MGB Electric Gauges Read High, Any guesses?

[Taftster]

1968 MGB, has electric gauges for Fuel, Oil Press., Tach, and Temp. All read high by about 20%. Tach reads 4500 at 3000 actual rpm.
Fuel reads Full till about 3/4 tank. Oil is always at 75, and Temp is always midway between N and H. Any guesses? Being that it is across the board, I am not thinking that the individual gauges are the problem.

 

[PAUL161]

Not sure about the 68, but my 72 had a voltage limiter mounted on the firewall in an area above the drivers feet and I believe it was for controlling voltage to the instruments. Been a few years since I built that car so memory is a little foggy. Worth checking out. PJ

 

[Mickey Richaud]

Good chance it's the voltage stabilizer, mounted on the cockpit side of the firewall. Output should be around 9.5 - 10 volts. If its ground is faulty, you're probably getting full 12 volts to the gauges; it's grounded through its mount to the firewall. Check to be sure there's a proper ground; if so, then the stabilizer may be faulty. Good luck, and let us know what you find.

Oh, and if you don't already have it, here's a screenshot of the wiring diagram for a '67/'68 MGB.

Mickey

 

[DrEntropy]

Moss Motors offer a solid state instrument voltage regulator that works. +10V constant to the gauges.

 

[PAUL161]

Thanks Doc for letting me know my memory isn't quite as bad as I thought it was!

 

[DrEntropy]

Thanks Doc for letting me know my memory isn't quite as bad as I thought it was!

HA! You're welcome, Paul.

You KNOW memory is like a hard drive; only so much room. What was on yesterday's breakfast table is unimportant when compared to things like where/what voltage stabilizer works for LBC instruments! I'd need to open the toolbox to get the right part numbers, but just know they're in the circuit and where to get a good one!

EDIT: Just opened the box, the Moss part number is: 131-555, Original BMC number: BHA4602 (negative ground).