Temp Gauge

[roadking77]

I am pretty sure there was discussion regarding this subject before, but heregoes.....

Car:1977 Trans Am 400 - Mild build, stock radiator, a/c not hooked up, Q jet carb, Auto trans-
My temp gauge works sometimes, but not accurately. Sometimes it barely shows, sometimes it pegs. I had the radiator flushed recently and it was running about 220 +. Last weekend the air temp was 100, I went out for a short drive and the gauge showed above 220. I put an infrared therm on the heater hose and it was just at 180, as well as a direct shot to the radiator also showing 180 deg. I am fairly certain the gauge is not reading correct.

Suggestions?

[Jur5664]

I have the same on my 1977 Formula 400, probably the gauge or the sending unit are inacurate. i do not bother about it 

[b_hill_86]

You may have a short in the sender wire. When it pegs, it may be grounded out. Grounding the sender wire with the ignition on should cause the gauge to peg to full hot. Not sure why it would barely show at times unless you also have a loose power wire/printed circuit problem.

As for the temp difference, it’s possible it’s simply a sender issue in that the typical parts store senders are kind of a one-size-fits all and not calibrated properly. The consensus is that a lectric limited sender is the go-to fix. I put one in my car and my factory gauge is now on par with my mechanical. They advertise as having the senders correctly calibrated unlike others.

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https://www.lectriclimited.com/vehiclesearch/result/index/year/128/make/20/model/58?cat=734

[5th T/A]

Kerry your timing is pretty good because I just resolved a problem on my 1980 TA with inaccurate temperature gage readings.

To some extent I agree with Brian that a partial short could cause the gage to read higher. A dead short will cause the needle to go off the scale. It wouldn't hurt to follow the wire back from the sending all the way back to the fire wall. I think your sending unit is on the driver's side cylinder head as shown in the first photo. If not routed properly the wire could be grounding out being too close to an exhaust port. A short would cause the gage to go full right off the scale and an intermittent short would probably cause the gage needle to flutter.

Temp sender location on driver's side cylinder head. If it is not located on the cylinder head, then it will be at the front of the intake next to the thermostat.

IMG_0254 by Lawrence Alexander, on Flickr


If the sending unit wire was broken or disconnected this is what the temperature gage would look like on a warmed-up engine. Ignition on, but engine not running.

gage with sender disconnected by Lawrence Alexander, on Flickr



If the sending unit wire had a dead short to ground, this is what the gage would look like. Notice the needle is off the scale. Ignition on, but engine not running.

Gage with grounded wire by Lawrence Alexander, on Flickr

Next time it reads high you could disconnect the sending unit wire and see if the gage goes to zero. Any reading above zero means a bad gage or partially shorted wire. But, I wouldn't be surprised if it's a bad sending unit. I bought mine from LectricLimited, part number 08993107. I believe your car will use the same part number, but double check.

I run a 180 degree thermostat on my car, typical operating temps are 184 - 187 degrees (from my FAST EFI display). Next chance i get I will try to get an Ohm reading on the sending unit once the car is warmed up. This may help you in determining if your sending unit is bad or not.

[roadking77]

Thanks for that info. Its a beautiful weekend to drive and look at the problem. Will give me something to do this afternoon or tomorrow.