1973 Duster - Fuel and Temp Gauge issue

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Harveybos

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Hi
We're trying to revive a 73 Duster 318 that's been off the road for many years.
It's running now but we have a problem with the fuel and temp gauges. With the ignition on, and engine off the temp gauge is on bottom stop and the fuel gauge shows fuel in the tank. When the car is started both gauges go right over to the stop.
We've tried another Voltage Limiter and put an additional earth wire from the cluster but no joy. The ammeter shows a quite high level of charge too - could it be a Voltage Regulator problem and it's overcharging?
Thanks, Harvey
 
It may be the VR but there are other causes and I've written about them here a LOT. One big problem is that the power path from the battery to the VR can suffer voltage drop, and so the VR power terminal sees low voltage, and 'tries' to ramp up charging voltage until the VR setpoint is reached. So by the time the setpoint is reached at the VR, the battery is boiling LOL

Here is how to check

VERY FIRST thing to try is with the engine running, pull the connector off the voltage regulator It should stop charging. This simply checks that the field wire is not shorted, and that the green wire brush of the alternator is not grounded

Turn key to run with engine stopped. Probe the ballast resistor to find the IGN feed from the bulkhead. This should be the terminals on one end which are jumpered together. Now with one probe on that terminal, either compare that point (to ground) to battery voltage, or put the remaining probe onto battery POS post. This will give you the voltage drop directly. You should read a VERY small voltage, not over perhaps .3V (3/10 of one volt). ANY drop there is added to the charging voltage of "normally" 14V So say, you read 1.5V there, the thing will charge at 15.5. [14V (vr setpoint)+ 1.5V (the voltage drop)]

Also the VR MUST be grounded and the ground point "at" battery NEG potential. You can check this, too. Make this test first with all acessories off, and again with heater, lights, etc powered on. Start the engine and run and fast idle to simulate "low to medium cruise" RPM. Stick one probe into the VR mounting flange, and the other onto the battery NEG post. As before, you should read a very low voltage, the lower the better. Zero is perfect Again, any voltage reading is added to the set point.

If these two points check OK, if the VR is grounded, and the battery seems good, replace the VR
 
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Thanks for the prompt response. The car is stored some distance away so it will probably be the weekend before I can check. I’ll post the results!
 
Just to know where the problem is not at... there is a white plastic engine harness connector right side of firewall. Contains alternator wires, temp sender wire, and more. If/When these connectors melt down, strange crossed circuits can happen.
 
^^THIS^^ and until you get things sorted, you might yank the instrument regulator out of there to prevent damaging the gauges.
 
Harveybos said:
Yes, will do!
Click to expand...
Managed an hour on the car today - battery voltage was 16.5v on start up. I think the Voltage Regulator is past it’s best - see photo..

5714AB08-98BC-425E-9A92-F760A84D3FBA.jpeg


CB8F47AC-F57F-44C1-A8E9-64F9E7C3291B.jpeg
 
Looks to be OEM. Nothing lasts forever. Best to disconnect battery and mount/ground the regulator before reconnecting.
 
Many thanks for all the advice! Fitted a new Voltage Regulator at the weekend. Cleaned the white connector block and the connector blocks at the back of the engine bay as well. Then discovered that the earth lead from the battery to the radiator support was broken, so repaired that & fitted an extra earth lead from the engine to the body. Going to do some more & re-test next weekend but think it's just about there. Fingers crossed!
 
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