[GPMESQ]

I have Rally Gauges in my 1969 GTO and my temp gauge is not working...the gauge does pop all the way up (below 100) when key is energized but not reading anything. Should I start with the sending unit? How do you go about troubleshooting the temp gauge?

My Factory radio seems to have no power...how do you go about trouble shooting the radio? Check power at fuse panel?

If you have not figured it out yet, electrical issues are my Achilles heal.

[paint guy]

Temp gauge: If the needle moves when you turn the key on, I believe you are getting power to the gauge. If you ground the lead to the gauge, the needle should move. If it does, the sender is suspect or is not grounded. Teflon tape used on the threads of the sender may cause insufficient ground.
Radio: there should be power to the radio when the key is "on' or "acc". If you can find power, but the radio doesn't work, there's your problem. If you can't find power, trace the wire back to the fuse block using a continuity tester, if OK, then check the fuse.

[GPMESQ]

Originally Posted by paint guy

Temp gauge: If you ground the lead to the gauge, the needle should move. If it does, the sender is suspect or is not grounded.

Stupid question: How do I ground the lead to the gauge?

I am trying to figure out if my sending unit works...any way to test it?

[Old Goat 67]

Check out this thread.

http://forums.maxperformanceinc.com/...d.php?t=721231

[GPMESQ]

Originally Posted by Old Goat 67

Check out this thread.
http://forums.maxperformanceinc.com/...d.php?t=721231

That thread is amazing...all questions answered...now I can test my sender and gauge.

[Peter Serio]

The only way to properly test a temperature gauge sender is to heat it in water. You will need wire up a test bench with a 12 volt car battery, the gauge and some jumper wires. I use a mechanical (brand new) temperature gauge in my water to give me a point of reference. You cannot simply heat the water, drop the sender into it and measure resistance (been there, tried that). Ohms law states: http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html

Therefor:

http://www.cdxetextbook.com/electric...s/ohmslaw.html

[GPMESQ]

Originally Posted by Peter Serio

Ohms law states: http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html Therefor:
http://www.cdxetextbook.com/electric...s/ohmslaw.html

"Your talking jibberish..."

So I can not simply use a multi-tester, set on ohms, to check it cold and heated? I need to add voltage with a battery?

[Old Goat 67]

Yes, you can, but if you have it immersed in water, you will probably get an erroneous reading because of an added parallel path through the water.

Charles

[Peter Serio]

In a nutshell; the electricity supplied to the sender via the green wire forms a portion of the resistance equation. So simply heating up a sender and measuring the changing resistance at different temperatures using an ohmmeter by itself is a waste of time.

[GPMESQ]

...I am still confused...I understand I can not check it in water but can I check it without adding voltage or not?

[Peter Serio]

My best advice here is that you seek the assistance of a qualified local restoration shop or a top notch mechanic who specializes in electrical problems on vintage GM cars.

[Old Goat 67]

Originally Posted by GPMESQ

...I am still confused...I understand I can not check it in water but can I check it without adding voltage or not?

Worked for me. Substitute known resistors for half scale and full scale to prove your gauge first, then check the resistance of the sender open circuit at operating temp with an IR gun.

[GPMESQ]

Originally Posted by Old Goat 67

Worked for me. Substitute known resistors for half scale and full scale to prove your gauge first, then check the resistance of the sender open circuit at operating temp with an IR gun.

I am heading to Radio Shack for resistors...also, can't I test the sender by leaving it in place, measure ohms cold and then heat it with my heat gun and re-measure ohms?

That will establish the sender is working and then I can check the gauge using the resistors?

[Old Goat 67]

then I can check the gauge using the resistors?

Well, I'd say yeah, close enough for ball park go-or-no go. Measure it open circuit resistance cold first, then same thing at themostat warm up.

Place a 90 ohm resistor from the end of that green lead to a good ground, gauge should read 1/2 scale -a bit left to right say maybe 1/16" to 1/8". A 45 ohm should cause the gauge to read full scale "hot."

Ball park figures I used on my 67 and it is reading the temp of the engine at about 100% accuracy compared to an IR gun shot on the water passage on the intake manifold. Close enough for me. I ain't putting the rover on Mars.

[Old Goat 67]

Further info for 68-69, see post # 11.

http://forums.maxperformanceinc.com/...37#post4537137

[GPMESQ]

Although I did not get to Radio Shack yet I did get back to the car today.

As I previously said with no power the gauge drops all the way past 250 and once the key is turned to on it pops all the way below 100. That means I have power to the gauge?

I then used the ohms meter to check the ground on the sending unit and it appears to be well grounded. Then I used the ohms meter on the sending unit cold and got a 796!?! I checked it twice!

I then warmed the car up to operating temp and check it hot and got a 95, so it would appear the sending unit is functioning, right?

If I understand this correctly this would suggest a problem with the gauge? I am still going to get the resistors and check the gauge as they are inexpensive enough.

Assuming its the gauge, do I have to remove the entire cluster to get to it?

[Old Goat 67]

Only thing I wonder about is why is the sender reading 796 ohms cold? But, who knows where that sender came from. 95 ohms warmed up sounds good, but why doesn't the gauge respond?

Sounds like you may have an open lead from the gauge to sender? Read that green lead on the sender while disconnected and open circuit, with key on. Should see 12 volts coming from gauge at that time.

If it is open, putting a resistor in place of the sender won't get a reading either.

Need to locate and fix the open circuit if that's the case.

Charles