[BN]

Recently installed a set of reproduction rally gauges from the parts place. The oil gauge works perfectly but the temp gauge never goes above the 130-140 area. Thought it was a bad sending unit so I replaced it with one from Lectric Limited with exact same results. Car has a new radiator that when installed the mechanic said “it’s running nice and cool now”. I thought maybe a bad thermostat but I have heat. Anything I could check. Could this be normal?


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[indymanjoe]

seems really low to me. Do you have access to a IR thermometer? You can get a cheapie from Harbor freight for $25.00 and shoot the water cross over.

i have this one.

https://www.harborfreight.com/Non-Co...ing-69465.html

[Peter Serio]

The problem that you are running into is that the gauges you bought are copies of GM parts that were made 1/2 way around the world; 48 years after GM made the car. You have to match the gauge to a sender which requires a way to both test the gauge and test the sender to determine accurate temperature display. Those reproduction gauges are made using thinner wire ( the wire windings which create a small electro-magnet) than GM gauges. Another thing they changed was to solder in a 100 ohm resistor in-between 2 of the posts. (The original GM resistor mounted externally right underneath the printed circuit and that resistor was in a range from approx. 82 to 85 ohms.)

The 2 coils work in opposition to one another. Both coils share a common power supply. One coil winding wraps around the poles and then thru that resistor to ground at the back of the metal housing. The other coil is wound 90 degrees out of phase from the first coil and then thru the PC into the dark green wire. Dark green wire to the sender and the threads of the sender complete that pathway to ground. When you apply 12 volts + power both coils are energized. One coil tries to tug on the pointer clockwise and the other coil tries to tug on the pointer counter-clockwise.

Electricity will always seek the path of least resistance so the pointer (that is attached to very center of a tiny disc magnet) That magnet is free to rotate approx. 120 degrees and will respond to changes in the the field(s) as current flow changes inside the wire winding(s).

You need to balance out the magnet fields so that the pointer is giving you accurate readings. The only way to do this is first repair the gauge to operate more in the fashion; the way GM built it. The most important step is to select a sender which has a true reading teamed up with the gauge. The sender and the gauge MUST be matched as a team!!! Since the sender is a sealed unit and therefor non-adjustable you have to test the gauge & repair that first.

I test the sender(s) with hot water (boiling) and jumper wires to ground using a 12 volt DC power supply. I do this not on the car, but on my workbench. That way I know what each line on the gauge represents. I always have a small thermometer in the water as it heats up.

[BN]

The Parts Place is sending one of their sending units to try. We’ll see.


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[Peter Serio]

The gauge, when working properly; the lines on your year gauge represent approximate temperatures in Fahrenheit as follows.


Topmost line = 100 degrees.
1/4 scale approx 180 degrees. (Thermostat open) Your car should always warm up, at a minimum to at least this line.

1/2 scale line = 210/212 degrees (boiling). This is your danger line, above this line you are fine, going past this line is a sign of trouble.

3/4 scale line = 230 degrees.

bottom line = 250 degrees.