I have been trying to get a trans temp gauge in my 89 working for a couple of weeks now. I put the gauges including , boost and egt where the message center resided. I buttoned everything up last week and took it for a spin. Old used boost and egt work ok but the tt gauge did nothing. I took everything back apart and checked good for keyed 12v at the gauge posts. I grounded the sending unit wire to check the gauge sweep and the gauge pegged and stuck. I left the sending unit in and the manufacturer gave me a new gauge thinking it was a defective gauge.

Now the gauge will sweep and return when grounded but still has no reading when warm. I just happened to turn on the headlights and the gauge pegs returning to 0 with headlights off. When I check voltage at the gauge posts the voltage goes from 14+ at idle (to fast but that adjustment comes later) to 0 when I turn the headlights on! I'm not sure where I got the 12v it was a red and black that originally powered the now missing message center, or radio.

What the heck would cause it to loose voltage with the addition of headlights and just as important, why doesnt the gauge work when the headlight are off? I'm guessing the sending unit is OK because it is a "slow" 3 seconds or so, sweep to peg unlike an immediate peg when I ground the sending unit.

Jeeez I can hardly understand this when I read it back I hope you guys can figure out what I mean.
Tim

 

Sounds like you have a power problem, the gauges draw so little power fuse it and hook to battery, try running a wire to battery as a test, mess cent wire may be for lights,

 

Is there any possibility that you could have reversed the power and illumination wires?What brand is the guage?

 

Once you get power to it all the time, don't be surprised if you do not get a reading. Most gauges don't start to register until 140 degrees, and you won't get the trans that warm at this time of year where you are unless you work it hard.

 

I finally got time to work on my gauge power problem. I just needed to run a better ground. I don't get any reading (it starts at 140 like Wanna said) at a steady interstate 70mph cruise for 30 mi with outside temp in the 50's. This is the first time I have had a TT gauge, will it register just highway cruising with no load?

Tim

 

Not with 50 degree temps. Block off the radiator with cardboard and let it idle in gear for about 5 minutes, you should be able to get it to move then.