Guage issues

[Surfnuke9]

My fuel guage was dead when I bought the truck a few weeks ago. Temp guage worked fine.

I checked the resistance from the fuel sender and it varied with tank level, I disconnected the sender wire from the sending unit with the key on and there was no response from the guage, so I purchased a working used guage and replaced the old one.

Now the fuel guage reads 3/8 of a tank regardless of level and when I connect the blue wire that runs from the fuel guage to the temp guage, the temp guage pegs high and gets very hot.

Any thoughts?

[Desoto61]

How did you go about verifying the fuel guage was working specifically? With the unit in the tank or removed?

I don't know the specifics of these particular systems but from playing with others the questions I would try to answer are:

Are the resistance levels in spec for the sending unit? Corrosion or a bad connection could mean that everything is shifted up by some amount, throwing off your readings.

Have you verified your ground path from the tank to the frame? If you checked between the guage and the tank it may be fine but the guage reads to body ground which might have issues.

What is the voltage being sent by the guage? Is it within spec? Is it the same at the guage as it is at the tank?

Is there an electrical damper (capacitor or inductor, can't remember which) on the circuit that could be bad?

Without knowing what the correct resistance range for the fuel sender and temp sender are I don't know that the temp guage is a good indicator of anything as the resistances could be drastically different so guage behavior won't be the same.

[Surfnuke9]

Have not checked the tank to frame ground. Stupid not to yet. Could be the answer.

voltage at the guage is fine at 12.5 volts. Guess I should pull the sender out of the tank and check it all out. all I did before was pour in 5 gallons of gas from a can and read a different resistance value after adding 5 gallons.

dont know about the damper (?) in the circuit. Will pull out the schematic again and look for one.

Thanks

[Desoto61]

Have not checked the tank to frame ground. Stupid not to yet. Could be the answer.

voltage at the guage is fine at 12.5 volts. Guess I should pull the sender out of the tank and check it all out. all I did before was pour in 5 gallons of gas from a can and read a different resistance value after adding 5 gallons.

dont know about the damper (?) in the circuit. Will pull out the schematic again and look for one.

Thanks

I'm not familiar with your particular system. Does the gauge send out 12 volts? Chrysler used a secondary regulator later on to reduce the voltage to about 4 volts for all the gauges. That regulator was usually in the fuel gauge and fed the others. It was mechanical and could go bad too. If it was bad then the gauge won't read correctly.

I'd say to check the output at the back of the gauge, then at the sender. Then check the ground side, if all that looks good then pull the sender and check it out.

Again, some of the later chryslers (and many others) had an inductor (I think) on the back of the gauge to dampen the pointer movement as the fuel sloshes around in the tank. It's another source of potential problems.

[712edf]

Fuel gauge

I'm not sure what year vehicle your working on but the sending units in the 70s & 80s units work on the principle a varying GROUND resistance. When the tank is full the wire from the sending unit to the gauge is fully grounded. When the tank is empty the circuit appears to be open (not grounded). If you disconnect the wire from the sending unit to the gauge the gauge will read empty because it is ungrounded. If you take that same wire and ground it to the frame the gauge will read full if it is working properly.I believe Voltage is always present at the gauge when the key is on, but the sending unit provides the ground to complete the circuit. The sending unit itself must be grounded to the frame.

[712edf]

Also Dodge fuel guages work on a different resistance range than GM and possibly Ford.If your using an aftermarket/universal gauge it might not be compatable with your sending unit.Seems like Dodge sending units show about 7 Ohms resistance for a full tank and about 73 Ohms resistance when empty. Higher Ohms values means more resistance/less continuity, which in turn means less ground. Less ground shows up as lower fuel level on the gauge. Voltage remains constant.Its the presence/absence of a ground that moves the needle.At least this is correct for 70s & 80s trucks.

[712edf]

If grounding out the wire from the tank to the instrument panel doesn't give you a "Full" reading with the key on then your problem is in the gauge/instrument panel area, not the sending unit. If grounding out that same wire does give you a "Full" reading then the sending unit is faulty or not grounded itself.

[Surfnuke9]

I read the manual again and I feel dumb.

There is a constant voltage limiter built into the fuel guage that sends 5-7 volts to the temp sender. The manual says it will be seen as a cycling voltage if the limiter is working properly. My original guage did that so it is probably just fine.

What I have found is that there is no ground strap on the fuel line coming from the tank. Most likely my problem?

thanks for the replies.

[Surfnuke9]

original fuel guage re-installed, grounding strap added to the fuel line.

temp guage works again as it should.

Fuel sender removed and checked out. varies resistance just like it should.

My problem was a float which was full of fuel.

For the time being, I coated all the soldered joint with JB Weld after draining the float and re-installed.

Fuel guage works like a champ.

Thanks for the responses.

[Desoto61]

Glad you got it worked out. The float issue ran through my head but you talked about adding fuel and seeing a change so I wasn't sure.

If the JB weld doesn't hold up look around for the stuff they use today. It's a synthetic material so no chance of it leaking really.

[JimmieD]

The electrical charging system and gauge wiring are the only two weak points in early Chrysler products & Dodge trucks. There is a known problem with the Charging/Amp gauge circuit which can potentially cause an electrical fire or worse. It is strongly suggested to modify that circuit, eliminate Amp gauge and replace with Volts gauge.

In addition the voltage regulated gauges suffer from age defects, primarily in the mechanical 5VDC voltage regulator for gauges. There is a DIY conversion to a modern solid state voltage regulator for gauges and it's pretty simple to do.

It's quite lengthy to explain so I'll post links to several websites offering specific directions for a fix. I don't like to lead someone to another site but this stuff is complicated and can cause a lot of serious trouble if not attended to.

Richard Ehrenberg Alternator repair:
http://www.allpar.com/history/mopar/electrical.html

Richard Ehrenberg Gauge Voltage Regulator repair:
http://www.allpar.com/history/mopar/electrical2.html

Mad Electric Amp gauge elimination [also click 'Tech' for further diagnosis/repair]:
http://www.madelectrical.com/electri...p-gauges.shtml

Many Mad Electric pages have a flag at bottom to click for Page 2. Good solid info there. Check this link also for further info:
http://www.madelectrical.com/workshop.shtml

Mopar Muscle [tries to keep up with Richard Ehrenberg, good luck M/M....]:
http://www.moparmusclemagazine.com/t...air/index.html

Alternator upgrade parts:
http://www.alternatorparts.com/chrys...grade_kits.htm

Enjoy yourself....

[Surfnuke9]

Thanks JimmieD.

I think it was one of your old posts that made me go to the allpar site where I did read the voltage limiter conversion procedure.

Good stuff you posted and all now saved in my favorites for printing later.

[JimmieD]

Hah, thanks for saving it, I wish everybody would! I have the Radio Shack part numbers here for voltage regulator upgrade:

Catalogue number 276-1770a, #7805 +5VDC 1 Amp Electronic Voltage Regulator

Cat. 272-1014 axxial lead 22mfd electrolytic capacitor [35V maximum potential]

Cat 276-1363 Aluminum Heat Sink for TO-220 packages.

All of this stuff is made in the same places now so Radio Shack is as good as any overall, better than some...

[Surfnuke9]

Very nice. thanks for the part #s

[712edf]

I am bumping this thread..Sat Nov 8