Help needed - '76 Temp gauge wiring
 

I'm finishing up the assembly of a basket case '76 911S that is cleaning up into a really great driver. All the electricals are online but two things - the e-brake indicator light won't shut off (I'll start another thread if I can't figure that one out) and my temp gauge is non-functional/pegged.

When the key is turned, the gauge pegs. I've checked continuity from the temp sensor to the gauge, no problem. It is on a black/green wire. The ground on the gauge seems fine, all the other gauges are functioning. There is a solid green wire on the back of the gauge that appears to be open. I've traced it to a similar green wire in the engine bay wiring harness - it isn't connected to anything. The green wire is on what I believe is position #2 on the 14-pin connector. The black/green wire to the sender IS connected to the sender.

Where is that green wire supposed to go? Is that why my gauge is staying pegged? Please help! I would like to know what temps my fresh engine is hitting during break-in...

Thanks!

Dave:

Pin #2, a solid green wire, in the 14-pin connector is used in Sportomatic cars only for the oil temperature warning lamp. It's found in non-Sporto cars but not connected to anything. Ignore it, or do as I have: it carries the current for my monster low pressure warning lamp from an adjustable pressure switch set to turn on the lamp at hot idle.

But back to your gauge problem. As was stated here many years ago by the Guru Warren Hall a/k/a "Early_S_Man" the TEMP gauge sensor has a negative temperature coefficient, meaning it pegs LOW when the wire breaks. NTC= resistance decreases with increasing temperature, moving the needle up. (The PRESSURE sensor, on the other hand, pegs HIGH and resistance goes higher with pressure. Warren said it all here: http://forums.pelicanparts.com/showthread.php?s=&threadid=264777&highlight=gauge+ pegs)

OK, so it's pegged HIGH, so that means that you have a low resistance somewhere. That means either the sender is shorted internally, or the sender wire, which is a green wire with black stripe, is grounding out somewhere. So the drill is to measure the continuity of the black/green, and if it's OK, check one end for continuity against the body of the car.

Most likely the sensor is kaput. Smile big if you buy one from Pelican, knowing that you don't have to pay $200 for the SWB version!

p.s. the E-brake indicator light is probably the sensor wire grounding out on the brake handle or somewhere around under the boot.

Good luck!

According to this thread, the green wire might be the A/C clutch wire. Ignore it just the same.

http://forums.pelicanparts.com/porsche-911-technical-forum/356525-how-do-i-route-signal-line-gauge-dash-engine.html

Thanks John. I swapped out the sensor with a random sensor off my bench and a known good one from another engine - gauge pegged both times, with the extra green wire either connected or disconnected.

Then checked continuity (again) from sender connection to back of gauge - no problem. Then disconnected the green/black wire from both the gauge and the sender and checked it against chassis from both ends and at the 14 pin connector. No shorts.

Now what? This is getting strange....