74 SB Fuel gauge issues HELP! please

[elkgrovejohn]

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)?

[KTPhil]

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.
_________________
Current Fleet:
- '71 Fastback
- '69 Westfalia
Retired:
- '67 Beetle
- '65 Beetle (x2)
- '65 Bus
- '71 Squareback

[OTTO 1303]

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
_________________
74 Super Sun Bug - OTTO
74 Super - DANZIG
74 Super - HERZOG
16 R-Line

[BenJAMin]

Did you replace the vibrator?

[elkgrovejohn]

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?

[elkgrovejohn]

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

thx

[BenJAMin]

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

[elkgrovejohn]

[BenJAMin]

yes, for what it costs I'd replace it just to try to eliminate it.

[Blue69Baja]

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
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original..

[elkgrovejohn]

[Blue69Baja]

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.
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original..

[elkgrovejohn]

[Blue69Baja]

[elkgrovejohn]

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.

[Blue69Baja]

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.

Jim.
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original..

[Blue69Baja]

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
_________________
69 Baja with a 1914...
72 Super 100% orig...W/ factory air... 56,000mi
Even the spare is original..

[elkgrovejohn]

[elkgrovejohn]

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

[elkgrovejohn]