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74 SB Fuel gauge issues HELP! please
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elkgrovejohn
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PostPosted: Mon May 06, 2024 4:15 pm    Post subject: 74 SB Fuel gauge issues HELP! please Reply with quote

Hi all,

First post here. have a 74 VW SB convertible. working on fuel gauge. getting erratic readings, gauge drifts around from 1/4 to 3/4 of tank no matter what is in gas tank. replaced sending unit, old one had dead spots. bench checked new sending unit 10 to 85 ohms, installed, filled tank, sending unit reads 11 ohms with gauge wires removed. sounds good so far... unfortunately, upon connecting gauge, it continues its drifting around behavior. remove sending unit wire (yellow), gauge reads empty....thats good, ground sending unit wire (yellow) gauge reads full.....so seems like gauge is good. Any thoughts on why my gauge is behaving in this manner (drifting)?
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PostPosted: Mon May 06, 2024 4:45 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

If the sending unit is not grounded well, the gauge can be jumpy. Some add a separate ground wire, if one isn't already there.
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OTTO 1303
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PostPosted: Mon May 06, 2024 7:19 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Check ALL connections between gauge and sending unit, including ground.
Even the slightest increase in resistance at connections will cause gauge to read incorrectly.

The only resistance that gauge should see is the sending unit

Good Luck
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BenJAMin
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PostPosted: Mon May 06, 2024 7:42 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Did you replace the vibrator?
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elkgrovejohn
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PostPosted: Tue May 07, 2024 8:29 am    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

I have not replaced the vibrator.....given the gauge behavior when disconnecting the sending unit.....gauge shows empty....when grounding the sending unit gauge shows full tank....

am i correct in understanding that this means the vibrator/gauge is working?
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elkgrovejohn
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PostPosted: Tue May 07, 2024 8:32 am    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

I measured resistance from the sending unit (ground terminal) to chassis and found 0 ohms. as expected.

thx
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BenJAMin
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PostPosted: Tue May 07, 2024 9:14 am    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

On my car that fixed it. i can't pretend I did any tests but that was what fixedit.
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elkgrovejohn
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PostPosted: Tue May 07, 2024 2:44 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

BenJAMin wrote:
On my car that fixed it. i can't pretend I did any tests but that was what fixedit.


Was your fuel gauge exhibiting the same behavior?
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BenJAMin
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PostPosted: Tue May 07, 2024 7:10 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

yes, for what it costs I'd replace it just to try to eliminate it.
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Blue69Baja
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PostPosted: Wed May 08, 2024 10:07 am    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

The little screw that attaches the OEM vibrator or the aftermarket regulator to the speedo has to be in place and tight! That is the ground that makes the regulation circuit work.

If you ground the sender wire do it only long enough to see the gauge go to full. You need someone to watch the gauge! If you ground it for more that about 20 seconds it will damage the fine wires inside the OEM vibrator and it will be toast.

Erratic operation, like what has been pointed out, will cause erratic needle action.

Here is a link to my channel. Information there might be of some help.


https://www.youtube.com/channel/UC_2W_OgL6dEgsA5VVBrKObA

Jim
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elkgrovejohn
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PostPosted: Wed May 08, 2024 4:15 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Blue69Baja wrote:
The little screw that attaches the OEM vibrator or the aftermarket regulator to the speedo has to be in place and tight! That is the ground that makes the regulation circuit work.

If you ground the sender wire do it only long enough to see the gauge go to full. You need someone to watch the gauge! If you ground it for more that about 20 seconds it will damage the fine wires inside the OEM vibrator and it will be toast.

Erratic operation, like what has been pointed out, will cause erratic needle action.

Here is a link to my channel. Information there might be of some help.

Thanks Jim,

Got the regulator out, screw was loose. terminals on the regulator rusty, time for a new one....stay tuned.





https://www.youtube.com/channel/UC_2W_OgL6dEgsA5VVBrKObA

Jim
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Blue69Baja
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PostPosted: Wed May 08, 2024 4:56 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

If it is the metal type it is a vibrator I would clean up the terminals and see if it works.
The aftermarket units will work but the needle will be sluggish upon turning on the ignition switch but should work ok with the OEM gas gauge.
Good luck.

Jim.
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Even the spare is original..
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elkgrovejohn
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PostPosted: Fri May 10, 2024 5:48 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Blue69Baja wrote:
If it is the metal type it is a vibrator I would clean up the terminals and see if it works.
The aftermarket units will work but the needle will be sluggish upon turning on the ignition switch but should work ok with the OEM gas gauge.
Good luck.

Jim.



Here is a pic of the vibrator, looks a little crusty, especially around the rivets.


Image may have been reduced in size. Click image to view fullscreen.
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Blue69Baja
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PostPosted: Fri May 10, 2024 7:25 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

elkgrovejohn wrote:
Blue69Baja wrote:
If it is the metal type it is a vibrator I would clean up the terminals and see if it works.
The aftermarket units will work but the needle will be sluggish upon turning on the ignition switch but should work ok with the OEM gas gauge.
Good luck.

Jim.



Here is a pic of the vibrator, looks a little crusty, especially around the rivets.


Image may have been reduced in size. Click image to view fullscreen.


Haw, that is a bit corrosive.
If you have a dvm measure the resistance from tab to tab...
If you read <>18 ohms then I would use a wire wheel on a moto tool or fine sand paper to clean it up. Then I would solder it if the rivet will take solder.
Moisture in the old vw's was not good.
I did a rewire of my Baja years ago. The old wires were oxidized from the terminal lug, up the wire for an inch or two.

Here is a link to my rewire of the Baja.

https://www.thesamba.com/vw/forum/viewtopic.php?t=373119&highlight=rewire
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Even the spare is original..
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elkgrovejohn
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PostPosted: Fri May 17, 2024 3:30 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Image may have been reduced in size. Click image to view fullscreen.


oh no.....got the new regulator installed. Now the fuel gauge reads 1/4 tank steady.

recap

New sending unit, bench test showed 15-80 ohms.
Tank is 7/8 full, sending unit reading 25 ohms....seems right.
ground on tank./sending unit is good, added additional ground as well.
When I removed the active lead from sending unit, gauge reads empty, as expected.
When I ground the active lead on sending unit, gauge reads full, as expected.

Any thoughts on what I can check....try next?

much appreaciated.
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Blue69Baja
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PostPosted: Fri May 17, 2024 5:12 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Measure the voltage of the black wire attaching to the regulator. Looking for above 10vdc..

Measure voltage at the regulator input to the gauge. It should be 5.x volts dc.

Let. If the voltage into the gauge is low then there is something going on with the regulator.
Lets start with this.

Where to read the 5.x volts. Red line.


Image may have been reduced in size. Click image to view fullscreen.


Jim.
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Even the spare is original..
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Blue69Baja
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PostPosted: Sat May 18, 2024 3:15 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Not sure if you did the previous tests/measurements but I will add what your next step should be.

Remove the sender wire from the gauge tab.
Power off. Ignition switch to off.

We want to measure the resistance of your gauge.

Ohms scale to the lowest reading on your DVM. Usually 200 ohms.
Touch the leads together and check that the DVM reads close to 0 ohms.
If it read 3 or so... we can subtract that reading from the resistance of the gauge.

You don't have to remove the Regulator to measure the gauge resistance if you can reach the gauge tab where the regulator attaches.
Put the DVM probes on both gauge tabs and see what the resistance is.

We want to see above 16, preferably 18... Out of about 30 OEM gauges I have worked with they all read above 16 if they are good.
Aftermarket gauges will usually read around 30+ ohms.

If you read around 14 then your gauge has been fried by the testing to see if the gauge can go to full by grounding the gauge to sender lead to test! You can not leave that ground on the gauge for more than about 5 seconds.. If you do that any longer the fine nichrome wire that is wound around the bimetal bar in the gauge to start shorting.

Jim.

If it reads 14 then it will be a problem,

Jim
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elkgrovejohn
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PostPosted: Sat May 18, 2024 6:02 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Blue69Baja wrote:
Measure the voltage of the black wire attaching to the regulator. Looking for above 10vdc..

Measure voltage at the regulator input to the gauge. It should be 5.x volts dc.

Let. If the voltage into the gauge is low then there is something going on with the regulator.
Lets start with this.

Where to read the 5.x volts. Red line.


Image may have been reduced in size. Click image to view fullscreen.


Jim.




Black wire 10.8vdc
Voltage to gauge 5.3v
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elkgrovejohn
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PostPosted: Sat May 18, 2024 6:04 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Gauge resistance 12.3 ohm ....uh oh....now what?

Last edited by elkgrovejohn on Sat May 18, 2024 6:08 pm; edited 1 time in total
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elkgrovejohn
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PostPosted: Sat May 18, 2024 6:07 pm    Post subject: Re: 74 SB Fuel gauge issues HELP! please Reply with quote

Blue69Baja wrote:
Not sure if you did the previous tests/measurements but I will add what your next step should be.

Remove the sender wire from the gauge tab.
Power off. Ignition switch to off.

We want to measure the resistance of your gauge.

Ohms scale to the lowest reading on your DVM. Usually 200 ohms.
Touch the leads together and check that the DVM reads close to 0 ohms.
If it read 3 or so... we can subtract that reading from the resistance of the gauge.

You don't have to remove the Regulator to measure the gauge resistance if you can reach the gauge tab where the regulator attaches.
Put the DVM probes on both gauge tabs and see what the resistance is.

We want to see above 16, preferably 18... Out of about 30 OEM gauges I have worked with they all read above 16 if they are good.
Aftermarket gauges will usually read around 30+ ohms.

If you read around 14 then your gauge has been fried by the testing to see if the gauge can go to full by grounding the gauge to sender lead to test! You can not leave that ground on the gauge for more than about 5 seconds.. If you do that any longer the fine nichrome wire that is wound around the bimetal bar in the gauge to start shorting.

Jim.

If it reads 14 then it will be a problem,

Jim






Gauge resistance 12.3 ohms.....uh oh....what now?
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