Temp gauge wacky

[kgroombr]

I have been tinkering with this on and off for six months. I can't get my 77 Vega GT, with the 140, temp gauge to read correctly.

Symptoms:
At cold the temp gauge is pegged down where it should be.
As engine warms up it pegs out at 250 and then goes down and steadies out at 210 after about four of five minutes of reading 250.

What I have done:
Replaced sending unit, no change.
Temporarily replaced gauge with a known good one from another Vega I have that reads correctly. No change.
Installed a new grounding strap on the engine.
Checked ground on the instrument cluster (all good).

I have used an IR temp meter and check the actual temp of the engine and it never goes above 194 degrees (even at the sensor area) during the whole time the engine is warming up and the gauge reading 250.

Other problem that may be related to this. My tach reads about 100 rpms high and I notice that it drops slightly when I turn the headlights on or crank up the blower motor.

I am dumbfounded. I followed the wiring diagram and didn't see anything that would cause this problem.

Any help would be greatly appreciated.
Thanks,
Ken

[Monza Harry]

Ken is it possible that you have an air bleed problem (in the coolant system), I had a POS that used water and the crazy gauge thing notified me I was low on water. The other thing that comes to mind there is a voltage regulator in the gauge system for the gauges (they are all 9V except the volt/amp, although all the gauges would go ape S T if this was solely the issue) that could also be a part of the problem as you mention the head lights, etc. So my next thought is the printed circuit on the cluster perhaps you have one line damaged in the temp circuit, causing this. I haven't been under dash in along time so maybe Spencer, Senior Savage, or others who enjoy sitting in the seats upside down will be able to help you locate this and trouble shoot your problem, I have a feeling that your problem is more likely electrical (Tach, head light and other clues you indicate) Just some idea's to banter about. Good Luck and keep us posted Please. Harry

[heinz057]

do you have the sending unit for a gauge and not a light ?

[kgroombr]

do you have the sending unit for a gauge and not a light ?

GT with temp gauge.

I said earlier it levels out about 210. It actually levels out at about 220.

I am pretty sure there isn't any air in the system. I changed the coolant about a year ago with 50/50 and at the same time I changed the thermostat thinking that was the problem. I purchased the car late last year and it has been doing this since they day I got it running. No leaks, and no overheating of any sort, as I can open the radiator cap at any time and it never boils.

I have kept it full and have the right amount in the overflow bottle. I only had to add a little (maybe half a pint?) during a 5000 mile period.

Thanks all for reading. What would help, and I have googled quite a bit for this information is how to use a multimeter to test the sending unit. I found some info, but it doesn't discuss what the reading should be, only to see if it is changing.

Ken

[spencerforhire]

You seem to have covered all the bases here; you've tested everything you can and eliminated other factors that would cause a gauge to read high. When you say you've tried another sender, I presume it was a new one from the GM dealer. This could be the source of the problem; over the years, they have made dozens of sensors and they do not all produce the same resistance at the same temperature. GM is notorious for improperly cross referencing parts; if the sensor is from a different year or a different car it could be wrong. Chances are the previous owner installed the wrong sender and now you have too. I guess you need to find an original sender from the exact year Vega as yours to test this theory.

[SeniorSavage]

sounds to me like maybe your sender is too close to an exhaust manifold or header?

just a thought ..

[cosvega76]

Sounds like a ground problem to me. Check the grounds between the instruments and the body, and the body to the block.

When I was a lot younger, I did a valve job on my dad's Rambler. Everything went together fine, but when he went to work with it, he said the temp gauge showed it was overheating, but didn't act hot. I had forgotten the ground strap between the body and the block, and it was grounding through the temp gauge.


Chuck

[SeniorSavage]

thats a good lesson for everyone Chuck - thanks for mentioning it.

[cjbiagi]

It's always a good idea to add extra ground straps between the engine and chassis as well as other subsystems and the chassis. Bad grounds account for a lot of strange problems. Although you did say that you added grounds already......

[marco_1978_spyder]

I also suspect ground. Try making a makeshift ground directly from the gauge's ground post to a good ground source and see if that clear's up the reading's.

If that doesn't change anything, I'd look into Heinz's suggestion and make sure the sender is the right one for the gauge equipped car's.
htth

[kgroombr]

Thanks all,

I will check the grounds. I have a sending unit from a engine that read correctly when pulled. I will try that next. I will keep you posted.

Ken