Thread: fuel sending unit MELTED

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Howdy

I put in a new sending unit in my 73 charger, and it didn't work..bummer.

Finally got back to it, and I see that it has a dead spot. I tear it apart and find the plastic lining behind the wound resistor wire was melted.

Looking a bit deeper, I find the tank has good ground, and the guage is sourcing 12V. ( 11.4 )

I look a bit deeper, and find the voltage reducer is present on the guage, but obviously it isn't reducing. The Haynes manual shows it to be a simple resistor.

Is there more to the reducer than a resistor? I thought it was unusual for a resistor to fault as zero ohms, don't they usually fail open? Could I have gotten the wrong sending unit?

I disconnected the temp and oil pressure sensor until I get this sorted out. (and the fuel guage ! )

I'm a bit surprised no one chimed in on this.

Ok, I'll just sludge through on my own.

I have ordered a solid state voltage reducer off eBay, it's supposed to be universal. Also have a new sending unit coming.

Best part of this is, I can replace the reducer without pulling the gauges, and the sending unit without pulling the tank.

I can't say that I've ever seen one do this.. But I think it was said earlier, the gauges run around a 6 volts range. And I see you found a voltage reducer so once in place you should be good to go.

There's a dash gauge voltage regulator required for MoPar's. Take a look at Rock Auto Parts - they're about 35-38 bucks for the good Standard branded item. If they fail, you will have gauge and sender problems - or at least on a Ford it will.

RockAuto Parts Catalog

"STANDARD MOTOR PRODUCTS Part # VRC601 More Information About this Part {Click Info Link for Alternate/OEM Part Numbers} 12 v to 6 v Voltage Reducer for the Instrument Cluster Dimming; 12 Volt; Contents: Regulator
w/o Rally Package"

I had this typed up and my ISP evidently crashed and I never got back to it

Ok, an update for ya:
I pulled the old voltage reducer and cracked it open. As I mentioned earlier, the schematic shows it as a resistor, but would more accurately depict it as a variable resistor, at least that is how I think it is supposed to function. It consists of what I'm guessing to be thermally reactive metal arm with a contact on one end, and a thin coil of presumably resistive wire around the arm. Definitely reminds me of an old external voltage regulator.

I installed the electronic voltage reducer, and discovered the hard way that It's not reverse bias protected, oops! Easy to make a mistake installing something when you can't see what you're doing.

I ordered another, I'll see if I can pay more attention this time.

Don't you hate it when you let the smoke out? 8-)

Update

Swapped in the new electronic gizmo. No function of any gauges.

I concentrated on the fuel guage.. confirmed 5 V to guage. Confirmed 5 V to sending unit. Confirmed ground to tank. Tried grounding the guage output, got some needle movement. The needle was off the indications, to the left of E Nd when I grounded the output, it went almost to the E, then returned to the left after I un- grounded it.

On top of that, the first gizmo which I thought I had fried, puts out 5V on the test bench.

I can only suppose that I'll have to order a stock type reducer..

Another look at the stock reducer, and it appears like it would act as a circuit breaker, and reduce voltage as a resistive function.

This sounds like the polarity is reversed coming off your electronic gizmo.

The reason,

“ Tried grounding the guage output, got some needle movement. The needle was off the indications, to the left of E "
If wired correctly a ground there would produce a full reading, you do have the sender disconnected?

If not, grounding with the sender in play the extra load killed the voltage causing what you saw.
To verify turn off the power does the needle go to the same position?

Just fyi;
" unusual for a resistor to fault as zero ohms, don'tthey usually fail open?"
When a resistor reads 0 ohms it is shorted and could be the reason the sending unit went south. To get an accurate reading on a resister you need to lift one side and then take the reading. If not the complete circuit is in play and the reading will be incorrect.

Got a question the original reducer when you power it up what are your readings 12 or a reduced amount? Not being an expert on ferds or mopower I do not know what the working value should be. Please tell me what year and car I will see if I can locate that information.

My point is as long as that gizmo is reducing the 12 v it is working. Most cases it is either going to fail open and therefore 0v or short and passes 12v.
So here is what I am reading; now no gauges are working, could be that 5v is too low.

Problem started with
1 the gauges were working, but the fuel sending unit melted
2 found a resister on the back of fuel gauge
This tells me that there are two circuits in play. The primary circuit reduces the voltage to the whole instrument panel. The fuel gauge has its own working voltage cause of that resistor.

If I were doing this I would first verify whether or not the original reducer is working. That one that supplies the voltage to the complete instrument panel, get that fixed they were working.
Then I'd turn my attention to the fuel gauge and go from there, the sending unit is nothing more than a variable resistor as you indicated.

Good stuff.

First. Since I am getting the 5 v to the guage, the polarity is good.

It does seem as if it needs higher voltage: the needle moved in the correct direction when the guage output was shorted to ground, it just couldn't make full deflection to reach F.

I didn't mention this, but in troubleshooting the melted sending unit, I got full deflection to F by shorting the sending unit, with the original reducer in place. This has me thinking the melted sending unit was not high enough power rated.

I think the sending unit goes to zero ohms. Maybe I'll pull it and check. Anyway, zero ohms is zero ohms, having the sending unit in parallel wouldn't cause higher amp draw.

I played with the old reducer. The little wire which is wrapped around the contact arm is in bad shape, and was burned in half where it grounded to case. I unwrapped a loop, and re- grounded it to case, then put power to it, with no load. Made me wish I had an analog meter to watch it. Anyway, looked like the contact was cycling open and shut, I was getting a cycling voltage from 1-3 v.

The Haynes diagram shows the reducer powers all the gauges, I'll take another look at that too.

Also it's a 73 charger, and I think it has the rally gauges.

Looks like rock auto doesn't have the rally guage voltage reducer.

BUT, it looks as if I can swap the case from the old one and make it work.

I did some reading around and it looks like 5v is correct, could not find any schematics for the dash. But I did stumble on a couple of guys hammering on, I think the same problem you are having. Could be enough info there to help you along the way.

Trying to repair the sending unit windings will be a pain, you'll be better of just grabbing another unit. The senders are standard for instance Chevy is 0 - 90 ohm , looks like Dodge is going to be 240 empty ohm, 33 full ohm; this was for a 79, lower link.


Have a look;
Gas gauge or voltage limiter


Fuel sending unit :
Dodge D100 Parts > 1979 Dodge D100 AUTOMETER FUEL LEVEL SENDING UNIT240 empty ohm, 33 full ohm; Includes installation hardware; This fuel level sending unit fits the standard 5-bolt mounting flange; Adjustable for 5 in. to 24 in. deep tank; Does not

Good luck ask if I can help or be a sounding board, sounds like you have the skills for this.

Thanks for the thread. Biggest thing it revealed is that electrical gremlins can be a bear.

At this point I have a theory:

Original sending unit disintegrated. ( rust )

Voltage reducer failed, sending full voltage to guages.

Oil guage isn't connected, so it was unaffected. Temp guage works, but reads low. New tank sending unit melted from the 12 V.

Stock voltage reducer operates above 5 V, so the electronic gizmo I installed isn't powerful enough to operate the gauges.

I agree that's why I pasted the link, seems as thou Dodge electrics are a bit quirky. The fuel gauge is one of the most talked about from some of the searching I did.

Maybe take a ride back down to that junk yard you found and look around for some early 70 dodges. The fuel gauge and senders are more then likely electrically the same. If you find a sender and the ohm measurement is in range 240 empty ohm, 33 full ohm, move it to the sender you have. If the tank fitting is the same and the float arm to short/long a little cut and past will do it.

The old reducer

I ordered the non- rally reducer, and will swap the cases.

I'm concerned about power rating. As near as I can make from the Haynes diagram, the rally cluster adds an additional guage, but I suspect it's two additional gauges.

I'm considering leaving the temp and oil pressure guages disconnected.

Maybe I can come across a rally reducer in the future