The temperature guage in my 88 doesn't work. I turn the car on and the needle drops below the bottom line and never moves but when I turn the car off the needle moves back to the bottom line. I have recently replaced the circuit board for the guages and put a new temp sensor on but left the old wire. What could be the issue?

 

I experienced something similar on an 84 back in 1990...turned out to be an easy fix. Mine wasn't a gauge that I recall...it was a dummy light, and didn't come on when the car overheated. The mechanic told me all he did was take out the cluster for inspection of the printed circuit and it checked out good.....so he reinstalled it and lo and behold it worked. It was just not getting a good connection to begin with.
Im assuming you replaced it with an exact part and not another printed circuit from another year.
PS...This is when I was a stupid teen.....I never told the mechanic that I had the cluster out and it was me who didnt get it slid in all the way upon reinstalling it...lol

 

lol I replaced the circuit board with one from a camaro parts website. I'm hoping to not have to rip the cluster out again. It's a huge pain in the butt

 

Dang Speedo Cable!!!

 

I'm not too familiar yet with the wiring for these third gen cars, but for coolant temperature sensor readings, their function is based on the resistance of the sensor based on the heat it is exposed to.

For our coolant temperature sensors (someone correct me if I'm wrong), they are negative temperature coefficient sensors meaning the hotter the sensor gets, the less resistance it has, in turn, making your gauge have a higher temperature reading.

So in your case, since the gauge is going lower, like it is very cold, you must have too much resistance somewhere in your circuit. If this is true, I would check to make sure the connection on the sensor is good, and look over the wire to make sure it is not corroded anywhere. Check over places where the wire loom is broken, very rarely does a wire go bad in protected parts of the loom. However, if the wire has been exposed to the elements long enough, it can be possible the whole wire is bad.

If you do find bad parts of the wire, just splice in a new part (make sure to at least use heat shrink around the splicing to keep water/dirt out of it.)

Possibly, something isn't connect so there may be a break in the wire or it's not connected properly to the back of your instrument cluster. An open loop like that would be infinite resistance, making your gauge peg down the lowest it could go.

 

LOL, just today I'm going through the same thing with my '86 IROC. The gauge has been acting up lately, sometimes it would work and sometimes it would stay below the bottom mark. I removed the sensor connector on the head and grounded it to the the block to check the gauge, the gauge almost pegged out hot, but not quite; which I thought was odd, because I thought it should peg out if grounded. The connector that the wire hooked to the sensor was pretty loose so I replaced it with a good connector and it started working again.

So I thought I had the problem fixed, but on the next trip into town, it worked all the way there, but after shutting it off and starting it again, the gauge was on the bottom mark again and didn't move from there all the way back home. I'm starting to think the gauge is bad, but I guess I can pull the gauge cluster and check the back for a loose or bad connector there. I guess I'd better start surfing the web for a replacement gauge.