Temperature gauge puzzle

Followed Briano's instructions on here to find out what's wrong and I'm stumped. I currently have the gauge itself out on the bench and it seems to check out fine. Battery across the back gives 75% of the way up to hot. The voltage stabiliser is fine (fuel gauge works fine). Took the face off the gauge and metered the connection points on the back to their termination points inside the gauge - all good. I've checked the sender wire from the engine compartment up to the cluster and it's good. Before removing the gauge I tried disconnecting the sender wire - no different. Am I missing something obvious here?

Well I would of put the +9 on the positive side of the back of the gauge while it was still inside of the cluster, and negative on the other if it worked then I would say you have a broken run from the connector to the gauge and not a gauge issue itself.  Just because you have 12V gnd, and 10 on the Stabilizer doesn't mean that because your fuel gauge is working the mylar is intact to your water gauge.



As the sender sends a sense indication, that is to allow the Negative side of the cluster, then I suspect the wire or the connection to be flaky.  If you have "Plumped" your connector with a piece of electrical tape on the plastic tongue under the mylar, then I would measure between the mylar pin and the Gauge.  it isn't unusual for the mylar to get broken.   You can feed the gauge direct from the wire side of the connector via an insulated male/female spad connector, to the back of the temp gauge…

Just for giggles, you may want to remove the globs of solder while you have it apart clean the whisker wire and re-solder them with good solder as a Cold solder joint will occasionally test good, but apply some current and it goes south.

I have had both broken traces on my mylar that I had to back feed, 12Vin on the black where the mylar would bend up as I slide the connector on, and I have had broken traces on the 10V stabilizer.

But usually it is the gauge that is the flakiest part of the puzzle.

Those pesky Electrons…..
 

Do you have a relay for the coolant level warning  (the red flashing led)? On mine it is relay 43 (but can also be 42) and is in relay plate position 18, row of additional relays on top, far right position. If this is faulty or the relay pins are wet, it will cause the temp guage to go straight to max and led flash. Pull the relay and see what the guage does.

LeeO said

Do you have a relay for the coolant level warning  (the red flashing led)? On mine it is relay 43 (but can also be 42) and is in relay plate position 18, row of additional relays on top, far right position. If this is faulty or the relay pins are wet, it will cause the temp guage to go straight to max and led flash. Pull the relay and see what the guage does.

No, the car is a 1986 - I believe just before the time when the red flashing led was also used for level sensing via some relay trickery.

Finally nailed it! After an hour with the multimeter and after removing the nuts and washers from the gauge mountings to the Mylar, it would seem that the sender input track in the Mylar is shorting somewhere in the Mylar itself. I cut away the Mylar from around the sender input post, insulated it from the surrounding Mylar tracks with a plastic washer and wired it direct to the yellow red sender wire having cut said wire from the input connector to the cluster.