Hi, on a 1985 244GL LH2.2, recently switched my harness to a newer one. Now my coolant temp gauge is backwards. Meaning it's trying to push the needle down instead of up.

Any reason why this is happening?

I did tests. I connected a DMM to the sensor itself and get backward voltage when I connected red wire to the sensor blade and black wire to ground. It reads -V.

I did a test on the gauge by connecting the yellow wire to ground and turned on the ignition. This made the needle go all the way up. Then I stopped, so I wouldn't burn out the gauge.

Any reason why this is happening?

 

which harness did you switch? there's a lot of them throughout the car.

the coolant temp sensor is resistive, the voltage comes from the instrument panel. the circuit is really simple, coolant temp sensor is connected to a yellow wire which is connected to instrument panel connector 31 (the full circle connector) pin 2

 

Switched the main harness, it goes to: alt, dist, ICU, ECU, IAC, TPS, fuel injectors, coolant temp sensor, knock sensor, oil pressure sensor, LH temp sensor, grounds, half of the ignition coil, 25 AMP fuse.

Also cleaned, sanded and sprayed all engine compartment grounds.


The only thing that isn't working correctly now is the coolant temp sensor. Which seems to be reading backward. Making me think the yellow wire is different from the donor (87 DL)

I hooked up a DMM to the blade (red wire) and ground (black wire).


I read a negative voltage while it was running. That doesn't really sound right does it, it being resistive means I have voltage running through my block?

 

my 87-88 greenbook shows the same yellow wire to the temp gauge sensor going to instrument panel 31/2. there's a connector drawn in the middle but not labeled or shown.

oh wait, that connector IS shown on another page... its a 2x4 (8 pin) block conector, temp sensor yellow wire on pin 2, between pin 1 (black, oil pressure sensor) and pin 3 (red, alternator D+). the drawing is poor, but this connector is shown near the center of the firewall. 85 and 87-88 greenbooks show this as the same

 

looks like instead of supply ground via the sensor, you are supplying a voltage on that wire somehow. The gauge is actually 10V (from a step down regulator in the cluster). If the gauge is trying to go down when you turn on the car, that means that instead of supplying a varying resistance to ground, you are likely supplying 12V (-2V differential) to the wire that should be (approaching) ground.... which makes the needle go down instead of up.
The fuel gauge shares the same +V as the temp gauge... is it working correctly?
Its strange that removing the lead and placing it on ground made the gauge move as it should, this would indicate the wiring/gauge are fine... have you tried to see if you head measure +12 in reference to ground... you would have to have a lot of missing ground wires for this to happen. But worth a check...

I would look at the wiring harness at the large connector on the firewall, oil pressure, alt and temp (and more) go through this connector. I am betting a short from the Alt + wire to the temp gauge wire.

 

Thanks for your insight.

That grey box connector was kind of messed up so I hardwired those wires.
The thing is, another guy is having the same or a similar issue, except his fuel gauge isn't working also.
I'm currently presuming his issue is the voltage stabilizer, since both his fuel and temp gauges aren't registering. He also did the temp gauge test, and was successful like I was.

So I'm assuming our gauges are indeed working. I know both mine work, I only know his temp gauge works. (he could indeed have an issue with the sender at the prepump)

Oh yeh, yes my fuel gauge is working.

When I did the voltage stabilizer test in the bentley manual, it's not right. But, my fuel gauge works. When I put 12.6V (from battery) to the 2 bottom pins, I get 12.6V at the top pin. (the one that's supposed to be outputting 10V) Now if I put 12.6V to just the bottom pin, ground the middle pin, it gives 11.6V to the top pin. Which seems like how it's supposed to be. But isn't what the book is stating.

The main thing about this is we both switched main engine harnesses with newer harnesses without the biodegradable defect.


So I'm leaning toward the idea something is different with our harnesses, but I can't tell what.


We've been trying to figure this out here: http://turbobricks.com/forums/showthread.php?t=298349

That's where we were told about the grounding test.