oil press rally gauge problem
 

Hi everyone I need help Bad lol I have a 69 gto it had idiot lights and I went and put in rally gauges I did change the 2 wire up t the circuit board everything works as it should my problem is the oil pressure gauge here is what I did so far put in new sending unit 3 so far when I take plug off gauge goes to 80 I ground end of plug gauge goes to 0 fine gauge ok I start car oil pressure fine go for a ride all seems ok I bring car home shut it off and turn key on without starting the gauge wont go back to 0 stays about half way start car gauge goes all the way up to 80 shut car off take plug off put it back on all seems fine its driving me crazy is there another wires I am suppose to change ?? I even tried to ground sending unit out and still not correcting problem tried 3 new sending units all same thing any help will be greatly appreciated

If you have checked your printed circuit and the dash wiring and all of that is good. Also check and make sure the metal gauge housing is grounded to the "-" body of the car.

You don't say but are these original 1969 GM gauges or are they modern aftermarket replica parts made in China or Taiwan?

Most likely your oil pressure gauge is bad, you could replace it or if you want to PM me and send it to me I can rebuild it for you. Those gauges will go bad after 50+ years. If it is an original GM gauge very often I can repair them, it all depends on condition.

There is a resistor on the back of all 3 gauges, it is put in the circuit as a baseline so that it gives each gauge something to compare itself to. The sender of each gauge being the variable. It is not common but sometimes that resistor will go bad. Either short out or open circuit. A bad resistor on the back of one of these will most certainly cause that gauge to read incorrectly.

GM used 2 different kind of resistors, the older ones were wire-wound on a piece of woven asbestos mat. (These resistors will get warm during normal operation.) The newer ones are a carbon "footprint" on top of a ceramic insulator. After manufacture the newer design resistors were tailored, measured and then color coded to an ohms range span. The color epoxy paint on top also serves as protection for the carbon footprint.
On the older wire-wound resistors; when those get old sometimes the ohms value increases by 10 or 15%. The ceramic ones can go bad after 50 years. Often due to weather exposure or intense heat over many summers.

Ok Thanks yes they are original rally gauges there is no place to put ground on unit I thought it gets grounded through engine what should I do soldier a wire on the case then ground it ?? I will have to remove gauges in the winter I like to enjoy the car for now just hop it doesn't burn up in the mean time Thanks for the reply

1969 GTO Dash gauges, Ground paths
 

The ground on 1969 gauges is from a pathway off of the housing via tiny flat pieces of steel overlapping to bridge the gap in-between the housing and the sheet-metal skeleton of the dash pad. Running any GM car or truck gauges with absent or a weak ground at the dash cluster can easily burn up the coil(s) inside of the gauge. GM car & truck Gauges need TWO grounds, one at the dash cluster and the other ground is at the very end point of each sender. The 12 volt + power for the gauges is provided via the key switch thru a 10 or 15 amp fuse in the fuse block.

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Thanks Pete just went out and check it again when I turned the key on the gauge went half way shut key off pulled the plug off sending unit turned key back on and gauge goes to 0 all straps are on for grounding even if I take a wire from the neg battery and touch gauge cluster base it does nothing if I could some how have the gauge to read 0 I would forget about it and put in a manual gauge and say the hell with electric crap seems to me its still holding voltage after I turn the car off other wise what would pulling the plug off and back on for it to work Thanks again for your help wise I was close to you I would just pay you to fix it Ed

It sounds like a bad gauge to me. Either that or you do not have it wired properly. 50+ year old gauges are generally going to be bad from age. I would like to see the gauge cluster for testing, if one gauge has gone bad the others are probably not far behind.

OK Pete maybe I will look to buy a cluster send it off to you to rebuild and this winter I will change it over lots of work to remove lol I will be in Touch Ed