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Couple issues, hopefully someone can help
#1
My 73 Mach 1 is having an odd problem lately. The car runs great doesn't overheat but with the headlights on the temp gauge goes all the way to H. With the lights off it climbs back down to the middle. The engine ground is good and clean, I cleaned up the headlight grounds as well. Anyone encounter this problem? Is there suppose to be a body ground on the 71-73's? Thanks
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#2
Justinc1973;64221 Wrote:My 73 Mach 1 is having an odd problem lately. The car runs great doesn't overheat but with the headlights on the temp gauge goes all the way to H. With the lights off it climbs back down to the middle. The engine ground is good and clean, I cleaned up the headlight grounds as well. Anyone encounter this problem? Is there suppose to be a body ground on the 71-73's? Thanks
Not likely that is an issue, but to answer your question: There is a strap from engine to frame...not sure where it is located. I think if you didn't have a good frame ground you would have other issues with lights, radio, etc.

Other things to try:

1. Measure resistance of ground from battery to the sensor. Should be 0, or under .2 for sure.

2. Measure voltage from sensor contact to battery ground, lights off, engine on.

3. Measure voltage from sensor contact to battery ground, running/parking lights on.

4. Measure voltage from sensor contact to battery ground, headlights on.

5. Measure voltage from sensor contact to battery ground, high beams on.

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351C Bold Manners, Brash Attitude
Favorite Teams: Michigan Wolverines and Whoever Is Playing Ohio State.

When I drive past a herd of cows, the cows MOO at me
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#3
Im wondering if it has something to do with the fact that when you pull out the switch it also turns on the lighting behind the temp/amp/oil gauges. Im willing to bet it has something to do with the headlight switch where it controls the lighting for the gauges. Try this, pull the switxh on amd turn the headlight knob to dim/brighten the lights, see if the gauge moves. Thats the only thing i can think of that has activity between the light switch and those gauges.

"Do it once, do it right!"
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#4
Justin,
My Mustang does the exact same thing.
I figured a bad ground as well. If it is a bad ground somewhere, I haven't it yet.
I bought a new headlight switch hoping that's it, but haven't put it in yet.
if it solves the problem I'll let you know.
if you happen across the solution, please post it so I know what to check.
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#5
Welcome to the site, Justin. I'm sure that someone will come along soon with a solution to your problem. When you get a chance, post an intro in the Introductions section and we can give you a proper welcome. Good luck!

Doc

Doc

[Image: 6y14ea.jpg]

Project started 8-7-10
Completed: All new suspension, rebuilt 351C H Code bored .030 over with mild cam and intake, new 3.50 TracLok, custom exhaust system
Current "mini-project": interior upgrade Undecided
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#6
Justinc1973;64221 Wrote:My 73 Mach 1 is having an odd problem lately. The car runs great doesn't overheat but with the headlights on the temp gauge goes all the way to H. With the lights off it climbs back down to the middle. The engine ground is good and clean, I cleaned up the headlight grounds as well. Anyone encounter this problem? Is there suppose to be a body ground on the 71-73's? Thanks
Do the fuel level and oil pressure gauge change a small amount with the headlights on as well? If they do, it sounds like something is causing the output of the Instrument Voltage Regulator to change (the IVR supplies the voltage to these 3 gauges). It could be the IVR itself. Does the same thing happen if you cause high current loads from other things such as engaging the cigar lighter and horns? If so, it could be a charging system issue. Has the regulator been changed out to a new style solid state or is it the original style electro-mechanical? If it is a new style, make sure the case of the regulator has a solid ground where it mounts to the fender apron. These are just shots in the dark at this point. There is a wiring diagram on this site under the "DATA" tab that might be helpful to you. Let us know if you find the problem. Chuck
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#7
Here's what you should try.

Turn on the car/lights. Take a jumper cable and attach it to the negative post on the battery, then attach the other size of the one cable to a body ground. (Shock towers can be a good choice). Look at your guage, if it reads correctly then you have a bad body ground.

'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)

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#8
The body ground is a bare braided wire that goes from the back of your intake manifold to the firewall.

Doc

[Image: 6y14ea.jpg]

Project started 8-7-10
Completed: All new suspension, rebuilt 351C H Code bored .030 over with mild cam and intake, new 3.50 TracLok, custom exhaust system
Current "mini-project": interior upgrade Undecided
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#9
Could be a bad connection on the circuit board where the dash light trace or the ground trace is contacting the temp gauge trace.

Other than that, the temperature wire is completely separated from anything related to the headlight switch.

Let me check your shorts!
http://midlifeharness.com

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#10
I tried providing a body ground using a cable from the negative post to the fender apron, no change. The gauge will also go full H with just the parking lights on as well. The fuel and oil gauges don't act erractic with the lights on. I will have to do some more testing. My car only has a battery to engine ground, but everything seems to work properly (besides the temp gauge) maybe I will add a body ground just to be safe. My voltage regulator has been updated to a solid state, I cleaned that ground as well. Nothing yet. Thanks for the ideas, I will try a few more things.
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