THSCC Forums

I still suspect one of two things are making the gauge read high. One is that the ground point on the gauge is not at the same absolute potential as the ground on the sending unit. One way to check this is to connect a wire to the ground point on the gauge and another on the sending unit ground and see if there is any voltage difference between the two points. The sending unit might be off as well. Disconnect the sending unit from it's wire and take 2 of your 25 ohm resistors in parallel and hook them between the sending unit wire at the sending unit and ground. The resistance will be 12.5 ohms which should give almost full scale on the gauge. Crank the engine and see if the reading changes much. If it does then I suspect we are back to having a grounding issue.

A third possible cause is vibration. I had a problem with an electric gauge on my old 210. With the sending unit mounted to the block with a T fitting to retain the original pressure switch I got erratic readings and the brass fittings failed due to vibration. I then mounted the T fitting and sending unit on the fender and ran a thick walled teflon line from the block to the sending unit and got stable readings. This also meant the sending unit was grounded to the body and not the motor. So far the teflon line has been in service for almost 30 years. I thought of this because my VDO gauge setup on my 510 came with a hose to remotely mount the sending unit.

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George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

purchased a decent multimeter off of amazon. Pulled the gauge and tried to get an ohm reading, but wasn't successful, so just buttoned it back up to chase another day. So whatdoyouknow... stupid gauge works as it should. Only thing I can figure is there wasn't a good connection for the sending wire to the gauge. It no longer climbs over 100psi at start up, 90 max and then settles down into a more expected operating range once it warms up. Go figure. Anyway, it did prompt me to buy a new multimeter, so I'm good there =)

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Rodney

'08 Bullitt mustang, CAM 7
Autox VP '09-'10, President '11-'12, interim President 2nd half of ‘14
proud recipient of the Bowie Grey service award '12
Now just a guy driving a mustang....