Hello everyone,

I just put a new engine in my 1985, it is running great. However the temprature gauge is malfunctioning, it reads normal temps at idle. I started the car cold the gauge was reading LO, I drove it less than a 1/4 mile and the gauge went to 260. the coolant is full and the T-stat opens around 185 degrees, any ideas what could be causing this? A bad ground maybe. thanks for any guidance

 

What does oil temp read when it shows coolant at 260?

Are 1985's pressurized? If so, did you get all the air out?

If it is a ground issue it is not in the CTS circuit. It will be with the computer itself.

The CTS circuit has two resistors in it. One is a fixed resistor something like 1,000 Ohms. The other is the sensor itself. As the engine heats up the sensor's resistance drops so that the voltage of the CTS circuit raises. If it were a grounding problem with the CTS circuit then the temp would forever read "Lo".

If it were a failure of the fixed resistor allowing more voltage on the circuit than normal, the system would start off with a high temperature reading and you'd never see "Lo".

Check the ground at the comp.

 

Check/test the coolant temp guage sender located between #1 and #3 sparkplugs.

 

what is the proccess to test the gauge sensor? I did chase wires and found that that wire was burnt, so I repaired it and the same thing happens. I can check for A ground problem at the computer, Where can I get a diagram for this car? Like I said before It works Fine At idle, I will bleed the system of air again. This is annoying I just want to drive my car! Thanks for the input, I will keep you posted.
Eric

 

Make sure all the air is out of the cooling system. You have to remove the radiator cap (engine cold) and idle the engine at 2K for a few minutes until it's hot. Add coolant as necessary.

Remove the connector from the guage sender. Engine running, if the guage says LO (cold) even though the engine is warm, the wiring is good so replace the sender.

Remove the connector from the guage sender. Engine running, if the guage says 299*(hot), check for a short in the circuit (drk green wire) back toward the instrument cluster.

If the circuit and connections are good, replace the instrument cluster.

Here this might be easier. It's from my 86 FSM, and my sender is located between #6 and #8 plugs. The 85 was different.



Everyone needs a FSM for your year car!

 

The computer has nothing to do with the temp gauge on an '85. A whole car wiring diagram should come with the FSM for your car, in addition to the electrical trouble shooting manual. As RRT vette said, your temp gauge sender is in your left (drivers) side head between the #1 and #3 spark plugs. A couple of simple tests are to disconnect the wire from the sender, then with the ignition on, check the gauge. It should read "Low". Now ground the wire that connects to the sender. The gauge should read full scale, (300°? Maybe only 260°). If that checks out, you know the gauge is OK. Now you can ohm the sender at different coolant temps to be sure the sender is OK. The specific ohms at each temperature isn't so important as the fact that they are different at different temps. Don't just replace the sender, unless it rests bad, because it sounds like it is OK, and that would just be throwing money at it. Based on your report, I'm guessing that there is either an intermittent short or open circuit in the wire from the sender to the gauge causing your problems. I'll bet there is a place the wire can short out, that you missed in your repaors. ALSO! Make sure there is no Teflon tape on the sender where it screws into the head. As you can see from the above, the gauge works off of the varying current that the sender sends to ground. If you insulate the sender or even compromise the ground to the head, it will cause problems with the gauge. If you need a sealant, use a liquid or paste that won't insulate the sending unit.

RACE ON!!!

 

Well here is the current situation now the oil temp gauge went to 320 as soon as I started the car, and the temp gauge starts at lo and climbes up without the wire hooked up to the sensor. Grounding the wire to the sensor did not make the gauge reading change. Do I need a new digital dash?

 

^^damn dude don't go to extremes. I am sure it has nothing to do with the Dash.

Now, if your dash is faded and you need an excuse (for the wife) to put one in..........Ok.

It is your dash, replace it now.

Highlight that last sentence and print it for the little lady.