D**N Temp Guage - Help

Jcon · 06-08-2002, 05:44 AM

My temp guage hasn't been working. I traced it to a bad ground last night. Ran a new ground wire and now the damn needle jumps to the top of the guage and stays there as soon as you turn the key.

As stated in last nights post I am an electrical idiot.

Help

PS - The gauge and sender a brand new

dad911 · 06-08-2002, 10:28 AM

From the back of the guage the green/black wire IS NOT GROUND, it goes to the oil temp sender. If you ground that connection, it will peg gauge. Make sure the sender is the correct one for the gauge. Wrong sender will make gauge read low/zero.

Jcon · 06-08-2002, 10:41 AM

I didn't ground the green / black wire. But if that wire is grounding somewhere else (frayed etc) would it produce the same results?

Thanks

Early_S_Man · 11:09 AM

Jeff,

Open sender lead = pegged gauge

Grounded sender lead = zero gauge reading

Clear??? Disregard any other 'hints' that suggest otherwise from posts!!!

OK ... you have half of the system working! The 'needle jump' is indicative of an open to the sender! Try grounding the connector at the sender location with the ignitionswitch on ... if the needle does not go to zero ... you still hve an open somewhere. Try a temporary ground at the gauge input lead just to make sure it is working correctly. Two likely sources are that 1/4" female Faston connector at the sender (broken strands inside wire at crimp) or a bad connection at the 14-pin connector to the front of the car on the relay/fuse/CDI panel in engine compartment. Clean the 14-pin connectors with isopropyl alcohol and an old toothbrush on the male side and pipe cleaners in the receptacles of the female side. The connectors are silver-plated ... so be careful not to scratch or abrade away the plating!

Test the wire to the sender again after cleaning the 14-pin connectors. If still no 'zero' indication, wiggle the wire just 'aft' of the connector ...while firmly holding the Faston to ground. If intermittent or still open, cut off the connector and strip the Green/Red wire about 3/16" and test the gauge again by grounding the bare wire. If you now have a zero indication, install a new female connector.

If still open go back to the 14-pin connector from the main wiring harness to the front of the car and find the Green/Red wire at Pin 4 ... test with test clip to ground ... zero indication obtained, now?

Let us know how the testing proceeds??? Good luck!

dad911 · 06-08-2002, 03:05 PM

Both My '78 & '71, I just went to garage, shorted sender wire to ground, and Temp gauge pegged at 300. Unplugged/open = Low reading.

My Oil Pressure gauges work opposite --> Open = Pegged gauge.

Jcon · 06-08-2002, 06:34 PM

Warren & Alan - Thanks Guys

Alan, I think you're right. I have a seperate round VDO oil temp gauge that I hooked up to test. With the sender left disconnected it reads zero. When I ground the sender post it pegs.

As it turns out, the new guage is bad. It pegs no matter what you do. I even tested it on completely seperate leads. I ran new hot / ground and sender wires to make sure the results were the guage. Sender post disconnected - it pegs, sender connected it pegs ( New sender wire and connectors running from sender outside my car and in thru the drivers door) - Sender post grounded it pegs. I just bought this damn thing.

Last piece of help. For now I'll install below the dash the round 2 5/8 inch VDO I have. How do I route the connection to the sender. I have the power and ground hooked up, but I can't see where I can route the sender wire from the engine bay and get it back inside and under the dash.

I'm trying to make it to an SCCA autocross tommorrow so any help tonight would be very appreciated.

Early_S_Man · 06-08-2002, 07:50 PM

Jeff ... obviously, I misread your post (or had brain fade) and was talking about the oil pressure circuit, since the pic and wire color codes didn't apply to the temp gauge circuit!

The temp sensor is a negative temperature coefficient (NTC) device, meaning the resistance reading should drop as the oil temp rises. Unfortunately ... the wonderful Bentley manual doesn't give any values for the sensor ... they seem to think all you ever have to do to the gauge is install new illumination bulbs!

Are you getting a resistance reading in the 1000 -2500 Ohm range at the Green/Black wire to ground at room temperature?

Also, are you certain that you are connecting the wiring to the gauge properly? The two gauge sections are 'inverted ... the temp gauge has ground at the top, the +12 Volt connection to the left, and the 'G' sensor connection to the right as you look at the back of the gauge. I suggest retesting both your old gauge and the new one ... a 5000 Ohm variable resistor (potentiometer) can be used to replace the sensor for bench or in-car testing.

Jcon · 06-09-2002, 02:54 AM

Thanks Warren,

I'll re-test it this morning. I'm almost positive I had it right last night. I actually removed the temp guage itself and tested it on new power / ground and sender hook ups. It's worth another look though. Hell most of my really good screw ups happen when I work in the garage late at night and I'm "positive" I've done it right.

I'll let you know how it turns out - thanks for the help.