@joe_padavano

I am stumped here.


1967 Olds Cutlass

Here is what I have found in my testing.

I have an all bodies Olds factory service manual but can’t find any of the specs or testing procedures for the gauge or supply voltage at the dash.

I have 3 gallons or so of gas in the tank.


My assumption for fuel gauge dash power diagram of contacts, sitting in the seat looking at the dash:

(+ positive when key on) ( sender input)


(Ground)



Here are my tests that I performed:

SENDER TESTS:

1. Ohms of resistance at the sending unit connector output on the fuel tank = 19.6 ohms

2. Ohms of resistance at the back of the gauge on the connector pad from the sending unit brown wire = 19.8 ohms

From the two above tests, I confirmed the sending unit wire and signal is correct going to the back of the gauge connection


For the next test, I tested for power and ground at the connection points in the dash panel that touch the back of the gauge

3. The voltage being sent to the Driver’s side upper left connection pad square with the key on should be the voltage supply pad to the back of the gauge = 8.0 V

with key off, no voltage at the pad

4. The ground pad at the bottom of the triangle shows full continuity to ground on a jumper across my tester


The tests I performed on the gauge:

Looking at back of gauge in hand, my assumption for connections are:

(Sending unit input) (+ positive input)


(- Ground connection)



5. Sender pin to ground pin = 98.8ohms

6. Sender pin to positive pin = 45 ohms

7. Positive pin to ground pin = 145 ohms


Testing the gauge in hand:
The Gauge pegs the full mark when a 9V battery is connected across the positive and ground terminals. I tried it with a 100ohm resistor from the sender to ground shows no change(as it already shows 98 ohms resistance across that path in my test.

Edit- added later this evening, more testing with different resistors!

I just tested with a 50 ohm resistor that was tied between the sending unit peg and the ground, and the gauge shows ~ 1/2 full.

With a 15 ohm resistor, it shows about 1/4 full, so the gauge must be ok, from my deduction.

(Edit update!) I now believe the fault may be in the dash wiring somewhere.

As soon as gauge is attached to the dash, it shows no voltage to power strap with key on, and the gauge doesn’t move from anywhere the needle is moved to manually.


Can anyone confirm what the voltage should be at the dash where the gauge contacts it?

Has anyone done the tests like I have above on a working gauge setup to allow me to compare my test info?

Thanks in advance!

 

Try reading this thread it is pretty good. I used to try to give electronic advice but realized I had forgotten most everything I learned in 35 years working in that field (I hate getting old). I will suggest this though. I think the gauge and the sending unit in the tank are in a parallel circuit so I don't think your gauge would neccesarilly be bad based on your measurements.

https://classicoldsmobile.com/forums...6-olds-141607/

 

I've never performed the tests that you have done other than measure the resistance of the sender. They are not parallel, the + side is 12v, the sender (negative side of the circuit) is a variable rheostat of 0-90 ohms based on the level of fuel in the tank connected to ground. The other connection is chassis ground.

There is a pink wire that should have 12v on it when ever the key is on. The tan wire runs back to the tank. If you ground the tan wire the gauge will go to E if you disconnect it will go past full with the key on.
Back at the gas tank sending unit there are 2 wires, one is grounded somewhere close, the other is the tan wire that runs to the gauge. The same principle applies, if you ground the tan wire the gauge goes to E, if you remove it the gauge goes past full. If the wire at the tank passes this test, it's usually the sending unit.

 

I updated my info for more testing just performed….

50 ohm resistor on the gauge shows about half full, and 15 ohm resistor shows less than 1/4 when testing the gauge in hand with a 9 volt battery.

I now believe my dash wiring has a fault or issue.

 

It seems your gauge is good. Connect it all back under the dash and find the connector in the trunk. Perform the test as described above.

 

I don’t believe I am getting the correct voltage on the dash where the gauge contacts touch.

Testing the connection where the sending unit goes to the gauge shows the correct ohms as compared to the direct reading at the sending unit connector.

 

It should be close what the battery voltage is.

 

Sorry, I was at Carlisle all week. The sender should range from 0 ohms (empty) to 90 ohms (full). Your terminal labels on the back of the gauge are correct (bottom terminal is ground). As noted above, the power feed from the dash should be battery voltage (approx 13.8v with the alternator charging).

 

Thank You both for confirming the voltage that should be supplied from the dash (and for confirming the ohm range empty to full, Joe).

I will have to do more digging in the dash and on the printed circuit board, it looks like.

 

Can anyone confirm that one of the new rally-pac printed circuit boards is the same for the Speedo side and fuel gauge as a regular cutlass with nothing on the right side?

Whenever I get time soon, I will pull out the gauge panel and check the traces again on the printed circuit. If not that, I will trace the wire and the clip connector that plugs into it and see if I can find the issue.

Also, who makes the best upgrade harness for these 1967 cars?