1966 chrysler fuel gauge testing (NOT sending unit)

darth_linux

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I'm starting to run out of ideas for bench testing my fuel gauge, and am just about ready to give up and put it all back together again and live without a gas gauge.

Here's what I've done so far:
* tested original voltage limiter - 12v goes in, fluctuating voltage ~5v comes out
* tested new "original style" limiter (Standard VRC601) - 12v goes in, fluctuating voltage ~5v comes out
* plugged either limiter into it's socket on fuel gauge circuit board, applied 12v to the input tang that the condenser plugs into (per FSM) - gauge needle does not move, fluctuating voltage ~5v at pin 3 (dk. blue) over at the plugin near the ammeter (voltage outbound to sending unit)
* plugged either limiter into it's socket on fuel gauge circuit board, applied 12v to pin 2 (black) near ammeter (incoming 12v) - gauge needle does not move, fluctuating voltage ~5v at pin 3 (dk. blue) near the ammeter (voltage outbound to sending unit)

So, the voltage limiter functions, the circuits between the inbound 12v and the outbound 5v work (I had to resolder pin 3 as it was loose). But, I'm getting no needle movement at the gauge, probably because there is no sending unit to provide resistance. I was thinking that a full 5v to the gauge with no resistance would make it read "full" but maybe I have it backwards. Does a full 5v make it read "empty?"

I think the only thing left for me to do is momentarily ground out pin 3 (dk. blue) while 12v is applied to the voltage limiter and see if that gets the needle to bounce. If it doesn't, I can confirm the gauge itself is bad, correct?


pin 3 resoldered
E8E7BCAC-B409-480C-AFEF-C1E8A960EF15.jpeg


FSM showing to connect 12v where the condenser plugs in
215AC591-14FF-461E-9610-CA0CDEB4CFA5.jpeg
 
UPDATE:

I went ahead and shorted pin 3 to ground while 12v was applied to the voltage limiter and I was able to get the needle to move to nearly FULL. It goes in intervals of about 2 seconds with this Standard VRC601 limiter so it took about 10 seconds to get the needle that far. I didn't want to risk frying something so I stopped after that. So, I know now my gauge works, my limiter works, and all that's left is to investigate the sending unit, which appeared to be newer.

I wasn't getting a signal to the sending unit when I had the car on a lift 2 weeks ago, and I now believe that the loose/broken pin 3 terminal was the problem.

I'm going to put that cluster back in and find out what happens! All of my LED bulbs lit up correctly and I'm lubing the speedo cable while I have it apart, so I'm crossing my fingers.
 
If you ground the sender wire at the tank and the needle goes to full you know that the gauge, regulator and wiring are all working.
 
If you ground the sender wire at the tank and the needle goes to full you know that the gauge, regulator and wiring are all working.
thank you, that’s essentially what I did. The gauge cluster is pulled out and on the bench right now, so I did basically the same thing.
 
So.....You verified your gauge works. Good. Now it is between that blue wire and your gas tank. You can verify the blue wire with an ohm meter. Should be zero or close too it when you check it end to end. Then with that ohm meter check between ground and the pin on the send unit. It should be somewhere between 10 ohms and 75 ohms depending how full your tank is and if the sender works or not. How about the tank itself? Good ground?
 
So.....You verified your gauge works. Good. Now it is between that blue wire and your gas tank. You can verify the blue wire with an ohm meter. Should be zero or close too it when you check it end to end. Then with that ohm meter check between ground and the pin on the send unit. It should be somewhere between 10 ohms and 75 ohms depending how full your tank is and if the sender works or not. How about the tank itself? Good ground?
I’m gonna reinstall the cluster today and turn everything on and see what the gauge reads. The sending unit has a good clean ground strap. I will check the resistance at the sending unit later today or tomorrow.
 
If you check the wire going to the sending unit under the dash to a good body ground, you can measure the resistance in that whole circuit. If it is in the 10 to 75 ohm range, all is good. If it is out of range, more checking is needed. This could save the effort of checking at the tank end. FWIW Lindsay
 
Once you get the cluster reinstalled and hooked up, you can turn the key to the "on" position and use a test light to the connector that hooks to the tank sending unit. Do this with the connector unhooked from the tank unit. You should get a flashing test light if everything is working as it should. A lot of gas tanks on older cars have a poor ground due to rust and corrosion. Rig a temp ground to the tank with some alligator clips. Hook up the connector to the tank once power to the lead is verified. If you do not get a gauge response with fuel in the tank, the tank unit is probably bad. A lot of the repop units from China have bad floats that leak and settle to the bottom of the tank.

Dave
 
60's Ford gauges operate exactly like 60's Mopar gauges. And even better, they use the same resistance values to indicate empty/mid/full. So...

To test them, you can use one of these GAUGE TESTER, FORD 1955-1986, 73-10 OHM FUEL GAUGE - #992-3FA - National Parts Depot
I use a Resistor Decade Box that I bought off eBay a few years ago. Same basic thing....

And here's how to use it. This channel has some really good stuff that applies to all cars, not just Fords... and the more I see, the more "crossover" between Mopars and Fords there is that we don't think about. Don't pay any attention to the first minute or so of clowning around.


My decade box.
8NytTLs.jpg
 
FINAL UPDATE:

Cluster reinstalled, fuel gauge moves but shows nearly empty for ~15 gallons in the tank. Sending Unit looked "newer" but it's obviously failed in some way.

Sounds like a good project for May, I'm pretty much done wrenching on the SS Anderson for April. I just want to drive and enjoy the car!
 
Once you get the cluster reinstalled and hooked up, you can turn the key to the "on" position and use a test light to the connector that hooks to the tank sending unit. Do this with the connector unhooked from the tank unit. You should get a flashing test light if everything is working as it should. A lot of gas tanks on older cars have a poor ground due to rust and corrosion. Rig a temp ground to the tank with some alligator clips. Hook up the connector to the tank once power to the lead is verified. If you do not get a gauge response with fuel in the tank, the tank unit is probably bad. A lot of the repop units from China have bad floats that leak and settle to the bottom of the tank.

Dave
I think this is the issue I have. The tank is 3/4 full, but I think the float is sitting near the bottom . . .
 
You could have a couple of different issues with your sending unit.
1) float is taking on fuel and sinking instead of floating
2) the part of it that creates the resistence could be bad

My reproduction sending unit would not go to full. I gentle bent the arm downward to allow the float to rise higher in the tank. Now my gauge goes to slightly above fuel and down to just about empty.

I still keep an eye on the trip meter. Way to heavy of a car to try pushing to the gas station.
 
You could have a couple of different issues with your sending unit.
1) float is taking on fuel and sinking instead of floating
2) the part of it that creates the resistence could be bad

My reproduction sending unit would not go to full. I gentle bent the arm downward to allow the float to rise higher in the tank. Now my gauge goes to slightly above fuel and down to just about empty.

I still keep an eye on the trip meter. Way to heavy of a car to try pushing to the gas station.
Thank you. I'm pulling the sending unit in 2 weeks (have to burn off some gas first) and I'll put an ohm meter on it and inspect the float too. Currently my trip meter and odometer only work very occasionally. Only when they are feeling happy. I just lubed the speedo cable real good, so I'm hoping with a few more miles put on the car, the odometer will wake up from it's slumber and start working again.

At the moment, I don't know how much gas I have nor how far I've gone unless I keep a little log book.
 
Thank you. I'm pulling the sending unit in 2 weeks (have to burn off some gas first) and I'll put an ohm meter on it and inspect the float too. Currently my trip meter and odometer only work very occasionally. Only when they are feeling happy. I just lubed the speedo cable real good, so I'm hoping with a few more miles put on the car, the odometer will wake up from it's slumber and start working again.

At the moment, I don't know how much gas I have nor how far I've gone unless I keep a little log book.

Don't hope too much. My odo is crapping out right now. Glad you reminded me of this issue. I'm gonna score a new one. Will see if any are left.
 
I finally gave up chasing a perfect sending unit. My gauges were on spec but my original sending unit wiper was worn out. The replacement sending unit would not go above 3/4 and I didn’t trust empty. So @Big_John s suggestion I got a meterMatch and calibrated my gauge and sending unit together. Now it is very accurate.
Fuel Sender calibration using Meter Match
 
Repo sending units read 3/4 when full, frustrating but it what it is unless you can find an original one. They are a Ford scale, how they are so cheap. Most times the float on the sending unit isn't floating anymore.
 
Repo sending units read 3/4 when full, frustrating but it what it is unless you can find an original one. They are a Ford scale, how they are so cheap. Most times the float on the sending unit isn't floating anymore.
Thanks. I’m pretty sure my float is sunk to the bottom because the gauge reads really empty when the tank is full, and as the tank gets more and more empty, the gauge shows more and more full. I think the weight of the gasoline on the float is pushing it down to the bottom and then, as I use the gasoline and there’s less and less weight on the float, the float rises a little bit. Only thing I can think of. Time will tell when I take the float out.
 
Just dropping this here for posterity.

1965 Chrysler 300 4dr hardtop 383 Auto

My fuel gauge suddenly is on the fritz this spring. Has worked as it should for the lifetime of the car.

Occasionally now when I start the car (or turn the key into accessory) the fuel gauge does not move... or moves after some time (*several minutes) of the car being started, but very slowly. Sometimes it works fine... sometimes nothing.

I ran a ground wire from the battery to the sending unit praying that was the issue. Turned the key on, no movement in the gauge. Damn. Not the sending unit.

Am fearing it is the Voltage Limiter/Regulator... that tiny little bastard tucked into an impossible to reach back corner of the gauge cluster. I hear it is a bi-metal gauge that when 12v hits it, it slowly "heats" up and then touches its other side to complete a resistor circuit... making the voltage to the gauges 6v. Why the hell didn't they just put 12v gauges into these cars... who knows. So when it fails it can stay open, sending a constant 12v. Bad news bears.

Think this is what has happened as I was driving the other day and Fuel gauge was not working and then ALTERNATOR gauge pegged hard to the top... and let out a big puff of white smoke... and now it dead in the water. Literally tinted the actual gauge glass with smoke from the inside too. The Fuel gauge also stopped working. Lame.

So the whole cluster will have to come out. Terrifying. Think I will just live with it for now. Nothing like having NO fuel reading.

**UPDATE
Oddly, I went for a drive last night and the Fuel gauge came on after driving for a while, so it is not totally dead. Just working when it wants. Fingers crossed it doesn't go up in smoke too.


s-l1600-1 copy.jpg
 
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My issue has been solved mostly. The float has been changed out and the gauge now works. It doesn't read accurately except when you first start the car. When you start it up, the gauge goes to the correct reading, and then slowly falls to about half of the correct value.
 
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