Two Wrongs make a Right! (Electrically Speaking!)   
Jun 2021 R. Kwas   [Comments Added]


Details of function of the Brake Warning Valve and Switch

Additional Information:
        Resetting the Brake Failure Warning Valve Switch

        Brake Warning Switches

Brake Warning Indicator Disassembled
Dual Brake Failure Sensing from the Volkswagen World


Sometimes, when I help an owner through an electrical issue, I come across a symptoms or situations which are truly entertaining to me(!), whereas they might be completely baffling or even infuriating to the poor electrical layperson.  This has just happened when discussing a '68 1800S of John F with Dual Braking System.  He mentioned that the Brake Warning Indicator was lit, noting that he did not have ANY Handbrake installed, and the wire, which would normally get a chassis connection by a Switch at the Handbrake, wasn't even connected (but hanging in the air, not dangling against the body either!). 

Brake Warning Indicators on 1800E and Amazons with dual Braking Systems.


Referring back to the Owner's Manual, I pointed out to him that this Indicator, when fitted in 1800E or 122/123s with dual Braking Systems, was an OR-tied Indicator, with either the Switch at the Handbrake OR the Brake Warning Valve Switch being able to light the Indicator by supplying a chassis return connection, after the internal shuttle moves to the side due to one of the two Braking Systems not making pressure.  This owner was actually aware of the BWVS input, and confirmed it was responsible for turning ON the Brake Warning Indicator, by disconnecting it, and observing the Indicator went OFF.  He also noted that non-self-resetting Switch was latched closed (with both Brake Systems in proper working order), and asked what the best way to reset it was (see below!).  So far so good, and nothing surprising...yet! 

See also wiring of Brake Warning indicator as implemented in the later 1800E:  Wiring Diagram Excerpt Brake Warning Light

Details of function of the Brake Warning Valve and Switch:

Little magic...just a simple and effective way of sensing a pressure difference between the two braking systems...!  Of course there is a precise pressure difference specified as a function of the counter-spring force vs. the cross sectional area of the Shuttle, and this is covered in the Green Factory Manual, but that is not necessary for a general functional understanding of the BWVS here.  

See also how VW does this in contrast.  

Inner workings of the Brake Warning Valve Switch, in Normal and Activated Conditions. 
The Orange contact is shown lifted from contact with conductive housing in the Normal Condition,
and in the Activated Condition, bridging from the external terminal to housing (chassis).


The Shuttle (Volvo calls it Piston, but I prefer Shuttle as it is able to move in either direction) within the BWVS, had moved to one side when this owner bled the brakes, and I advised him on the process of resetting the Shuttle (essentially just loosening the Switch and allowing the Shuttle to recenter itself from the springforce at either end, but see also below), but he noted that when he had the wire off the Switch, to extinguished the Brake Warning Light, the Tachometer would also quit working!?!  Now that Symptom is unusual!

This very unique interaction of normally unrelated circuits might be a real brain-teaser, because there is no wiring common to the two circuits to interact(!), but I expect it is caused by once again the most common cause of wacky symptoms:  A poor chassis connection...this time, I expect, in the area of the Tach [that's the most I can say at this time!] until he feeds back the actual findings and solution, I have generally explained it to the owner as follows, and I have also suggested he carefully poke around the area with an Insulated Poking Tool (for example, also known as a free, give-away chop-stick from the oriental restaurant of your choice, after you're done with your Fu-Yong!), and to note all findings, conditions and associated symptoms and corrections he undertakes, and will add this feedback here. 

General Explanation of Symptoms:  If Tach works only while the BWVS is closed and supplying a chassis connection to the Brake Warning Indicator, that suggests this is also the chassis return current path for the Tachometer, BUT, the Tach does not have a separate wire for this (which would be the first logical place to look for the poor connection resulting in the interaction of these two, normally independent circuits)...instead, it makes its return path by way of internal connection to the Instrument case, and externally, the securing brackets as shown here [yes, those brackets are critical to the function of several of these Instruments, including on later injected models, with the Voltage Stabilizer!]


1800 (Smiths) Instrument mounts are either two, single sided "J" shapes, or a single "U" shape, which straddles the back of the instrument.
In both cases, Thumbscrews tighten these onto on threaded posts on the instruments, to draw the Instrument securely into its hole in the Dashboard,
 but the electrical fun does not stop there(!), since the Instruments do not have individual connections and wires for their negative return current, these
 mounts also serve as the electrical return path for the instrument!  What does the reader of the SW-EM site think the author recommends to keep this
current path conductive and free from corrosion...?  
(Two pictures stolen from epay offers...Condition:  Good, gently used, and 50+ years old, but sold As-Is!)


I expect that somehow the normal, intended power return path for his Tach is less than good (I hope its not the circuit board to Instrument case connection, within the Instrument!), and chassis current connection is actually by way of the BWVS.  If I get enough findings and details from the troubleshooter, I may/should be able to more specifically pinpoint the poor connection...stay tuned! 

Intermediate Condition (...what we know so far!):  Two wrongs of a closed Brake Warning Valve Switch, AND likely a poor chassis connection at the Tachometer in combination, have resulted with a normally functioning Tachometer!  

[See also picture of some wiring needing a connection to chassis behind the 1800 Dashboard...this was simply added to the Instrument bracket (Fuel Gauge in this case!).  It is important wiring I'm certain, but it is not even specifically shown on the Wiring Diagram! ] 


Feedback from John F. after resetting Brake Failure Switch and restoring Tach function [and Conclusion]: 

        " here is what I did. First, I removed the brake warning valve/switch. The plunger was stuck in the closed (?) circuit activated position.
        I worked the plunger in and out several times and added some lubricant then reinstalled it.  The brake light went out, but I still did not
        have a functioning tach. [I rather expected this, because there was now no chassis connection, as had been there when the contact was
        closed to chassis!] So, I climbed under the dash and started looking for loose or disconnected wires or nuts. And low and behold one of
        the thumb screw nuts on the tach was missing, and the other was very loose. I'm not sure how that happened but when I tightened the
        one and added a new one, the tach worked fine. So, I guess you were right. Somehow, don't ask me how, the tach was grounding through
        the brake switch. [Indeed it was...electrical current is not intelligent, but it does follow the rules of physics and science, so can sometimes
        be crafty and downright spooky in finding a conductive path!]  The wire from the brake switch to the light looked good but it goes into the
        harness and is lost for most of its travel. Anyway. all seems well now..."   

Tip:  You might want to put a dab of ACZP under those thumbscrews, now that it is clear they provide an important electrical connection!

...and note that if your 1800 is a later Injected E or ES, and you're just reading this out of interest or for entertainment, this situation and advice STILL apply to you, because neither Gen 2 (Turquoiseface) or Gen 3 (Blackface) Tachometers had separate negative power wires, but only return their operating power to chassis by way of the Instrument Housing, and securing brackets.   


Additional Information:

Resetting the Brake Failure Warning Valve Switch:

Regarding the Brake Warning Valve Switches and preventing damage to the protruding plastic pin, I thought the following info was important enough to pass along verbatim!  The highly experienced Volvo Specialist Eric H. "Planetmen" on the vintage Volvo Fora (contact info below!) suggests the switch should be removed BEFORE the brake bleeding operation!  He says in this Thread (  ) on Turbobricks:  

            "...critical procedure is to remove the switch BEFORE the brakes are bled and to only re-install the switch
            after the brakes are bled.

            The plastic shaft that actuates the switch gets damaged each time the shuttle assembly inside the warning
            system goes back and forth.

            This chops off small pieces and shortens the plastic shaft to the point where it is not long enough to cause
            the switch to break its internal contact that would shut off the dash light. "


Brake Warning Switches, new vs old, damaged.

Neville Britto picture used with his kind permission.

New BWS on left and used one on the right, exhibiting definite signs of a damaged/sheared transfer pin,
consistent with not having been withdrawn before bleeding operation, as Eric warns! 


Links to Referenced and Related SW-EM Articles:

The Voltage Stabilizer of the later 1800E and ES models (with Blackface instruments) are mounted on the Instrument mounts, and use the mount for their chassis return current path as well!  This connection has previously been shown to also be critical to the function of the VStab!  See:'s%20Tachometer.htm  I have added the information learned here to the Tach Notes Article also.


External Links:

Eric Hamlet
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502 or 

An example of the "Craftiness of Electricity" to find a current path...this time, after a tree rips the electrical service wires clean off the house (but only disconnects one of the three!), and there are NO SYMPTOMS, and nothing seems wrong!!


Brake Warning Indicator Disassembled: 

Notice this Brake Warning Indicator is fully isolated from the panel it is mounted in, so will need two connections, as made somewhat obvious by the two terminals evident (an indicator which had one side of the lamp tied to the panel would have only a single terminal!).  ...and that spring is pretty impressive, and about 20 times heavier duty than anything found in a chinashit indicator...that's why this indicator will likely also last about 20 times longer than the chinashit indicator...just sayin'!  R. Epperson picture, used with his kind permission.  

2W Indicator lamps:   Phillips (Made in Germany)12913.


Dual Brake Failure Sensing from the Volkswagen World:  

The Volvo solution is what I would call a hydraulic solution.  According to this graphic, VWs used what I would characterize more as an electrical solution, employing two hydraulically activated SPDT (Single Pole Double Throw) switches and a transistor to control and drive the Brake Warning Indicator.  I show the VW solution here because it demonstrates and explains some interesting circuit design and implementation also incorporates a Bulb-check function which the Volvo implementation does not have (of course being OR-Tied in the Volvo application, we will see the Indicator lit with the Handbrake activated). 

VW implementation of a Brake Warning Indicator for dual braking systems.  Unfortunately, I only have the gallery location on thesamba VW site I can attribute this graphic to:  ...I expect it is originally from some VW service manual. 


External material sources are attributed.  Otherwise, this article is Copyright © 2021-2022.  Ronald Kwas.   The terms Volvo, VW, and Smiths or other suppliers, are used for reference only.  I have no affiliation with any of these companies other than to try to keep their automotive electrical products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here (like giving Lucas all the grief they so richly deserve!).  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! 

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 odd metaphor and probably wise-a** comment. 

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