Odd Fuel Gauge Issue

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1979bettywhite
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Odd Fuel Gauge Issue

Post by 1979bettywhite » Wed Jul 22, 2020 4:56 pm

So I keep having this odd occurrence with my fuel gauge. It will read fine for a while and read correctly. After about 20-30 minutes of driving, it will peg out past the full mark. Occasionally it will come back down, but fairly random. I have been through the wiring and that all looks fine. And of course like I said it will read fine for a while. I am wondering if it has something to do heat maybe? I am clueless at this point and I am tired of dropping the tank over and over (don't have access port in the rear for the sending unit etc.). Any thoughts?


letank
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Re: Odd Fuel Gauge Issue

Post by letank » Wed Jul 22, 2020 5:48 pm

when the resistance for the voltage regulator in the temp gauge heats up, it changes the reference value to the other gauges... I used to have the same with oil pressure... fine for hours, then drops and a few minutes later comes back up.... finally after changing the temperature gauge -because it has the CVR or constant voltage regulator-, it was fine for a few months then started to act up.... then I did the 7805 CVR as explained in the following post:

viewtopic.php?t=8334&start=20

Now the gas gauge is as correct as can be, the oil pressure is steady and the temp gauge is fairly precise...

My gas gauge was dropping after 50miles to 1/2 and to 1/4 after 100miles, ground is good, I cleaned up both extremities and I installed a new sender 4 or 5 years ago... 79 is plastic or metal gas tank?
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (227 Kmiles... the running test lab)
See Engine bits , Fuel and brake lines, Body work, Trany and TC


ferrum
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Re: Odd Fuel Gauge Issue

Post by ferrum » Wed Jul 22, 2020 5:52 pm

My fuel gauge did that also. It happened more in summer time. Since changing out voltage regulator, it hasn’t happened yet and gauge is much more stable reading.
'76 Cherokee Chief


Topic author
1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Wed Jul 22, 2020 6:27 pm

Thanks letank and ferrum for the input. letank - I am running the MTS poly tank in the Chief. Have checked the ohms at the sender before I buttoned everything back up last time. Was as correct as I could get it to show full and empty at correct levels and correct ohms.

Actually no gauges outside of the speedo and the fuel gauge are in use in my dash cluster. I am running the Sniper EFI system so my coolant and voltage are displayed there. I also have a mechanical oil pressure gauge as well as a voltmeter, and a back up temp gauge in a three cluster pod under the dash. All the OEM cluster gauges are unplugged and zip tied off in the engine bay.

Maybe not running the temp gauge in the cluster is causing an issue with the fuel gauge? I know on my CJ they are connected and the voltage is regulated down for them.

I will take a look at upgrading the voltage regulator. Seems some basic soldering skills are all that is needed to swap to a new one?

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Re: Odd Fuel Gauge Issue

Post by fulsizjeep » Thu Jul 23, 2020 5:53 am

We have a MTS tank in a 77 Waggy. I had to run a ground wire from the frame to the locking collar on the sender at the tank for it to work halfway reliable.
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Topic author
1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Thu Jul 23, 2020 6:13 am

How did you attach the ground wire to the locking collar? I despise removing the tank again, the sender is just not accessible with the tank in, but I guess I can do it, lol.


letank
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Re: Odd Fuel Gauge Issue

Post by letank » Thu Jul 23, 2020 9:13 am

1979bettywhite wrote:
Wed Jul 22, 2020 6:27 pm
Maybe not running the temp gauge in the cluster is causing an issue with the fuel gauge? I know on my CJ they are connected and the voltage is regulated down for them
you got it... no voltage regulator for your fuel gauge

If the gauge is working for a while, then you are getting the ground circuit closed.
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (227 Kmiles... the running test lab)
See Engine bits , Fuel and brake lines, Body work, Trany and TC


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1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Thu Jul 23, 2020 9:22 am

So are you saying I need to reconnect my stock temp gauge? Or provide regulated voltage some other way to the fuel
gauge?

fulsizjeep - I don't know why I asked about the lockring before, I guess my brain was not working last night. On the MTS tank there is of course a separate sending unit and pickup. I have it grounded with a wire running from one of the tabs to the frame. The MTS tank uses a series of metal tabs that keep the sending unit clamped to the tank squeezing the o-ring to seal it up.

So, yes, it does have a good ground currently. Don't know why my brain was not working last night, lol.


letank
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Re: Odd Fuel Gauge Issue

Post by letank » Fri Jul 24, 2020 12:20 pm

1979bettywhite wrote:
Thu Jul 23, 2020 9:22 am
So are you saying I need to reconnect my stock temp gauge? Or provide regulated voltage some other way to the fuel
gauge?
yes , our gauges work on 5 volt not 12, and this is what the temperature gauge does, regulate the voltage mechanically
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (227 Kmiles... the running test lab)
See Engine bits , Fuel and brake lines, Body work, Trany and TC


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1979bettywhite
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Odd Fuel Gauge Issue

Post by 1979bettywhite » Sun Jul 26, 2020 6:12 am

I just thought about this, but would the sending unit for the temp gauge being unplugged effect the voltage that is going to the fuel gauge? I would think that the temp gauge it getting a 12v source, just not a sending unit input, therefore not effecting the voltage it passes through to the fuel gauge?


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tgreese
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Re: Odd Fuel Gauge Issue

Post by tgreese » Sun Jul 26, 2020 8:49 am

No. No significant change.
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Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
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1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Mon Jul 27, 2020 7:49 am

Gotcha. So it sounds like it might be something in regards to my CVR. Since I am not using any of the gauges in the cluster any longer (other than speedo), I may just wire up an aftermarket 12v gauge and see how that goes. I monitor temp and voltage through the Sniper handheld, and have a mechanical oil pressure gauge. Could just throw the fuel gauge in the cluster with that one and see how that does. I guess my other option is to pull the cluster and install a new voltage regulator?

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tgreese
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Re: Odd Fuel Gauge Issue

Post by tgreese » Mon Jul 27, 2020 10:44 am

A popular mod is to abandon the constant voltage regulator (CVR) in the temp gauge and substitute a 7805 voltage regulator IC. Cheap and effective. See Michel's reply (letank) above.

Abandon is exactly how it's done - the temp gauge has a 12V in and a 5V out. You remove the 12V connection and put 5V directly into the 5V out connection. This then powers all three gauges with 5V.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green


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1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Fri Jul 31, 2020 9:25 am

Ok, researching this a bit more. I checked the resistance with the key off and I get a reading of about 12, which is probably close to right as the tank is about full. When I power the key on, I get 0 resistance, hence pegging past full. I have checked my ground at the sender and that seems to be fine. I can get a resistance reading with the key off by using that grounding point. Any other thoughts on this?


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Re: Odd Fuel Gauge Issue

Post by sirrus » Fri Jul 31, 2020 10:14 am

You shouldn’t measure resistance of the circuit with voltage - that will end up in either random readings or dead multimeter. Your multimeter itself applies small current/voltage to circuit you’re testing and measures voltage drop or current to get resistance. Having external voltage/current applied to circuit will really upset the measurement :)


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1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Fri Jul 31, 2020 10:23 am

Got it. So it seems if I am getting the correct reading testing the sender with the key off, then what is showing on my gauge should be correct? I know my ground is good at the sender and the new gauge.


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Re: Odd Fuel Gauge Issue

Post by sirrus » Fri Jul 31, 2020 10:46 am

It should be provided your gauge is good. If you have some resistors or potentiometer around, you can test the gauge with known resistance and see what it’ll show


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1979bettywhite
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Re: Odd Fuel Gauge Issue

Post by 1979bettywhite » Fri Jul 31, 2020 10:48 am

Thanks. The gauge is brand new (I know that doesn't mean anything these days), so I assume it is reading correct. Sadly I don't have a potentiometer around.

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tgreese
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Re: Odd Fuel Gauge Issue

Post by tgreese » Fri Jul 31, 2020 11:29 am

The TSM tells you to disconnect the sender from the gauge, and test each separately. There is a complete procedure you can follow.

You test the sender with your multimeter. It's a variable resistor that changes value with the fuel level.

You test the meter with fixed resistances in place of the sender. Jeep gave their mechanics a special tool for this, likely at great cost to the dealerships. You will need to come up with some form of resistance to do the test yourself; either a potentiometer or fixed resistors. Radieaux Shacque would be a good place to buy these, if they were still around.

Clip in the equivalent resistor between the gauge terminal and ground. Get some clip leads and resistors - to do this test, you're going to have to figure out a way to buy or borrow these materials.

https://www.radioshack.com/products/min ... 0332010245

https://www.radioshack.com/products/500 ... 0332255301
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green


letank
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Re: Odd Fuel Gauge Issue

Post by letank » Sat Aug 01, 2020 1:32 pm

tester I made in 1988, when the oil pressure gauge acted up... still works, go for the 1/2 watt resistor

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Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (227 Kmiles... the running test lab)
See Engine bits , Fuel and brake lines, Body work, Trany and TC

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