Strange Fuel Sending Unit Behavior - BJ70 (1 Viewer)

[dcruiser666]

Since I've owned my BJ70 I have driven without a working fuel sender. Always used the trip meter and filled up between 800-900km. I finally decided to replace it last week and it worked fine with correct readings since the install. Today was the first time I completely filled the tank and the gauge no longer shows a reading. The behavior is the same as it was with the bad sender - it rises during the pre-glow stage and then drops to nothing once the engine starts, no movement with power on. Curious if anyone has experienced this before? Seems like filling the tank may have shorted out the sender somehow. A bad ground doesn't seem likely to me since it was working fine before filling the tank.

 

[RhinoRig]

Probably a bad ground. That’s how the current flows and if the gauge sees no volts the needle doesn’t move. I’ve chased this in 2 80s and replacing the sending unit never helped. It’s in the wiring.

 

[dcruiser666]

I see, could I just splice in a new ground from the sending unit ground wire? I haven't looked at this wiring diagram, but seems like that could save me some hassle.

 

[CharlesHS]

Locate the harness connector between the sending unit and the vehicle harness, separate the connector and measure resistance (ohms) thru the sending unit and whether the ohms change depending upon the level of the fuel in the tank. Then check to see if there is voltage present to Pin 1 in the vehicle harness. As you stated.... there may be a short in the wiring or a bad ground as others have posted.
The last thing I would do would be to replace the sending unit prior to doing some voltage/resistance measurements.
I had a similar problem with my 1993 PZJ75 where the fuel gauge would barely move off of 2/3.

 

[dcruiser666]

Okay, I’ll do the tests. The reason I replaced the sending unit first is because the po told me he tested it and determined it was the culprit. I have no particular reason to trust/distrust this person but it did work when I swapped it. On your 75, when you say the gauge barely moved off 2/3, does that mean the gauge was stuck in that position or it didn’t move if you put more than that in the tank?

 

[CharlesHS]

When I added 30L (approx 8 gal) of diesel fuel to the tank I would expect the needle to move more than 1/8” past 2/3. I measured the resistance of terminal 3 to ground and terminal 1 to 3. The added fuel should change the resistance between 1 and 3.
Measured the resistance for a new sending unit at: Full (22 ohms), 1/2 Full (approx 55 ohms) and Empty (108 ohms).