[DogMeister]

'67 GTO with Rally gauges. Temp gauge stays at minimum all the time. There is continuity between the thermostat and gauge connections, 12V feed to gauge is good with engine running or key on. Gauge resistance reads 40k, and pegs to min or max depending on lead polarity. Feed from tstat to ground reads 600 ohms cool, 160 ohms at 150 F. Same readings at the gauge connection. The gauge housing has its own ground wire. Reversing the hot and signal leads on the gauge changes nothing. The same voltage feed goes to the oil pressure gauge and that reads fine. Disconnecting it has no effect on the temp gauge behavior.

The gauge spikes during crank.

I've tried 3 different thermostats.

I'm about ready to break out the 'fine adjustment tool.'

[Mister Pontiac]

You say '3 different thermostats.'

I'm assuming you are talking about the temperature probe? (that's what I call it)

If so, that was my first guess, but since you've tried 3 of them, likely not the problem. I'm chasing a similar issue in my 74 Bonne right now, so I feel your pain.

You mention the gauge spikes when cranking, so electrons are moving.

Maybe PM Pete Serio here on the forums. He seems to have a lot of great knowledge about the Rally gauges.

Hope that points you in the right direction.

[Peter Serio]

After 50 + years those old style GM gauges in the #4 pod (65,66,67) all have a 95 to 98% failure rate. Adding to the problem many of the modern aftermarket replacement senders that are commonly available are not made correctly for that old of a gauge.




I have fixed many GM (GTO) and Lemans temperature gauges over the past 30 years.

After I repair the dash gauge I choose an older (made in the USA) sender. You need and original GM sender, that or a car parts store brand sender that was made prior to the year 1990 or 1991 at the very latest. That was the last time those were made in the USA.

I test the sender and the gauge together, that is how I calibrate. Now GM wants money for the old blueprints, and you have to pay them in advance. Senders made overseas today have not been made according to the original blueprint because they do not want to pay for that. Odds are they have no running & driving 1966 or 1967 car over there to test them on nor would they even have the shop manual to your car.

I think they just guess. The other issue is they somehow envision about 20 to 25 years worth of senders (for 1960s and 1970s GM cars) with gauges are all the same, (because for a V8 application they all have the same thread pattern),= (1/2" X 14). I reality there are actually many different variations of temperature senders for older GM cars with factory option gauges! Today they just combine all of those older GM #s into one or maybe 2 aftermarket part #s; that is big part of the problem.

[Mister Pontiac]

Thank you for that reply, Peter. I figured you would have some good insight.

I had no idea the new temp sender's are so suspect, but it makes perfect sense. Like the OP, I've struggled with several different senders to no avail, yet the power, ground, etc are all good.

Question for you then... would you have a good sender for a 74 B body temp gauge? I'm willing to have a try with an older, earlier unit before pulling the gauge and sending it to you for repair/calibration.

[Cammer-6]

correct me if wrong but years ago they did make a correct aftermarket sender
has it really been 30+ yrs?

[bhill86]

Not sure about GTOs but Lectric Limited markets senders as being factory correct which is where I purchased mine for my 77 T/A. On the Firebird line the resistance value for the senders and gauge are different than, for example, 78 so they’re not interchangeable between these years. I had a parts store unit that was supposed to be correct that didn’t read correctly and the lectric limited one does. FYI.

[Mister Pontiac]

Originally Posted by bhill86

Not sure about GTOs but Lectric Limited markets senders as being factory correct which is where I purchased mine for my 77 T/A. On the Firebird line the resistance value for the senders and gauge are different than, for example, 78 so they’re not interchangeable between these years. I had a parts store unit that was supposed to be correct that didn’t read correctly and the lectric limited one does. FYI.

Thanks for this info. I checked with them too and they show they have the correct temp sensor for my 74 Full Size. I ordered one up to see if they're right.

[dataway]

Are the sensors just temperature based variable resistors?

If so, seems like a tuning pot in the circuit and you could tune the response to the gauge to whatever you want, long as the response curve of the sensor is reasonably close.

[Lee]

Originally Posted by Mister Pontiac

Thanks for this info. I checked with them too and they show they have the correct temp sensor for my 74 Full Size. I ordered one up to see if they're right.

Good luck. I ordered one a few years ago, and it was not accurate. I tested it against one I had bought (for MUCH less $) from AutoZone.

I installed each in my '73 T/A clone. Used a high quality IR temp gun to verify temps (marked a spot on my intake crossover, so I was checking the same spot each time), and mapped the ohms of each sensor as the temps rose to 180 (my t-stat setting at the time). Both senders were the same, and NEITHER were accurate.

I was able to buy a selection of resistors off of Amazon for cheap. Playing around, I was able to make a resistor pack to install between the sender and the gauge, which made "180" on the gauge actually 180.

I posted about this when it happened, probably show up in a search.

[DogMeister]

Originally Posted by dataway

Are the sensors just temperature based variable resistors?

If so, seems like a tuning pot in the circuit and you could tune the response to the gauge to whatever you want, long as the response curve of the sensor is reasonably close.

The problem is there is no response. You can't tune it if it's not there.

["QUICK-SILVER"]

Originally Posted by DogMeister

The problem is there is no response. You can't tune it if it's not there.

First post says:
"The gauge spikes during crank"

That sounds like the dash is wired for warning lights instead of gauges.

Cars with warning lights ground out the temp sender wire while cranking for bulb test.

That solid ground can jar the needle loose (cause it to slip) on a gauge.

This old thread has some good info about sending unit ohm values and resetting needles on a gauge.

https://forums.maxperformanceinc.com...it+ohms&page=2

Clay

[Mister Pontiac]

Originally Posted by Lee

Good luck. I ordered one a few years ago, and it was not accurate. I tested it against one I had bought (for MUCH less $) from AutoZone.

I installed each in my '73 T/A clone. Used a high quality IR temp gun to verify temps (marked a spot on my intake crossover, so I was checking the same spot each time), and mapped the ohms of each sensor as the temps rose to 180 (my t-stat setting at the time). Both senders were the same, and NEITHER were accurate.

I was able to buy a selection of resistors off of Amazon for cheap. Playing around, I was able to make a resistor pack to install between the sender and the gauge, which made "180" on the gauge actually 180.

I posted about this when it happened, probably show up in a search.

Thanks Lee. Yeah, kind of a shot in the dark, but I'll give it a try. All it can do is 'not work,' and it's already excelling at that. Heh.

I'll do a search for your resistor solution. That's a creative way to solve it.