prevents damage to threads while Switch is held in vice).  First, the bakelite Stationary Contact Assembly comes out, then a transfer pin with rubber seal and circlip. 


The Plunger is nothing fancy, and threads on the Housing would seem to be 3/8" - 14TPI (which my chinashit [which I did NOT buy or pay for!] thread-gauge did not have for some reason!). 

There is a lot of oily black munge in the housing...


The Contact assembly also is quite dirty, and corroded...


The central Contact Barrel with a very dirty Circumferential Conductor Band is extracted from the Contact Assembly.  Manner of function of the switch is thus clear...the stationary contacts are at different levels and CCB completes the circuit based on the amount the Plunger is depressed, which in-turn determines the height of the Contact Barrel and its CCB, so it becomes immediately clear, that Switch Adjustment is important.      


Under the Contact Barrel, the Return Spring is completely surrounded by a dirty mess, and in several pieces.  The debris does not seem to be oil based, suggesting that there was ingress of water for some reason.  The water contact to the Spring would seem to have corroded it, causing it to fail, and that would certainly explain why the Actuating Button/Plunger was not protruding the case too far. 


Cause of Switch failure, is a corroded and completely failed Plunger Return Spring, a result of ingress of what looks like it was water based, which caused the spring to rust and finally break.  Given the overall poor condition, there is little hope for rebuilding this particular switch and returning it to service...it looks like it lead a tough life, at least near the end...so will be resigned to the SW-EM Interesting Components Archive...maybe it should just be given a proper burial...!  [...doubt it!...I don't throw anything away!]

Conclusion:  This is switch of OK and adequate design, but clearly didn't do well with the exposure of the internals to water. 


Reference Information:

Switch Adjustment, from the BW Service Manual for the BW-35: 


External material sources are attributed.  Otherwise, this article is Copyright © 2022.  Ronald Kwas.   The terms Volvo, and you-know-who are used for reference only.  I have no affiliation with either of these companies other than to keep their products working for me, 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 donít, youíre 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 unique metaphor and probably (likely) wise-a** comment. 


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