[gennaro]

Ok all, this thing is getting to me . My temp gauge is not working , pegs all the time . So ... the first thing i did was check the voltage to the line going to sender its good . Traced it back to the cluster its good . Checked the tabs, good connection on both . I have removed the cluster and the printed circuit looks good and clean. I removed the gauge from the cluster used a known good sender and still pegs . I tested the sender on a known good after market gauge and seems good. So my only conclusion is the resistor or resistors in the gauge is gone. any thoughts ,(I think i covered everything)

[78w72]

can you clarify what voltage you checked on the line going to the sender? the temp sender doesnt have voltage going to it, it sends a signal to the gauge kind of like the fuel sender, its an ohms reading of resistance. if the gauge is pegged & not reading right with everything hooked up correctly & no breaks in the wire or printed circuit it could be a bad gauge or bad sender. should be a procedure in the service manual on how to check both.

also if you werent aware, 1981 has 2 temp senders, one for the computer which is in the intake manifold & the one for the gauge thats in the driver side head.

[Skip Fix]

Also the 81 senders use a different ohm than even as late as 78.

[Peter Serio]

I have fixed many of those gauges. The key to testing is to identify power in (12 volts battery +, key on), ground at the back of the gauge.
and then the signal out of the back of the gauge, thru the printed circuit. I call that wire the seeker wire (dark green) and then you must have the proper sender and that sender's brass threads complete that sensor circuit to engine block ground. The thick battery ground cable then transfers that back to the "-" terminal of the car's battery.

For 1960s as well as 1970s GM car there are many aftermarket, replacement senders with the threads that thread into the head or the intake crossover which are made incorrectly as far as ohms in relation to coolant temperature.

Those coolant temperature senders are all 100% sealed and are not adjustable in any way. They are made from a special interior part; it is called a thermistor.
The makers of such parts (especially if it's an "imported" sender) I guarantee you that they do not have access to GM's original electrical specifications to make a replacement sender. If in doubt I suppose they just guess.

One thing they love to do is take what would be, say 8 or 10 years of a GM car or cars and create one replacement sender which is not 100% true to any of the years or cars listed; almost like they "average" out all the parts #s and then make a "one size fits all." GM had many different electrical specifications for these coolant senders over the years. (Cars with factory gauges.) When the car is a brand new car, I am sure that GM knew exactly what each line stood for as far as temperature (even the unlabeled lines). However only the extreme lines ( cold-start and maximum overheat) and the 1/2 way line(s) were marked with degree numbers for the car customer.

To get one of these old temperature gauges to read right you have to know how it works so you can test it.

[gennaro]

I have 12v at pin 5 of the plug going into the printed circuit from key on, this pin 5 sends 12v to one of 2 studs on the gauge , 12v is then sent through the gauge to the other stud on the gauge which connects to pin 3 of the printed circuit which sends 12v to the green wire to the sender which is in my case is in the manifold ( 301 swapped to 400 ). i have the correct sender from the 301(which i tested on another gauge). So with the key on the gauge pins all the way to the right. I have the gauge on a bench right now , i did a test in the same manner and still pegs . If i put direct power on the pins of the gauge on one side or the other i will peg both way when i switch sides. So not sure if gauge is bad or what.

[gennaro]

Its been a while , I found another gauge cluster and still get the same simptom. I must be texting it wrong . Can anyone confirm im testing this properly ? my last post is a description on how im testing the temp gauge, thx

[i4abuygto]

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