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Discussion Starter #1 (Edited)
Hi all,

(Edit: Sorry the headline is incorrect, I missed it when I submitted - should read "1969 Mustang." Can a moderator change it?)

I have a 1969 Mustang convertible with a 302 engine. The other day I looked at the dash and watched the temp and fuel tank level gauges swing from a normal reading fairly quickly to very high readings. The fuel tank was near full, and the needle swung way, way above full. The temp was normal (lower end of the marked operating range), and at the same time swung high (to about the top of the normal range). Actual temp and fuel levels were fine, and the cooling system is full. Note that at the same time, the oil pressure reading rose somewhat (but not as dramatically). The alternator gauge stayed steady as a rock, right in the center as usual.

Funny thing is, a couple minutes later the gauges went back to where they were before - again at exactly the same time. The same series of events happened again later in the day - gauges swing up suddenly for a bit, then back down.

This smacks of voltage delivery issues, but I am only smart enough to know how the gauges work, not what would be causing it (so I am more theoretical than application-oriented here, hah).

What should I look for and where? I'm guessing voltage regulator on the alternator, a short somewhere, or something else I have not thought of. Any other things I should check?

Thanks, any help is certainly greatly appreciated. I have a rally coming up - my first one. Don't want to drive in it if there's a risk of some wires starting a fire or anything. :)

greg
 

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Look for a grounded signal wire from any one of the 3 gauges or the voltage reg. on back of the cluster is going haywire., my 2 guesses good luck
 

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Look for a grounded signal wire from any one of the 3 gauges or the voltage reg. on back of the cluster is going haywire., my 2 guesses good luck
There's a voltage regulator on the gauge cluster? Oh, fun. Time to break out the service books. :)

Anything else to check, or things to test? Thanks.
 

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Not sure about the 69 but most older cars are the same....

If you disconnect the wire at the gas tank and ground it to the frame the fuel guage needle will peg out past full. Same goes for the wire that goes to the temp sender (and oil for that matter). So, with that said, about the only place these two circuits are in close proximity to each other, and could possibly share a common ground path, would be behind the dash. You may take some comfort knowing that these are both ground circuits so there should be no worries about a fire. If these circuits find a hard ground the guages will peg and just stay there until you fix it. However, these circuits are finding an unintintional ground path and there may be a hot wire lurking close to them that is about to do the same thing. I would investigate the harness behind the dash and see if something has rubbed through the insulation.

Steve
 

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However, these circuits are finding an unintintional ground path and there may be a hot wire lurking close to them that is about to do the same thing. I would investigate the harness behind the dash and see if something has rubbed through the insulation.
Thanks, Steve - I'll start crawling up and around there and see what I can find.
 

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On the back of the instrument cluster there is a small metal rectangular piece with 9 volt battery terminals on it, your printed circuit plugs on to it and it is tight! pry it of one terminal at a time, if you just pull it off you will rip you printed board.
Bad news is you can crawl all day long but you cant see any of this with it installed, you must remove it to check it. fear not it sounds more scary than it is to pull out. Check some service books, you can handle it. Steve is right, they are negative (-12VDC) signal feed and other than give you wrong info, will not cause any harm to your car.
The signals come in as resistance and the instruments get 6 or 9 volts from the dash voltage reg. and the needles show you and me the actual values in temp, fuel levels and such. My 2 brothers and I all have 69 convertibles as well, fun cars! good luck
Joe
 

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I had a very similar problem last month with my fuel and temp gauges going all over the place. I pulled the dash, remounted the ground wire, and reinstalled the light switch harness plug. Everything worked fine after that.
 
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