Temp gauge higher w/headlights on

[jimincalif]

Driving through Santiago canyon with my headlights on yesterday, noticed the temp seemed a bit high. When I got out of the daytime headlight zone I turned the lights off and the temp gauge returned to normal, about a 15 degree difference. It is the same whether the headlights or just parking lights are on, its reading goes up.

I'm thinking there may be a high resistance ground connection somewhere for the dash that I need to clean up. When more current is flowing for the instrument lights, the resistance is affecting the gauge readout.

Anyone know where the relevant ground connection is? Mine is a 1980 FI.

Thanks,

Jim

[perthling]

I have the same issue on my '74. Haven't gotten around to checking my earths yet...

[mbouse]

yup... it sounds like a grounding issue. if you temporarily re-wire your headlights ground wire directly to the battery and run your test.. i bet the reading disappears.

have you added a separate grounding strap from the crankcase to the inner fender? if not, here is a great reason why you should. buy a 16" piece of black battery cable and attach it to the crankcase and to the fender.

factory ground pods on the inside of each fender supply the connection to the headlights and front marker lights, including the turn signal lights. rear factory ground points will vary depending on your model year, but i would check those as well.

the grounding pod for the gauge is located behind the gauge panel... although, i don't suspect that connection.

[spider2081]

My 81 has the same issue, reads about a needle width higher with the lights on. I haven't tried very hard to isolate the problem though. I did notice that turning the panel light dimmer down resets the needle to lower reading. I think mine is associated with the panel light circuit, of course they are on when ever the headlights are on. My first thought is the ground pod for the dash but I have remade that ground and it had no effect. I also removed the pods on the fenders and installed ring terminals so those grounds are also in good shape. I redid those grounds to cure signal/headlight light issues. Solving this issue is on my to do list, but if one of you solves it for me that would be great.

[manoa matt]

I got the same issue. Like spider 2081, it only moves the width of the needle. I have done the following:

Additional engine grounding strap on passenger side.
Additional grounding straps from cam boxes to body.
Cleaned ground pods at fenders and under dash.
Additional ground wire for gauges to dash ground pod.
Relay conversion for headlights.
Brown wire surgery.
Fuse box connections cleaned and new fuses.
ALL wires traced from source to destination to confirm connections, and conformance to Fiat wiring diagram.

Cleaned all contacts within the gauge sensor and dash gauge circuit. (Remember there is a "temp gauge resistor" in line in the circuit, its located up under the dash near the ignition switch in what looks like a fuse holder.) If the contacts around the resistor are dirty, the gauge will "read" a higher temp.

I'm not sure what else I could do to make that gauge read right, but at least it only moves about the width of the needle. The hardest part is remembering when the lights are on, it reads slightly higher. "Holy sh** I'm running hot! , nope the lights are on."

[jimincalif]

I'm not sure what else I could do to make that gauge read right, but at least it only moves about the width of the needle. The hardest part is remembering when the lights are on, it reads slightly higher. "Holy sh** I'm running hot! , nope the lights are on."

Holy smokes, just reading your list made me tired! I do have headlight relays and I cleaned the headlight grounds when I installed the relays. I will install another ground wire to the engine, this seems like a good idea for a lot of reasons. My instrument lights are not very bright even with the dimmer turned up max, this would seem to indicate the ground for the instruments is less than perfect.

[mbouse]

[quote="jimincalif]I will install another ground wire to the engine.[/quote]

make that a cable, not just a wire. get a replacement battery cable from the parts store with the ready-to-go crimped on ring terminals

[jimincalif]

make that a cable, not just a wire. get a replacement battery cable from the parts store with the ready-to-go crimped on ring terminals

Yes, understood, it will be a battery cable. Where do you recommend I bolt it to the engine?

Jim

[manoa matt]

A good place for an auxillary engine ground is on the passeger side on one of the air pump mounting studs. Either of the two lower studs for the mounting bracket, or the one upper stud for the tension bracket.

You can ground the cam boxes via one of the three studs on the back camshaft cover, connect to the firewall.

[mbouse]

i agree with matt, BUT... don't be tempted to remove any unused studs while you are poking around where the air pump used to be mounted. just hook up the ground strap.

[manoa matt]

The two lower studs go into the crankcase, don't remove these. The upper bolt that secures the tension bracket does not, this is where I attached mine. The bolt will bottom out in the hole so use some washers.

[jimincalif]

Thanks, mine is FI, so it never had an air pump, but I'll look for a stud to use in that area. I'm not brave enough to go removing things.

[spider2081]

I worked on my gauge today. As I stated earlier mine read higher with the dash lights on and especially with the dimmer turned all the way up. Today I took the car for a ride and got it up to temp. Then took out the gauge and soldered the ground terminal spade (the male spade that is riveted to the gauge) to the gauge case. I also soldered a separate ground wire to the light socket. I grounded that wire to the Ground spider under the dash. Took the car out for an hour ride and the gauge is no longer affected by the light circuit. Actually reads with the needle to the left of the 190 mark where before it was solidly on the mark with the lights off and to the right of the mark with the lights on. It is probably indicating about 10-15 degrees cooler with the engine fully warmed up.

[manoa matt]

Confirm those temps with a non contact thermometer

[spider2081]

Thanks Matt one of my co-workers has one of the remote temperature gauges and I will check my car with it. I don't know how accurate the Spiders gauge should be. With so few numbers on the face of a gauge its difficult to interpellate just what the gauge is saying. I think a needle width is probably 5 degrees and to be within 5 degrees would be a fairly accurate system. Probably why so many cars today have just a green and red band and no # on them. It's a top down day here in Connecticut. Hope it is for everyone else also.