Temp gauge changes when parking/headlight are turned on.

[Odoyle]

In my 77, whenever the parking lights or headlights are turned on the temperature gauge begins to read 10-15 degrees higher than the actual reading. The car is fully restored, entire wiring harness has been gone through, both temp sending units have been replaced, wires going to and from sending units have been replaced, inline resistor has been checked and cleaned, have double checked all ground are clean and secure. Everything else works fine... starting to think it might be alternator related, car has a 55 amp bosch alternator. Thoughts?

[Odoyle]

Update, drove the car today and noticed that when I press on the brake pedal with the left turn signal on, the flasher slows down dramatically, RH side is not affected, still flashes as normal. With the headlights on and brake pedal depressed the LH turn signal on the flasher just stays lit, doesn't flash. Trying to sell the car soon so would love to hear anyones two cents on this.

[spider2081]

First the temperature gauge.
The temperature gauge internal back lighting shares the same ground as the gauges needle. This ground is made with a rivt that holds the plastic light socket to the gauge case. it is a poor ground that causes the needle to indicate higher temperature than what it should. I have gotten some improvement by changing the temperature gauges light socket to a 2 wire socket and grounding the light to the grounding pod near the ignition switch. I think a real cure would be to open the gauge and make a better ground connection for the needle inside the gauge itself.

The turn signal issue. Many turn signal issues are solved by removing the turn signal bulbs, cleaning the bulb bases and the light sockets on the car. Also having a good strong flasher solves many of the issues. I like the NAPA EL-13H flasher. It is an electronically timed flasher works with incandescent bulb or LED's. I think they are close to $20 each.

[Odoyle]

Thank you for the insight, I'll work on fixing the grounds for my temperature gauge.

As for the turn signal, parking lights, and head lights, their all brand new, with new bulbs but will inspect for corrosion. I tried swapping my flasher unit for a genuine NOS flasher I bought some time ago, issue persists. Will look into buying the NAPA unit.

Checked battery, 12.4 volts off, with the car running voltage is 14.3 volts which seems healthy. Could it be a weak engine to body ground strap?

[18Fiatsandcounting]

Checked battery, 12.4 volts off, with the car running voltage is 14.3 volts which seems healthy. Could it be a weak engine to body ground strap?

Possibly. A new and fully charged "12V battery" is actually around 12.6 or so volts with no load, and the normal Fiat charging voltage is 14.3 to 14.5 volts. Yours seems to be fine, but it doesn't hurt to have more than one ground strap. I have often used the original body to engine ground strap but then added a ground cable from the battery negative terminal to the engine block (usually the motor mount bracket). This only works on model years when the battery is in the front.

And yes, the issue with the gas/temp/oil pressure gauges reading slightly higher with the lights switched on is common, and spider2081's answer is exactly true. An internal gauge grounding issue. I've even gone so far as to open the gauges and do some soldering to get good ground connections for the gauge movement as well as the gauge illumination light. It helps, but I still have a very minor needle movement when the lights are switched on.

-Bryan

[Odoyle]

Added a second ground strap last night, gauge now reads 10 degrees off with lights on instead of 15 so theres progress. Will remove the gauge and modify tonight.

What is really driving me crazy now is the with the headlights on and brake pedal depressed the LH turn signal on the flasher just stays lit, doesn't flash... could this be a faulty brake light switch? Something wrong with the ignition switch?

Already found a buyer for the car and he made a deposit, he is paying top dollar so I'm inclined to get this sorted, any thoughts are welcomed.

[spider2081]

Things I would try with respect to the turn signal issue.
Clean the light bulb bases and sockets. I like to put a slight bend in the top edge of the socket so the bulb base is scratched as the bulb is pushed into the socket.
Swap the flashers between the 63 and the 77 to se if a different flasher changes the problem.
I like the NAPA electronic flasher its part number is EL-13H close to $20 but I think its a good flasher for our cars.