My Vehicle: '86 240DL. After an approximate 7-minute drive in 52-degree weather, I noticed the dashboard temp gauge was just entering the red. I instantly turned the heater lever full and switched on the blower. Shut the engine off, coasted to a stop. A roadside investigation of the engine compartment revealed that the upper hose was warm/hot (normal) and the lower hose warm/cool, indicating that the thermostat appeared to be open and functioning proper. Back inside I switched on the ignition and headlights to check the temp. At this point I realized that while leaving the ignition on (engine off), the temp gauge would move from the middle (normal operating range), to the section just under the red zone simply by switching the headlights on or off. The movement of the indicator needle was slow, not instantaneous as if the unit might be grounding completely. Any ideas on this one?

Thanks a Ton!

You have a bad ground connection. Pull back the carpet at the bottom driver side of the console. It's hard to pull it back enough without tearing it, but look in under the carpet and mat at a point halfway down the side of the transmission tunnel and about in line with the front of the shifter. There is a wire screwed to the sheetmetal which needs to be tightened. Voila!

Tough to say without some serious troubleshooting.

1. The temp gauge signal is "conditioned" by the compernsating board. When this board gives trouble, the temperature gauge becomes erratic—it can peg to the top, stay at zero, or wabulate. A small (genuine) temperature change can cause the gauge to swing from one extreme to the other.
   

2. A bad or intermittent voltage regulator (aka "stabilizer," on the back of the cluster) can cause similar erratic behavior. This will also cause the fuel gauge to behave erratically (since both the fuel and temp gauges are powered by this regulator).
   

3. A bad ground or connection in the cluster can cause flakey (indeterminant) gauge behavior, and often as a result of other electrical loads. These loads will upset the ground reference.
   

4. And don't forget the ever-popular crumbling wiring harness. If the wire to the temperature sensor has crumbled insulation, a bare conductor might be intermittently touching ground........

I have been comtemplating a similar problem that only the temperature gauge and fuel gauge seem involved in. The temp gauge goes up, the fuel gauge goes down. It's like a see-saw.

Everyday, on the same road at the same speed, etc., the temp. gauge says different things. After reading the last post mentioning the voltage regulator on the instrument cluster I started to notice the fuel gauge fluctuations. I just replaced the thermostat and cleaned the threads on the temp. sending unit. I can't believe the engine temp. is actually fluctuating. My next plan (this weekend when I install my baby 2 inch tach.) is to:

1. clean contacts on temp. compensating board.

2. check voltage regulator.

3. run a new ground to the instrument cluster.

If all is operating normally, shouldn't the thermostat keep the engine at exactly the same temp. given the same speed, outsite temp, etc? Someone said that the whole range on the temp gauge is only about 15 degress so if you never get in the red to not worry about. But hey a guy's got to have a hobby , you know?

Robb

Robb --

The voltage regulator is shown in the pix below, and the second picture shows the location of the compensating board.

First, be certain the four nuts (two per gauge) are tight. The studs on the meters mechanically secure them, and also serve as electrical connections. Next, be sure the three regulator legs are shiny and properly (tightly) inserted into the socket. Finally, try a replacement regulator.

"...all is operating normally, shouldn't the thermostat keep the engine at exactly the same temp..."

I would expect the thermostat to keep the temperature within a narrow temperature range—probably +/- 10 degrees, or less. For the thermostat to open more (for additional cooling) the actual temperature must rise a bit. And likewise, when the engine load is reduced, the temp will drop somewhat, which cools the thermostat so that it closes. In any simple control loop, some error MUST exist to initiate corrective action.

The whole point of the compensating board is to minimize the needle swing with genuine engine temperature changes within this normal but narrow temperature range.

Don,

You are "the man". Much thanks for the info and pics! Ahh, Control Theory, I think I zoned out in that class - the teacher couldn't make it interesting for me....

I still wonder... I have had many cars that once the thermostat opens - boom it jumps to the same spot on the temp. gauge 365 days a year no matter how you are driving and stays there. Specifically, I now also have 2 Dodge Caravans that behave consistently on the same spot on the gauge all the time. Perhaps their gauges have a much greater range from cold to hot so you don't see the minute flucuations. I wish they'd put numbers on the darn gauges for us engineering types.

Thanks again for your detailed hints on the the compensator board and regulator. I'll dive into this weekend. Now that I've noticed that the fuel gauge seems to also be erroneously moving around I'll concentrate on the voltage regulator and ground but do the other stuff too just because I like to be thorough.

I'll post my results.

Robb

sounds curious,

In my brick i can fill up the gas tank from the red zone to nearly 1/2 of a tank simply by turning on the heater, but never noticed any effects to the temp gauge. I'm sorry, i realize this is probably not very useful information to you, but thought i'd share it anyway.

one thing to remember- the temperature range in the volvo 240 temp gauge (the green area) is only 15 to 20 degrees F (from bottom of green to the bottom of the red zone. do not worry about minor movement of the needle.

it sounds like you have a weak battery or a marginal altenator. a temperature sensor is a resister. resistance is a function of voltage and current. with the ignition on (and engine off), turning on the headlights dropped the voltage. The equation is V=IR, you change the voltate, you change the resistance (if the current is kept constant). this is why I think you have a bad battery. there should not have been a noticeable voltage drop in that short of time period. (with the engine off, there was no regulator/alternator to keep the voltage constant). also, a running engine keeps the voltage about 13+ volts. no engine on, the voltage drops to 12.5 or less volts. Have your battery checked.

Sorry, I didn't detail the information exactly, but with my scenario, the temp gauge moves up when turning on the headlights, not down. Also, the car has 158k & new battery. The sweep is approximately 1/3 the length of the gauge, from lights off to lights on.

Thanks again

If you have good grounds for the headlights and the dash then you may want

to check the ground wire for the alternator. Also if you see any flickering

on the dash like the idiot lights and even if you don't see any I suspect

you may have a bad voltage regulator on the alternator. Is your alternator

very old?

Dave 82 242ti 242kmi

I have the same problem and was instructed to change a small regulator behind the dash. Recently, I had a relay that malfunctioned. This relay controlled the headlight switcch on the dash. The harness leading into the relay looked like it was melting down. Now, i was thinking that perhaps this problem was somehow connected to the problem of headlight switch affecting temp gauge and fuel gauge as in my case. I still note the temp fluctuation after filling the petrol tank? This leads me to believe tha it is indeed the regulator behind the dash cluster. I was told this part is a bout a $25 item.

Alan