Intermittent Variable Instrument Lighting
first published 09/10 R. Kwas,  changes on-going  [Comments Added]


Permanent bypass of Variable Instrument Lighting 

122 Instrument Lighting
1800 Variable Instrument Lighting 
1800E/ES Variable Instrument Lighting


Note:  The information presented here applies to 122 and 1800 models with variations, which I've also tried to cover.  [1800 Variable Instrument Lighting has been added.  See:  1800 Variable Instrument Lighting  below.]

Background:  A Variable Instrument Lighting function is available on various vintage Volvo vehicle when Lights are ON, by turning the Lighting Control knob.  It is quite common however, for this function to be intermittent after so many years.  Symptoms of this are typically that the Variable Instrumentation Lighting remains OFF totally (while non-variable (fixed) “Heater Control Lighting” located just above those controls works fine), when the knob is pulled for Lights…when it “worked fine the last time Lights were ON”.  The simple reason for this change is that the knob may have gotten turned slightly as it was pulled when turning ON Lights, and this moved the slider of the variable control from the position where it was making contact, to where it is now not making contact.  

If the variable control is then turned to try to get the variable lighting to come on again or adjust them, they will often go through a little disco show as the slider contact makes and breaks connection…sometimes, just by exercising the control knob, the condition can remedied temporarily, but the problem is inherent and will return. 

(Applicable to 122 Model)  If on the other hand, non-variable “Heater Control Lighting” remains OFF when pulling out Lights Control Knob, and turning the knob, Instrumentation Lighting also remains completely without signs of life, Fuse 3 should be checked (it protects both Instrumentation Lighting, fixed and variable, and Parking Lights)…fuse should be checked both for being blown (uncommon), and more commonly for being oxidized at the conical ends, needing a “spin” to cut through this oxidation to reestablish contact (if this is the case, I suggest the reader spend some time perusing the ACZP and Gas-Tight-Joint pages!). 

(Applicable to 1800 Model)  All Instrument Lighting is downstream of the Fuse 3 and Variable Instrument Lighting Control. 

The root cause of this is typically a poor contact at the connection of the slider to the resistance element, caused by surface corrosion which inevitably develops on the exposed resistance wire after time. 

By design, the resistance wire needs to be open (within the control) and exposed to allow the control slider to make spring-contact as the knob is rotated.  However, since the control is not a hermetically sealed type, and unprotected as such, this uncoated wire is therefore exposed to atmosphere, and more-so to either side of where the slider contact happened to be positioned for a long time…one can guess the rest… 

Repair Options: 

Repair Option 1 (Most involved, and permanent, retaining the dimming function):  One solution which will retain the dimming function, might be to disassemble and clean the oxidation layer from the resistance wire, [This would involve a major disassembly, but I recently had acceptable result with only a partial disassembly and still applying the ACZP to the Rheostat wiping surface.  See:  Link ] and apply an agent which will protect the surface from the atmosphere in the future when reassembling…my favorite product for preventing corrosion on electrical connections comes immediately to mind here also (ACZP)…specific to this application, both factors critical to a reliable connection are addressed.  A film of it acts as a barrier to atmosphere, and the grease component will lube the slider as it is moved, yet it will not interfere with a good connection between slider and resistance wire because it is easily displaced by slider spring-pressure, allowing the slider to make a good contact to the clean wire below. 

I expect the displaced ACZP to protectively surround the contact area and make a Gas-Tight-Joint, which any electrical engineer (or reader of this site) knows is the most desirable condition to give long-term protection of any electrical connection on our vehicles.  Given that the fact that the control is typically set to the position of preferred brightness and rarely moved after that, I expect this solution would satisfy for long time…but opening up and working within the control might be a bit much of a project for the casual vintage Volvo mechanic. 

Repair Option 2 (Less involved, retaining the dimming function, but possibly still long-term satisfying):  Another possible solution is to spray Deoxit D5 into the control (unfortunately, since there is no real direct access to the interior of switch, a lot of D5 would have to be wasted wicking it down the knobshaft, to get good internal coverage), then rotate the knob to allow the slider contact to abrade away the oxidation on the resistance wire as it rotates.  Another generous dose of D5 will add another coating the newly cleaned surface.  I don’t expect this solution to be as long-lived as the ACZP solution, but I might be pleasantly surprised…the D5 does create an effective self-healing light film…getting the active ingredient to the wire is the difficult issue here…

Repair Option 3 (Permanent electrical bypass, Instrument Lighting set to maximum, and loss of dimming function):  Even more simply, if the reader does not mind loosing the variable dimming function, but does not want to get intimately familiar with the innards of the Light Switch (there is always a certain amount of risk of damage or other failure to reassembling the switch correctly, associated with disassembly), the variable function (and its intermittency) can bypassed externally, with a jumper connection between terminals 58a and 58b.  Since both terminals are populated and quite busy, it is doable, but a bit tricky to implement.  This results in all Variable Instrument Lighting being set to the maximum brightness.

The simplest way to implement this is by using paralleling terminals (an example is shown in FIGURE 1.).  These terminals are a combination of the typical female push-on part, but they also have an additional male terminal, allowing the new connection to be simply added.  Benefit:  With this implementation, no cutting or crimping of wires or any other change to the original harness is required, so this change is tracelessly reversible.  A simple 4” jumper with these connectors at both ends does the trick! 

Option 3A  A fixed brightness, but decreased from the maximum, can be implemented with Option 3, by adding a Dropping Resistor in series where the variable would have been.  Resistance value for brightness, and power rating, so that it doesn't get to hot, should be experimentally determined.  This is shown in FIGURE 2. below in Red. 

FIGURE 1.  Example of Paralleling Terminal for 1/4" push-on connectors.


Repair Instructions for permanent electrical bypass of dimming function on a 122 Vehicle


1.  Prepare a 4” bypass jumper with two paralleling crimps as shown in FIGURE 1. 

2.  Disconnect Battery before proceeding! 

3.  Remove Light Switch from dashboard, by unscrewing control knob, loosening retaining nut at control, then withdrawing control from dashboard, taking care with connected wiring. 

4.  Inspect terminals and connections at switch, and confirm these to be populated as shown in FIGURE 2.  (If different than shown, a non-standard condition exists and this must be completely understood before continuing with modifications listed here!).

5.  Install bypass jumper, by disconnecting connector (with two White wires) at terminal 58b, and replacing this connector with paralleling connector at one end of bypass jumper.  Place connector which was originally on terminal 58b onto male terminal of paralleling connector which was installed onto terminal 58b. 

6.  Repeat for terminal 58a (multiple Reds) for other end of bypass jumper. 

7.  Double-check all work before proceeding! 

8.  Reinstall Light Switch in dashboard, taking care with connected wiring. 

9.  Reconnect Vehicle Battery. 

10.  Check function of Instrument Lighting.  It should come ON at maximum intensity whenever Lights control knob is pulled from the OFF to second or third positions.  Fuse 3 still protects the Instrument Lighting (both fixed and variable) and Parking Lights circuit as before. 


Comments and inputs to this procedure are welcomed! 

122 Instrument Lighting:  Note the Lamp powered with the full and unreduced system voltage at Terminal 58b is the the FIXED brightness, and lamps powered by Term 58a are VARIABLE where voltage is subject to setting of the variable Resistor (Rheostat) or the externally added fixed Resistor.  

FIGURE 2.  Marked-up excerpt of 122 Wiring Diagram showing Variable Instrument bypass connection (shown in Red), from Light Switch terminal 58a (Red wires) to 58b (White wires).  Also shown in Red is optional Dropping Resistor used to implement a (fixed) less-than-full-brightness for the originally variable Instrument Lighting.

Actual value for the Resistor is quite subjective, depending on the owners preference of reduced brightness, but a range of 5-10 Ohms is a good starting value (rated at 5W), practical tests should be performed to determine acceptable level of brightness, and level of heat generated by this Dropping Resistor.

See also:


Related Links:


Excerpt from an e-mail exchange with an (early) 1800 owner: 

...My fuse box isn't keeping up as I am having intermittent light failure and no dash lights whatsoever.  


If it's variable dashboard lighting which is intermittent, [Correction:  All Instrument Lighting is variable in the 1800, and downstream of the Fuse 3!] it could be variable instrument lighting control of Light Switch (turning knob to vary brightness)...marked in RED below [above!] (see also:   ....written for a 122 but applicable to your 1800 with different color codes for wiring!). [1800 specific wiring diagram excerpt added below!]

Below, marked in ORANGE are Variable Instrument Lighting and Fuse 3.  Check that when you don't have Instr Lights to see it you also don't have outside marker lights...if not, Fuse 3/Holder should be suspect.  With Bat disconnected, clean FB and fuse with wire brush and reinstall with ACZP.  If installed fuse value is higher than 10A...or the original 35A Lucas Fuse, I highly recommend replacing it with a value no higher than 10 A!   See:

1800 Variable Instrument Lighting:  


Excerpt of carbureted 1800 Wiring Diagram showing Variable Instrument Lighting and associated Marker/Parking Light circuit. 
I have replaced the Light Switch section of drawing with a more accurate and detailed depiction (Light Switch shown in factory drawings in oversimplified and incorrect.  See:  1800 Light Switch Corrections   A disassembly, review and reassembly of the 1800 Light Switch is also presented at link.)

Note that all Instrument Lighting in an 1800 is downstream of Fuse 3, and also the variable control, so all Instrument Lighting is variable, unlike on the 122 shown above, where Heater Control Lighting is supplied from upstream of the variable control, so fixed at full brightness, and all others are variable. 

Note also that Fuse 3 should be replaced with a fuse value of 10A instead of the ridiculously high value of 35A specified in all factory documentation (undoubtedly assisted by the brainacs at Lucas).  See:  SW-EM Tech Bulletin 4  also:  Proving Everything Bad you Ever Heard About Lucas

Added to the wiring diagram above is also the Fixed Resistor which could be added, if Rheostat was hopelessly intermittent, to make Variable Instrument Lighting of the 1800, fixed, and at a level less than full brightness.  Starting with a value of 5-10Ohms (rated at 5W), practical tests should be performed to determine acceptable level of brightness, and level of heat generated by this Dropping Resistor.  Or, by making the Fixed Resistor 0Ohms (a simple connection with a jumper) the Instrument Lighting could be made Fixed at maximum brightness).  Since terminals of Light Switch do not have numbered designations to help with making connections to the correct location, wire color codes should be followed!

See also:


1800E/ES Variable Instrument Lighting: 

Note that Variable Instrument Lighting on the Fuel Injected models varies from the carbureted models, in that these vehicles have the Variable Lighting Control (43, Rheostat) separate and not as part of the Light Switch (44).

1800E '71 Wiring Diagram Excerpt showing Variable Instrument Lighting and associated components and wiring. 
Note that all associated wiring is highlighted in the above diagram in Red, but some is actually White...
observe actual Color Codes noted on the diagram and in the vehicle!
[...because it's difficult to draw a White wire on a white background!]

Note:  Fuseblocks of the injected 1800s are much less susceptible to corrosion, but not completely immune!  See also: 


My response to a Facebook Thread on the subject:

"Simple contact cleaners might work in the short-term to flush away corrosion abraded away by repeated turning of the switch, but the fix wont last, because these cleaners leave nothing behind, and that's what's needed to prevent surface corrosion from coming back...and it will!  I am the biggest advocate of long-lasting contact cleaner such as Deoxit D5, which does leave lubes and anti-corrosives behind (see:  ) but even it does not have the required staying power [with the heat generated on this Resistance, they would soon be be cooked away and intermittency would likely return!]...I think the only long-lasting solution is ACZP (see:  ) or bypassing variable option altogether. Cheers"


This article is Copyright © 2010-2020 Ronald Kwas.  The terms Volvo and Bosch are used for reference only.  I have no affiliation with either company other than to try to keep the vintage products of both working for me, and to help and encourage other enthusiasts do the same.  The information presented here comes from factory or my own wiring diagrams and is strictly my own experience and opinion, and can be used or not, or ridiculed and laughed at, at the readers discretion (but you might be sorry!).  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 

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 future revisions of this article...along with likely the odd metaphor and maybe wise-a** comment, as they come to me... 


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