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Temp gauge behaving badly
#1
tach 
I've found that my temperature gauge goes nuts when I turn my lights on.
It was a bit of a relief when I found the lights on / temperature buried hot connection. With the lights off there are no issues at all. The motor isn't hot, just the gauge says it is.

Anybody ever run into this?

I figure it's a bad ground or the light switch.
I ordered a new switch and I also ordered an LED kit, so I will have it apart soon.

Any ideas that I'm not thinking of?
Anything else I need to do while I have the dash taken apart?

Thanks!
Scott
  Reply
#2
I saw this similar type of symptom. It was a bad body ground.

Try this. Turn lights on, check guage and if it is reading high, take a wire (I used one side of a jumper cable) and create a body ground directly from the battery neg. Check the guage again.

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
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#3
Thanks for the tip Will.
I'll check out the ground situation tonight.

I just ran a new ground strap, as it seems the old one was both too long and not routed well. It was being chewed by the alternator. I don't remember there being a ground from the block to the body. Is that where the body ground would go, or just from the negative to the body somewhere? Should I need a new ground wire, what gauge wire do I need? Do I need a really big one like the one from the negative to the motor?

Oh, and PS......
Did it to me today without the lights on. Everything was cool all day, and then it went from middle of the gauge to as hot as it would go in about 20 seconds. I looked at the motor again to make sure, guessing it was still just the gauge but not wanting to gamble a warped head, and it seemed just fine. I shut the car down for about 2 minutes, then turned the key on but didn't start the motor, and the gauge was right back in the middle.

Thanks for the input.

Scott
  Reply
#4
Utard;25390 Wrote:I've found that my temperature gauge goes nuts when I turn my lights on.
It was a bit of a relief when I found the lights on / temperature buried hot connection. With the lights off there are no issues at all. The motor isn't hot, just the gauge says it is.

Anybody ever run into this?

I figure it's a bad ground or the light switch.
I ordered a new switch and I also ordered an LED kit, so I will have it apart soon.

Any ideas that I'm not thinking of?
Anything else I need to do while I have the dash taken apart?

Thanks!
Scott

Had a similar problem with my temp gauge ('71 Grandé). Only in this case the gauge would read barely warm when the engine was hot. Turned out to be a bad connection at the sending unit which is located just behind the distributor. There is an easy test to see if that side of the wiring is intact. Connect the positive side of a volt meter or 12 V test light to the connector and ground the negative side of the meter. With the ignition in ACC or ON position, the light should flicker or the volt meter should bounce indicating the IVR (instrument voltage regulator) and wiring are intact. The temp gauge works on resistance. There is a way to test the meter itself but need to check my resistance on that issue. You need a 10 Ohm and 73 Ohm resistor but I can't remember which one gives the high or low reading. Get back to you on that.

mike

[Image: 1_11_11_13_11_50_27.png]
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#5
I agree with with Will E's comment since I had the same thing happen to my 73 vert. The temp would rise to really high (almost pegged) although when measured with thermometer it was at 190 deg F where it should be. Turned out I did not have the proper grounding from the battery to engine with the mid-point ground to the apron. I fabricated a ground to the sheet metal and that took care of it. As a precaution, I added another ground from the rear of the block to the sheet metal.

My gauges always kick upward a notch (+1/8) when the lights are turned on. At least the oil and temp do (the alt barely twitches as it is). I messed around with things for months before giving up. I'm used to it now. Maybe someday I'll go back to the issue. At the time I changed out the gauge regulator, added grounding, etc. etc.

When you install the LED kit (you will love them) clean and repaint the inside of the gauge pods while you have it out.
  Reply
#6
Utard;25390 Wrote:I've found that my temperature gauge goes nuts when I turn my lights on.
It was a bit of a relief when I found the lights on / temperature buried hot connection. With the lights off there are no issues at all. The motor isn't hot, just the gauge says it is.

Anybody ever run into this?

I figure it's a bad ground or the light switch.
I ordered a new switch and I also ordered an LED kit, so I will have it apart soon.

Any ideas that I'm not thinking of?
Anything else I need to do while I have the dash taken apart?

Thanks!
Scott

A couple of words about the LEDs. Unless you remove the diffuser lens they won't be much brighter. The dimmer won't work and you can't use them for turn signals unless you install a different 'clicker'. We replaced the entire instrument panel with LEDs and these are the issues that arose.

mike

[Image: 1_11_11_13_11_50_27.png]
  Reply
#7
goodnigh;25614 Wrote:
Utard;25390 Wrote:I've found that my temperature gauge goes nuts when I turn my lights on.
It was a bit of a relief when I found the lights on / temperature buried hot connection. With the lights off there are no issues at all. The motor isn't hot, just the gauge says it is.

Anybody ever run into this?

I figure it's a bad ground or the light switch.
I ordered a new switch and I also ordered an LED kit, so I will have it apart soon.

Any ideas that I'm not thinking of?
Anything else I need to do while I have the dash taken apart?

Thanks!
Scott

A couple of words about the LEDs. Unless you remove the diffuser lens they won't be much brighter. The dimmer won't work and you can't use them for turn signals unless you install a different 'clicker'. We replaced the entire instrument panel with LEDs and these are the issues that arose.

mike

I took out my diffusers and the LEDs work great....also the set I have work fine with the flasher for the turn signals.

[Image: 1_14_01_12_10_54_29.png]

Ken

73 Convertible 302-4v-3spd yellow /white int

08 Bullitt 4.6-5 spd black/black int


All I ask is a chance to prove that money can't make me happy
  Reply
#8
Utard;25390 Wrote:I've found that my temperature gauge goes nuts when I turn my lights on.
It was a bit of a relief when I found the lights on / temperature buried hot connection. With the lights off there are no issues at all. The motor isn't hot, just the gauge says it is.

Anybody ever run into this?

I figure it's a bad ground or the light switch.
I ordered a new switch and I also ordered an LED kit, so I will have it apart soon.

Any ideas that I'm not thinking of?
Anything else I need to do while I have the dash taken apart?

Thanks!
Scott
Hi Guys,
I have the same problem with my car. Appearently it's common for this to happen. There is a small resister or relay behind the gagues that breaks down over the years. When the lights are on, which is usually the biggest power demand on the charging system, the resister is supposed to regulate the power to the gagues but over time it starts to break down and lets too much power to the gagues. Appearently it can be a pain to change. I actually changed all my radiator hoses, flushed the radiator twice, replaced the thermostat twice and put an electric fan on the radiator because I thought the car was overheating. I become suspicious when the temp gauge rose when I turned on the fan. One of the guys in the club had the same problem with his car. He fixed his but I'm not going to bother. I've found that after about 20 minutes driving with the lights on the gagues settle down. I hope this helps and maybe someone who's pulled the gagues apart can identify the relay or resistor. Appearently the part is available through someone like NPD or Mustang Unlimited.
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