Engine temp all over the place
 

Hi everyone, I'm new to the forums. I just bought a 1992 Volvo 240 sedan a couple of weeks ago, and aside from a few things it needs, it's in great shape!

One thing I've noticed is that my engine temperature gauge seems to settle at around a quarter of the way up during normal running. The engine itself seems to reach normal running temperature when you actually check it, but the gauge doesn't climb to normal.

Today, I was driving out of my neighborhood and the gauge started climbing rapidly, and stopped just below the red. I immediately pulled over and popped the hood, but the engine temperature felt totally fine (just like it does when it's running properly). I have plenty of coolant, and there are no leaks or steam. As soon as I pulled over I'd turned the car off, and turned electrical power back on about 30 seconds later, only to discover that the gauge had dropped all the way back down to 1/4, where it usually sits.

I started driving again, and about 10 minutes down the road it jumped right back to just below the red. I pulled off again, turned it off for a couple of minutes & looked under the hood, and everything was the same. I turned it on, the gauge went back to normal.

I decided to head home and park it, and as I was driving the gauge climbed to about halfway and settled there. Normally I wouldn't think about this, but given that the gauge usually settles around 1/4 I was concerned. I got it home and it's been sitting since.

It seems like the temperature sensor may be going bad, but what do you guys think?

Yep. That and Volvo temperatures are on crack.

One should be careful with engine temperature matters-easy to lead to Head Gasket and worse repairs! I'd change the sensor with a known good one to eliminate it as a cause, then look at the gauge/circuit board malfunction which is much more common and probably the cause of your troubles.

240 (and 740 and 940s) have a 'temperature compensation circuit' behind hte instrument panel... on 240s it was a separate little circuit board lpugged into the back of the instrument panel, on the 7/9's, its built into the instrument panel.

the purpose of this circuit is to keep the needle dead centered if the temp is in normal range. it creates a sort of dead zone, so if the temp is too cold, the needle reads low, too hot, it reads high, but anything in the middle its dead centered. darn near might as well have an idiot light instead of a gauge :-( this circuit can malfunction and lead to screwy readings, so thats something to consider as well as the temperature sending unit

I suspect Volvo put these compensators in to stop people from bugging their dealers with 'oooh, my car is too hot or too cold' when its really just at one end or the other of 'normal' based on driving conditions.

for those 240's where its a seperate module, IPD sells a kit that replaces that module with a jumper or something and lets the temp needle more accurately reflect the actual engine conditions.

anyways, thats just one more thing to check, in addition to the sensor and the wiring.

Pierce, you are spot on as to why Volvo came up with the board. The 140's don't have it; it's kind of nice to have a gauge that rises slightly as the engine idles. Idiot gauges are all the rage now...I know the Ford Explorer has an oil gauge that reads dead in the middle...it is connected to an idiot light sensor. Doesn't drop until op is down to 7 psi.

I would love to get my hands on a late 740, early 940 instrument panel (say, 91-93), so I could play around on my friends electronics bench and see if we can't come up with a single hack to duplicate what that IPD jumper kit does on the 240's that have the seperate module, i.e. turn the needle back into a 'real' linear gauge... my guess is, it would be pretty darn easy and just involve slicing a wire trace or two on the flexiboard and adding a jumper somewhere, maybe a resistor, but of course, the trick is, figuring out WHAT wire.

this panel wouldn't need a working tach or speedo or whatever, I just need the temp sensor part functional.

Here is a pictorial guide on how to defeat the temp compensation board from a very smart guy:


http://cleanflametrap.com/tempFaker.html

yeah, thats a 240 that has the seperate compensator board. AFAIK, the 740/940 has the equivalent circuitry on the main board of the instrument panel.