Gauges Read Low

[65_Rat_Merc]

I've replaced the voltage regulator twice and my gauges still read about 33% low. I've checked the output of both regulators and they output 5-5.5volts. The cluster and regulator are well grounded. I'm thinking about buying an adjustable unit to bring the readings up, do I need more or less voltage? Any other reason why they would be off?

[Groover]

I don't know the answer to this, but I'm good at random thought and I'm following closely for whomever has an answer.

Many moons ago, I spent a summer with the gauges in our 64. I took them all apart, tried different combinations of voltage regulators, calibrated and re-calibrated to get them correct.

viewtopic.php?f=9&t=5015&p=51735&hilit= ... ead#p51735

What you say is spot on. If the voltage regulator is correct, they should read correctly. You can calibrate them all a little higher from the back -- maybe you already know how to calibrate them?

In the end, my best guess is you have some other sort of electrical problem with the harness where you're possibly leaking some voltage (unnecessary resistance, etc.)? One way to test that would be to ohm out those wires. Another trick I learned (and ended up using) is that the old computer USB charging cords put out 5 volts and they're pretty stable. So you can grab an old school iphone charger or whatever and cut the end off. Use the correct wire and apply the 5 volts to the gauge directly. Then take the same 5 volts from the usb and apply it from wherever your sending unit would be (oil pressure or water temp, etc.), and see if you get the same reading. If not, then there is something wrong with the wiring or some additional resistance. If the gauge reads the same, then calibrate them all up a little or as you suggest, increase the regulator. if you have a variable regulator for testing, this would all be much easier.

Always be very careful with applying voltage directly to the gauge and if the needle pegs, remove the voltage immediately. They won't take full power very long before burning out.

[65_Rat_Merc]

That's a great idea using a phone charger! I'll have to give that a shot to test the gauges themselves, tho I'm leaning more on the side of a problem somewhere with the harness as I have my doubts all 3 gauges would be off by the same amount at the same time.

[tomb22]

I would look at the grounds between the motor and the frame. Make sure the motor has a good ground to the battery.
Tom

[65_Rat_Merc]

Engine to battery ground is good, engine to body is good as well.

[tomb22]

Did the gauges ever work? Maybe the the sending units are the wrong voltage. Try replacing the temperature sensor on the engine.
Tom

[65_Rat_Merc]

The 3 gauges with senders have always read low, and I have replaced all sending units.

[tomb22]

You say the controllers are putting out 5 to 5.5 volts. I don't know where you are checking that but I would check that voltage right at the gauges themselves. If you have 5 volts at each gauge on the power side (black wire) then check the voltage on the sender side of each gauge with the car not running but ignition on. When checking these voltages go directly to the negative side of battery. lets see what you get.
Tom