Voltage Stabilizer Study                                                                            05/02  Revised 1/09 R. Kwas

General:   In order to assure accuracy of the instruments and independence from vehicle voltage system variations, a strategy of using a  voltage regulator (stabilizer), was employed on late 1800 (injected) and 140 Volvo vehicles.  In the late sixties, this was still implemented using a thermo-mechanical module which contained a cycling element.  The original design was OK for its day...bimetalic springs are predictable, repeatable and reliable, but combining these with a heating element, with its connections of dissimilar metals, starts to reduce that reliability...add a few decades and the design will develop problems and fail.   

Exerpt from Volvo Service Manual for 1971 1800E


"Constant", vs. Average, vs. Continuous.  According to the factory manual, the output switches from 12V to 0V, with the long-term average value being 10V (this is incorrectly called "constant" in the manual text, possibly due to quirks in translation from Swedish).  This average voltage value is a function of % Duty Cycle, defined as the ratio of ON voltage and OFF voltage time.

Solving for duty cycle  = 100X10/12  = 83.33%

In other words, the output is 12V, 83.33% of the time, and 0V, 16.67% of the time, typically the on time is XX and off time is XX, but the actual time is noncritical.  As long as this ratio is correct, the long-term effective voltage is 10 +/- .2V.  This is why a failed unit can successfully be replaced by an electronic regulator with a fixed (continuous) output of 10V.  A Voltage vs. Time graph follows: 


VS output voltage plotted against time axis.


VS Wiring:  As can be seen in the wiring diagram following, terminal B of VS is the IGN Power input from Fuse 4.  Terminal J is the output and this terminal powers items 27 (Fuel Gauge), 29 (Oil Temp), 66 (Coolant Temp).  It is quite important for the VS to have a good electrical connection at its mechanical mount, as in addition to mounting, this also supplies a current path to the chassis for the internal heating element.  A loose, poor electrical connection just at the mount would result in non-function of the VS! 

Failure of VS will affect only the meters powered by VS!  I have heard of meters being damaged by a continuous 12V from a failed VS, but I have not confirmed this as a typical senario...since the instruments are being powered by 12 and 0V continually alternating, I rather doubt that applying 12V continuously could result in damage...whacky readings sure, but not damage...any reader having confirmed info to the contrary is welcome to contact me and set me straight on this! 


Exerpt from factory 1971 1800E Wiring Diagram.  Voltage Stabilizer and instruments supplied by it are highlighted. 


Voltage Stabilizer plus internal details. 

The most common failure mode for the VS is failure to cycle and stuck closed (steady voltage output terminal equal to input) when the heating element wrapped around the bimetalic element fails to heat the bimetal spring.  This can be caused by the element having burned open, corrosion at the connection end(s) of heating wire preventing current flow, or corrosion at the chassis tab which is crimped over the board edge and makes contact when case is crimped over that, also preventing current flow. 

If you feel adventurous enough to gently uncrimp the housing, and do an internal investigation, you may be able to determine the exact cause of failure, and correct this...otherwise a replacement is due.  Preferring a more reliable, modern all electronic solution, I would never reinstall a thermo-mechanical unit.  This is an ideal use for a highly reliable, fully electronic regulator putting out a continuous (truly constant!), not just average 10VDC.  The instruments draw only a small amount of current so represent a minimal load...a 1Amp regulator would essentially be only loafing along totally understressed.  I suggest using a completly electronic 1A regulator, such as an LM7810 (fixed) or LM317 (adjustable, set permanently to 10V). It takes a bit of electronic micro-construction, but I have even fit these into the original metal case, so when reinstalled, the upgrade is not even obvious without a close inspection, or electrical test.  Sorry, no pix available of this... externally, it wouldn't even look any different from the unmodified unit anyway! 

Links to discussions on the Subject:



The term Volvo is used here for reference only.  I have no affiliation with this company, other than to try to keep their products including subsystems working for me, and to help other enthusiasts try to do the same.  The results presented here are from my own experience, and can be used (or not, and used strictly for your amusement!) at your discretion.  As with any recipe, your results may vary!  As always, if you can supply corrections, or additional objective information or experience, I will consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and maybe wise-a** comment. 

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...the Washington Post wants you! 


B A C K! ...to Service Notes