Temperature gauge goes up when lights are on

[knightrider2000]

Got a Strange one for yall!

1988 Mustang GT Convertable 52000 miles
The Temperature gauge goes up about 10 degrees and rests on the 210 mark when the headlights are turned on. When the lights are turned off, it immeditly falls back down to normal, bout 200 or so. Any ideas on what could be causing this??

Thanks

Matt

 

[aleborjas]

need more chasis ground check and clean all the grounds and check if you have one from the engine to the fire whall, batery to body and one under the starter selenoid comes from the inside cables need to have good ground that ones is from inside acc like dash i had that one loosen and my fuel level whent crazy!

 

[Grn92LX]

Yep, thats a ground problem and a common stang problem.

 

[knightrider2000]

Car is 100% stock, seeme to happen after it got jumped off at one pointwhen the batt was dead. Any idea which ground could be the culprit before i start hunting grounds?

Thanks Again!

MAtt

 

[superhuaman]

This may sound weird.... I think it's because of the voltage drop when you turn on the headlights. See, the coolant temp sender has more resistance when cold, and less when hot. So when you turn on the headlights, the available voltage drops. But the sender stays the same resistance, so the temp gauge reading is different. To keep the same reading with different voltage, you would need a different resistance. I would check all the grounds and the instrument voltage regulator. Also, remember that when you have the headlights on, the instrument cluster illumination is on also. This may have something to do with it. Hope that helps some...

 

[BlackGT89]

I have also had this problem for years, but it hasn't gotten worse and I've pretty much gotten used to it. I too have heard the instrument voltage regulator is the culprit (check the grounds also), but I don't know where the regulator is located or even if they are available anymore.

 

[jrichker]

Grounds are important to any electrical system, and especially to computers.

1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. The wire goes into the harness at the starter solenoid.

4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or head bolt.

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.

See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103#volt for help troubleshooting voltage drops across grounds

 

[FORCED2DV8]

Grounds are important to any electrical system, and especially to computers.

1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. The wire goes into the harness at the starter solenoid.

4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or head bolt.

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.

See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103#volt for help troubleshooting voltage drops across grounds

I swear you must have the full Ford manual in your head or right on your comp. You provide excellent answers....

 

[5spd GT]

I have the same problem but my temp guage peaks when car is off and lights are on (headlights). I wish we knew the exact ground (if there was one) that effects the temp guage

 

[aleborjas]

all that is ground problems!

 

[superhuaman]

I have the same problem but my temp guage peaks when car is off and lights are on (headlights). I wish we knew the exact ground (if there was one) that effects the temp guage

Well my temp gauge creeps up very slowly after I shut the car off, with nothing on.

 

[5spd GT]

Well my temp gauge creeps up very slowly after I shut the car off, with nothing on.

Mine goes up real slow as well...but with headlights on. Lol...we need to get this figured out

 

[jimilane]

My car was doing the same thing.

It was the ground wire that goes from the negative side of the battery to the engine block. It was loose at the block and the wire itself was corroded.

Also, after car had run for a while and warmed up. It was hard to crank like a starter dragging or battery dead. The longer you tried to crank, the worse it was. If the car was cranked for a long time, smoke would start coming form the smaller grounds along the chasis! How about that? After the car cooled down a little, it would crank fine again.

Sounds like you might have the same problem, since you needed the jump. What was the cause of you haveing to jump sart the car? Was your battery really dead when you had to jump it? It might have been this same problem occuring.

All these problems were caused by the negative battery wire.

Hope this helps

Jim Lane

 

[5spd GT]

JimiLane - Is that the one that goes to the side of the timing cover? If so where can I get a new one at? Do they make ones just for our car or do I have to get a universal one?

 

[jimilane]

Yeah, it connects to the driver's side at the timing cover.

Just take the old one off and take it with you to the parts store and match up the length with a new one. They are less than $10.

Check to see if yours is bad first though. It still could be another ground somewhere.


Jim Lane

 

[5spd GT]

Thanks Jim, if they are that cheap I'll just get a new one anyways to see

 

[HISSIN50]

That particular ground is succeptible to galvanic corrosion.