I am working on a 70 Roadrunner convertible. I had to replace the fuel sending unit. someone had installed an incorrect ohm sending unit. I researched to find the correct ohm sending unit and installed it. The fuel level is almost full physically and I am getting a reading of 66 ohms at the guage cluster however the guage still reads empty. Do I have a bad board or bad guage? Any help would be greatly appreciated. thank you for your time.

 

This seems to be "status quo" for "repop" senders. This complaint is "all over" the A body forum. I cannot speak for the exact reading, but this sounds typical, IE the sender does not track like the old one. In fact, one member over at FABO is working on a prototype "scaler" device

In the meantime, the only thing I know is to "fiddle" it as close as you can. For me, way more important to show "empty" fairly accurate. Bend the sender if necessary. Add a series resistor if necessary.

BUT THINK about this!!!! Have you checked gauge accuracy?

(I do realize you are showing the sender is off)

You can check your gauge accuracy (all three gauges are the same) with test resistors, or by manually moving the sender.

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

These are not "written in stone" I'd say 70-75, 20-25, and 9-13

For example, you can go to RadioShack and buy a package of four 100 ohm 1/2 watt resistors and wire all four in parallel. This will give you a 25 ohm resistor which when used to replace the sender, should give you a 1/2 scale reading

Also I try to get you guys to think of these 'as a system' end to end. Cluster designs vary, as do the problems, but some examples of the problems:

The Voltage to the cluster MUST be "solid." This of course is supplied by the ignition switch. Some cluster designs are grounded "by accident." It's a good idea on most PC board clusters to add a ground wire to the board and bolt it to the column support or other good ground.

Some cars use a harness connector to pins on the PC board at the cluster. These pins can be broken loose at the PC board and must be repaired.

Some cars use a "socket" which the IVR plugs into. On my Dart, the brass contacts were not making contact with the board traces, and I had to solder jumpers across

The IVR itself is suspect. Buy an RTE or other solid state replacement, or build yourself one

The gauge studs on some cars use "fake" nuts. Consider replacing those, and loosen / tighten several times to scrub the board connection clean

The gauge themselves can be out of cal

In the case of the temp / oil gauges, both senders go through the bulkhead connector, so are subject to problems there

All senders can have poor connections right at the sender terminals. These can be corroded up in the molded wire connector

As you have found, the senders can be out of calibration

By the way, "back then," my 70 440-6 car was and remains, my all time favorite car. If I ever win the big one, I'll have another

 

Thank you for the info. all my other gauges work properly but the fuel guage. I have the correct ohms going to the pin connector for the sending unit to be correct. however the guage moves from completely empty, below the e line, when ignition is on it sits on the line and goes no further. is this a bad fuel guage?

 

Ground the sending unit wire and see what happens. Is your fuel tank grounded?

 

Can you re-state this? You are saying what? I'm guessing?

You have proper power to the gauge from the IVR

You have the sender adjusted to 10 - 15 ohms and connected to the sender connection of the gauge, to ground, and the gauge only reads near empty?

This could still be a bad connection, and was what I was getting at when I was talking about "thinking of a system."

Think of the circuit path

Ignition switch............harness connector.......PC board..........IVR........gauge stud..........through the gauge........out the sender stud..........back out the PC board pin and connector...........through the kick panel connector to the rear harness...........to the rear..........to the sender connector...........to the sender...........to the tank shell...........to body ground.

Every single one of those points is a possible bad connection or trouble spot. IF YOU have checked calibration of the temp gauge with resistance, and found it OK, then the ignition switch, PC connector, IVR is probably good

THIS LEAVES a considerable amount of "stuff" "down the path."

Do you have the cluster out of the car?

 

Is there anyone any one that might have a wiring diagram for a 70 b-body roadrunner rallye cluster panel? Its a satellite car, and I'm trying to match up the rallye panel to it, any help would be appreciated. J.R.

 

http://www.moparfreak.com/wiring/70ChargerA.jpg

http://www.moparfreak.com/wiring/70ChargerB.jpg

http://www.moparfreak.com/wiring/66to71.html


try these... 70 charger should be the same. The links for the 70 satellite/RR/gtx is broken.

 

i have seen a artical on how to correct an out of adjustment system. but takes some electronic work. ill try to hunt it down.