1969 Rallye fuel gauge test

[MikeyM]

Hi all,

'69 Bee Rallye dash, fuel gauge isn't registering. Zero. With key on, I took a test light and went from the wire/plug that connects to the sending unit to ground and the light pulsed. I then measured the voltage between that same plug and ground and was getting between 2-7 / 2-8 fluctuating. I also connected the sending unit lead to a new sending unit then grounded the rim of the sending unit and was able to make my test light dim and brighten as I moved the float and the light still pulsed. So this tells me the voltage limiter built into the dash is working, yes? But is the gauge bad? I read a post about loosening and tightening the screws that hold in the fuel gauge for better connectivity and that cleared up one guys' issue. That's my next move, next chance I get. Any ideas?

Thanks,
Mike

 

[Sinitro]

EZ to test...
Ground the fuel sender wire lead back @ the tank..
And ignition ON, the fuel guage should read full..
If yes, then everything up front is fine problem only leaves
the sending unit/rheostadt.. When you ground the fuel sender wire lead with the ignition ON,
you are bypassing the tank's sending unit, sending a "full" signal
directly to the gauge. If the gauge responds by going to full, the gauge, voltage limiter
and the wiring from the fuel gauge to the sender are all good.

Just my $0.02...

 

[BLK 68 R/T]

EZ to test...
Ground the fuel sender wire lead back @ the tank..
And ignition ON, the fuel guage should read full..
If yes, then everything up front is fine problem only leaves
the sending unit/rheostadt.. When you ground the fuel sender wire lead with the ignition ON,
you are bypassing the tank's sending unit, sending a "full" signal
directly to the gauge. If the gauge responds by going to full, the gauge, voltage limiter
and the wiring from the fuel gauge to the sender are all good.

Just my $0.02...

Just don’t ground it for very long…

Only long enough for the gauge to read. It should move almost immediately when you ground it as stated above. If it doesn’t respond within just a few seconds it’s not going to.

 

[MikeyM]

Thanks for the replies. So if the gauge does NOT respond but I still get fluctuating voltage/light, like I did with the test light, that means the circuitry is good but the gauge is bad? Yesterday, key on, I put a test light between the sender plug and the tail light nut in the trunk and saw the light fluctuating. That serves the same purpose as grounding near the tank, correct? I pulled the sender wire back through the trunk to get to it easier.

Thanks,
Mikey

 

[BLK 68 R/T]

It could be a circuit board issue or a loose/broken pin on the circuit board. If all that checks out good it’s most likely the gauge itself.

 

[MikeyM]

Ok. Thanks much.

Mikey

 

[Sinitro]

Hmmm..
If U think the guage is bad that I doubt...
But it is EZ to test, take a AA battery and a couple of test leads, and connect it across the guage terminals.
The guage is a 5V DC so the needle should move up about 1/3 of the way indicating about 1.5V.
If the guage's needle doesn't move then yes the guage is bad.

Just my $0.02...

 

[MikeyM]

Hey.. good test, thanks. I'll give it a shot. I grounded the fuel sending unit with my test lght, key on, and although I got the pulsing light, no needle movement.

Well bummer - I did that test as you suggested and no needle movement. I pulled out the cluster a bit and tilted it downward then connected the test leads to each lug, making sure the leads weren't touching metal. Tried the battery both ways and nothing.

Just for S&G I tried it with a 9V batter, no love. All this was, of course, key off but the leads directly across the fuel gauge posts.

So... in summary... 1) key on, grounding sender wire at tank to tail light screw, I get a pulsing bright light but no needle movement. 2) with key off, I jumped each terminal of a 9V battery to each post of the fuel gauge - no needle movement.

Dead gauge?

Thanks,
Mikey.

 

[MikeyM]

Well, here’s an update… After reading about the blue wire near the driver, kick panel, I grounded my Test light from that terminal, with the harness still connected, to the dash. When I turn the key on, I get the flashing light and my fuel gauge goes up to 1/4 of a tank only. Also, I attached my bolt meter to the Test light shoved into the socket in the harness, which goes to the back of the car. It’s reading between five and an Also, I attached my volt meter to the Test light shoved into the socket in the harness, which goes to the back of the car. It’s reading between 5 1/2 or so volts and zero. Should that reading be between zero and 12? I know the limiter is supposed to put out 5 V to the dash.Any ideas? I wouldn’t think I need to disconnect the harness that goes to the back of the car. Could this be as simple as just replacing the old unit with a solid state voltage limiter which will supply a constant 5 V to the gauge cluster?

Thanks, Mikey

 

[dadsbee]

You ground the wire off of the sender.... not the sender terminal. Lose the test light..

 

[MikeyM]

Thanks DB. So do you mean just run a normal wire instead of the test light?

 

[dadsbee]

Yes, just use a jumper wire and hook the sender connections wire end to a good ground. Turn the key on, fuel gauge should head for full. Turn off key before it gets there..

 

[MikeyM]

DB, you’re a genius! Thank you so much. The gauge moved all the way to full and I pulled the wire just before. Hopefully replacing my sending unit and cleaning off all the connection points along the path will bring this thing back to life.

Thanks again,
Mikey

 

[dadsbee]

So your gauge works!

Do you have an ohm meter? If so put the leads on the spot you just used as ground and the metal face of the sending unit to make sure that it's grounded. Should read zero.

Then put the lead on the sender stud. Pending how much gas is in tank it should read between 10 and 74 ohms. If not your sender rheostat is toast.

 

[MikeyM]

Gotcha. Thanks again. The sending unit currently in the tank is leaking so I have a new one ready to go it. Once it does, I will ground the sending unit to the fuel line via hose clamps and measure all that.

Mikey

 

[BLK 68 R/T]

Gotcha. Thanks again. The sending unit currently in the tank is leaking so I have a new one ready to go it. Once it does, I will ground the sending unit to the fuel line via hose clamps and measure all that.

Mikey

Just be prepared for the new sending unit to not read correctly on the gas gauge. It’s a known issue with all the sending units. Also, use your original lock ring if at all possible.

 

[MikeyM]

Thanks, I've read about that issue. Apparently, this sending unit is supposed to be accurate and I've bought things from Van's before. They say they've redesigned it so I guess we'll see: Fits 68 69 70 Charger Coronet GTX Roadrunner B-Body Fuel Tank Sending Unit 5/16 | eBay .

And if it doesn't read accurately, I've read on a couple forums that this dan/should correct it:
https://www.tanksinc.com/index.cfm/page/ptype=product/p roduct_id=748/category_id=-1/mode=prod/prd748.htm .

Mikey

 

[BLK 68 R/T]

Thanks, I've read about that issue. Apparently, this sending unit is supposed to be accurate and I've bought things from Van's before so I guess we'll see: Fits 68 69 70 Charger Coronet GTX Roadrunner B-Body Fuel Tank Sending Unit 5/16 | eBay .

And if it doesn't read accurately, I've read on a couple forums that this dan/should correct it:
https://www.tanksinc.com/index.cfm/page/ptype=product/p roduct_id=748/category_id=-1/mode=prod/prd748.htm .

Mikey

Hopefully it works out good for you.

 

[MikeyM]

Thanks man, me too. Maybe this might turn out to be the go-to sending unit. We'll see.

 

[MikeyM]

SUCCESS!! And a huge thank you to all of you. So I got the new sending unit installed with new rubber hose and properly grounded. Before I started, I jacked up the car by the diff then supported it with stands under the frame to let the rear axle drop. Turned out I had plenty of room. The old unit was a bit difficult to get out but the new one literally slid right in. And I did use the OEM lock ring (thanks Blk 68 RT). I siphoned out about 4ish gallons and I think I had a couple gallons or so left. Once done, I poured the 4ish gallons back in, connected the battery, key on and BOOM - 1/4 tank. That was about what I guessed was in there. So off to the gas station I went to fill up. It only took about 5.5 gallons until the auto-stop on the gas pump popped so I continued to fill it slowly while holding the little rubber seal on the nozzle back. I managed to get about another 1.5 gallons in there so probably about 12and when I turned the key on it read between 3/4 tank and full. I didn't want to go anymore and have gas blow out of the filler neck - this is only the 2nd time on a full tank of gas.

Is there a secret to filling the tank to full from the license plate filler neck? Does the pump typically shut off early?

Thanks again, everyone. Have a great weekend. Next - finish up the dash and interior light wiring.
Mikey