While restoring my 63 Vette, I discovered the temp gauge was not working and sent it off for repairs. When it came back, I checked and adjusted the calibration on the gauge before putting it back in the car. I did that by simulating the sending unit with a variable resistor set to 125 ohms and installing the needle so it pointed to 180 F. I reinstalled the gauge and ran the engine. The temp gauge rose steadily until it settled on 240 F. I shut off the engine and checked the sending unit resistance - it was 125 ohms. An infrared temp sensor showed the thermostat housing at about 180 F. This all points to an incorrect temp gauge. .... So, I disconnected the sending unit wire and connected it to a variable resistor set to 125 ohms. With the ignition switch on, the gauge read 180 F !? I reconnected the wire to the sending unit and again turned on the ignition switch. The temp gauge started at 180 and then steadily rose to 240 within about 10-15 seconds. I've repeated this experiment several times and the result is always the same.

I'm baffled. The gauge reads correctly when I run it from the calibrated resistor. The sending unit is creating the correct resistance for the temperature. But, when I connect the sending unit to the gauge, the gauge reads correctly for a moment and then steadily increases to read at least 30 F too high. Anyone have any ideas as to what is going on?

 

Can you tell us whether an increase or decrease in resistance is needed to cause the guage to rise? I assume that you checked the sender sesistance while hot in the manifold and that you check the 125 ohm resistor setting by grounding it to the intake.

 

Sounds a bit like a short in the temp sender.

 

When I was working on an earlier gauge problem, SWCDuke made me aware of a document called "Troubleshooting Corvette gauges." It provides a table of resistance readings and corresponding temperature indications. My sending unit matches that table very well (i.e., as resistance falls, temp indication rises). My gauge matches the table too as long as I use a calibrated resistor instead of the sending unit. (One lead from the resistor connects to the sending unit wire and the other lead is grounded to the engine). The problem occurs when I connect the sending unit to the gauge.


[Modified by kellsdad, 9:32 PM 3/26/2002]

 

I would still be suspicious of the sender. You can get a replacement sender at Autozone for $5.99 (Wells TU 5). I'd try one just to see what happens. Since your gaufge is basically "pegging", it indicates a ground in the circuit somewhere. If the opposite were happening (reading low), I'd guess that you had thread sealer on the sender and were not making good contact, but with the high reading, something has to be grounding out somewhere. :confused:

 

Do you have this document available? I would be very interested in to have it. I also have problems with my temp, batt and gas gauges.

Thanks in advance


[Modified by tapio, 7:44 AM 3/27/2002]

 

You have mail.

 

...all i know about electricity is that it somehow comes out of the wall....if everything is calibrated (did you try a new sending unit?) then it sounds like a bad ground

 

Knowing how bad grounds are a very common problem, I thought of that one too. I ran an extra ground wire to the instrument cluster near the temp gauge. It had no effect.

Any other ideas?

 

Kellsdad,

I too would look at the sending unit. It's a cheap and easy test and needs to be eliminated as a source for your problem.

Also, I'd love to have a copy of those calibration tables as my gauges have been erratic for quite some time.

Thanks,

T-bone

 

Kellsdad,

On my 65 the temp gauge was reading 20 degrees higher than the actual temp (IR gun). I used a meter with a variable resistor to add resistance until the gauge and the IR reading were approximately the same. Once I found the right resistance I went to the local electronics shop and purchased a resistor to match, spliced it onto the sending wire and everything was fine. Keep in mind that this technique calibrates the sending unit/gauge for the sweet spot not across the whole range of temp readings.

 

I assume that you're using a digital ohmmeter to check the resistance of your sender in the block. I also understand that you're touching one ohmmeter lead to the sender tip and the other to the intake.

I would do side-by-side resistance checks with the ohmmeter of the variable resister (set on 125 and grounded to the intake) and the hot sender (at a gauge indicated 180 degrees, installed in intake).

In other words, start up the car with your sender hooked up to the gauge. When the temp gauge reaches 180, shut off and quickly take an ohm reading of the sender with the gauge wire disconnected. Then ground the variable resister to the intake and measure the resistance (when it is set on 125 ohms) with the same ohmmeter.

 

Jerry -
I did that very thing and the resistance was the same both ways. I'll try a replacement sending unit later today and if it solves the problem, I have a working theory. When I check the sending unit with the Multimeter, it passes a relatively small current through the unit to the ground to measure the resistance. I suppose its possible that when I connect the temp gauge to the sending unit, it passes a larger current through the sending unit and could aggravate a short circuit in the unit. I'll post the results with a new sending unit tonight.

 

KELLSDAD, I FEEL YOUR PAIN. i have had a similar problem for three years with my 64. the gauge heats up in a normal fashion and stays at 180 for about 15 minutes. then, it creeps up to well over 250, and finally buries itself. now, before anyone howls, i had a radiator shop infrared the system and i am running at 180 even with the gauge bottomed out at HOT. it seems that somthing is shorting out the gauge. i have replaced three sending units, but the same thing always happens. so far i am learning to live with it. i have my own infrared unit to confirm my car is not overheating. THIS SPRING, I AM GOING TO BYPASS THE HARNESS WIRES AND RUN A WIRE DIRECTLY FROM A STEADY 12V SOURCE, TO THE GAUGE, AND FROM THERE DIRECTLY TO THE SENDING UNIT TO RULE OUT SOME KIND OF SHORT IN THE HARNESS. beyond that i don't have any hot ides. yes, i have used variable resistors to calibrate my gauge. sounds like we have the same problem. any thoughts???

 

Tom,

I'm having similar symptons with my '66 gauge. I've done similar troubleshooting with as you have described using the troubleshooting document that you emailed me. My gauge will peg all the way to the right when I first hit the ignition after the car has not run for awhile. I short the temp sender lead to ground and it will correct itself back to the cold side. The gauge will then steadily rise past normal operating temperature as the engine heats up and indicate that the engine is running in the red. I know the cooling system is fine as I can drive the car for extended periods with no overheating problems. Occasionally the gauge will correct itself back to the correct reading (around 180) or will flutter between 210 and 240 but most of the time it is reading in the red zone. The fluttering is what bothers me as we know the temperature does not change that quickly.

I ran 12 volts direct to the positive terminal of the gauge of the gauge with a pot connected between the sender input terminal of the gauge and ground. The gauge still experience some fluttering but the needle was in the right spot according to the table in the troubleshooting guide. Since I bypassed the harness and the sender I think I might have a gauge problem unless the pot I used was to sensitive. I have a restored gauge which I plan on bench testing before I go to the trouble of replacing my original. My only other theory is a bad cluster ground but everything else in the cluster is working fine. I'm looking forward to your post on the temp sender change.

 

Been there, done all that. I just installed a parabolic temp gage on the lower l/h side of the dash. Works perfect since it's not electric. :lol:

 

Try a temporary wire from the sending unit to the gauge. I suspect the bulkhead connector has corrosion and is causing unwanted resistance in the circuit. This is a very common issue with mid-years.

 

Well gang, I installed a new temp sending unit and the results are ..... (drum roll please) ...... (the tension is killing you isn't it?) ............ dramatic, but not complete improvement. With the new unit, the temp gauge reads 200 F when it should read 180 F. That's about a 40 F improvement over what I had before. I determined the gauge would read correctly with 25 ohms of additional resistance, so I picked up a resistor at Radio Shack and spliced it into the wire to the sending unit. The gauge is right on the money now. I suppose I could have removed the temp gauge and repositioned the needle, but there are few things I hate more than removing and reinstalling the instrument panel. Thanks to all for your comments, questions and sympathy.

 

Could you shot the document on guages to me also? The_Oman49@hotmail.com

 

I'm glad it is working to your satisfaction. HOWEVER, you may (or may not) have created a bad situation with the resistor. The purpose of the temp gauge is to alert us to an overheat situation. Those gauges are not a real high tech piece of equipment. In fact, they are about one step above an idiot light. By adding the resistance in the circuit you have gotten it to read correctly in one range, but will it read correctly as the temp heads North? Maybe it will, but I have read reports from others who claim that theirs did not. I would block the radiator long enough to create a mild overheat and check the action of the gauge before I felt entirely comfortable. Hopefully you'll find that it does what it needs to do and you can get on to something else.