on the way to work today i saw my fuel gauge slowly going towards empty like i had a quarter sized hole in my gas tank. i kinda freaked out. then i realized i had been driving in 70 degree weather for half almost half an hour yet the engine mysteriously had not managed to warm up all the way? factoring in the random jumps from the rpm gauge, i determined theres some crazy wiring problem. if i hit the brake the rpm gauge shoots up to like 3k and alot of the needles are just dancing. does anyone have any idea whereabouts the problem wiring might lie? i really dont like driving around on fake empty.

Behind the two gauges (temperature and fuel level), there's a gizmo (scientific term) that ensures a reliable (steady) voltage to these two gauges' circuits. Good news - if you're handy with soldering and circuit boards, it's replaceable!

Symptoms: (1) your two gauges fluctuate, (2) they are affected by stepping on the brake pedal, and (3) they both (gas and temp) are plagued by this problem....
It's the voltage stabilizer -- this is the only thing that the two gauges have in common, and this is consistent with your symptoms.

Why you have a voltage stabilizer, and why do we think this is the problem (and I'll keep it very, very simple -- engineers, please be tolerant of this simplicity):

Both gauges rely on the same principle, Ohm's law. Both gauges work by measuring how much electricity flows through them (current), and what affects that current are two things, resistance and voltage. Both gauge's "senders" operate by changing their resistance (either based on engine coolant's temperature or the depth of fuel in the tank). So that leaves voltage as the remaining factor that can affect the gauge's readings. But we want the gauge to only be affected by the resistance in their senders, so we want to keep the voltage constant.

Unfortunately, the voltage from the car's electrical system varies -- when you've just started the car, after a long drive, and most important, when you use accessories (even stepping on the brakes). So this would make the gauges' readings change all the time. So the solution? Ensure that these two gauges always get the same, reliable, steady amount of voltage! Thus, we have the voltage stabilizer, which takes the car's varying voltage and makes it steady and trustworthy (until it breaks, like in your car).

Hope this helped.

thanks ken! you're right - that definitely explains the behaviour. the temp gauge and gas gauge both appearing low would indicate a lower voltage than normal i guess. i assume they have their own smoothing caps too considering they're so slow reacting which could be why they're not jumping like the rpm gauge is. if its something like a 78xx i probably have parts on hand to replace it! i'll check it out when i get home. my main concern was running out of gas so i guess i'll just go straight to the pump and fill 'er up to be sure. thanks again for the insight!

handsome greg
http://xdissent.com

Hi, again.

I was able to find it in my manuals (dated 1990) ... it's a IC Voltage Regulator, typical three lead chip. And its specified output should be 10 +/- 0.2 V. Only thing I couldn't find out is its capacity (current or wattage) -- these green manuals assume that you're using Volvo parts, naturally!

You probably can replace it easily, as long as you use a chip with a high rating.

Good luck.

It may not be an IC chip...

I remember that the older 240s had a electromechanical device, not solid state. Maybe the newer ones like your '86 have changed over, though don't be surprised if it's still such an antiquated design.

I have heard of folks that converted their panels to using an IC -- you just have to determine what the proper voltage is -- it was less than the battery voltage (to ensure stability), but I don't remember exactly what -- maybe 10 volts?