Found this info for who ever gets into this situation again. Very helpful: What I did for the blown or inop gauges and the high volts from Alt.
The fuel gauge should have the following resistances ...See pic below
- S to Ground 68-72 ohms S is on Pax side
- S to I 19-21 ohms I is on drivers side
- S to A 19-21 ohms
- I to A Zero
- I to Ground 49-51 ohms
- A to Ground 49-51 ohms
The fuel sending unit wires are located on top of the gas tank where they are hard to get to without dropping the tank. The fuel sending unit should have a pink wire with voltage on the isolated center post. The other black wire on the sending unit with a tab style connector is a ground to the frame. Make sure it has good contact.
To be sure the problem is not the gauge, you can momentarily short the pink wire on the output of the sender to ground, and this should show up as FULL on your gauge. DO NOT
hold it for very long in this position. If the gauge does not move from EMPTY either the wiring has an open circuit (no voltage, or no connection to ground) or he gauge is bad. If it does move, the sending unit is bad.
The sending unit can be checked with an ohmmeter to measure the resistance between the round sender post (pink wire) and ground. It should be:
If the resistance falls in this ballpark (depending on how much gas you have in the tank), then the sending unit is fine. If it shows infinitely HIGH resistance, then the sending unit could be bad OR the wire from the tank to the gauge could be open.
The gauge can be tested with the resistances listed above. Run an appropriate resistor to the S terminal of the fuel gauge and to ground and check the readings.
The temp gauge has the following resistance ...
- S to A 19-21 ohms S is on the drivers side
A volt meter can be used to measure the voltage between the A terminal of the Temp gauge and ground. It should be pulsing and averaging about 5 volts. If it reads 12 volts the Jumper Strip/Regulator is bad. If it reads 0 volts, it has been burnt out.
The sending unit can be checked with the following resistances between the post and ground...
high resistance 300- 400
Beginning of Band
End of Band
If an appropriate resistor is connected to the S terminal of the temperature gauge and to ground, the above restances can be used to check the gauge. Use a resistor close to the specifications above to simulate the sending unit
Back Side of Speedometer with Fuel and Temp Gauge
The jumper/regulator strap runs from a voltage regulator inside the fuel gauge (terminal A) to the temperature gauge (terminal A). A voltage reading at terminal A of the temperature gauge should be approximately 5 volts. A 12V reading here means you have a bad regulator.
The meter movement in all the gauges have a built-in dampening mechanism which keeps the needle from bouncing around. The dampening mechanism is basically some thick grease on the movement's pivot points.
The reason all the meters are dampened is because the senders do not have a very constant resistance. If you put a good testing meter on the sender while the motor is running you will see the resistance bounce all over the place. The dampening averages out the reading. This also is why it takes a few seconds for the needles to come up to position instead of snapping to a reading.
To be sure the problem is not the gauge, you can momentarily short the wire from the output of the Sender to ground. If there is no resistance, your gauge should read 80 psi. DO NOT
hold it for long in this position. If the needle does not move from zero psi then, either the wiring (open circuit) or the gauge is bad. If it does move, the sender unit is bad.
It is very common for the sending units used with the 258 and 232 engines to be inaccurate. Make sure you have a good connection to the sending unit
t is easiest to test the sending unit by temporarily plumbing in a good mechanical gauge.
From the passenger side, left to right in the picture ...
Testing the voltmeter is easy, you just need a good 12 volt connection to the (+) post and have a good ground to the (-) post.
- Volt Meter gauge GND terminal (Center) - Black wire (Ground)
- Volt Meter gauge terminal (right) - Red wire (Ignition-on hot 12v)
- The other wire goes on the LH side or passenger side.
All in all somehow every one of the sender units is bad. The gauges check out OK.
HIgh Volts: I polished every ground I could find, removed the alt bracket and polished the contact areas, poliched the pwr cable conections from alt to battery as well, and the volts are down to 14.5 still slightly high but within limits. Interesting that I found some resistance in the wires at the firewall connection. Some wires were also pushing out and not making good contact. I put a small bolt in the firewall and connecting the under dash ground wires to that, then direct to the battery. Will test that later.