P30 sending unit reading half but empty and gauge says FULL

circleD · April 29, 2014, 03:57 PM

I ohmed out my sending unit at the top of the gas tank and had 43.6 ohms which means 1/2 full or empty on the P30 with 70 gallon tank. The gauge on the dash reads past full and I have put 10 gallons of gas in the tank plus visually inspected it with my camera. When I put a ground to the sending unit the gauge doesn't move and NO I haven't went behind the gauge and checked it yet. When the camper was parked 3 years ago, the PO said it had 1/2 a tank. I need thoughts about what is happening here.
1. Bad sending unit
2. Bad gauge
3. All of it

Stripe · April 29, 2014, 04:12 PM

70? How did a 33 foot RV get stuck with a 70? Mine is 80....

I'd say clean the sending unit.. Inside and out..

circleD · April 29, 2014, 04:31 PM

IDK? That's what is on the label on the tank i?? . I'm trying to narrow it down due to time and money. If I need to replace the unit and get a new gauge then I would rather buy new ones when I drop the tank so I can get it fixed sooner.

legomybago · April 29, 2014, 04:45 PM

I would go play with the gauge at this point.... check resistance going into (if any) to the gauge...Sounds like the gauge is not getting any resistance if it's reading past full. If you have 43.6 coming out of the sender, than we know the problem is in between the sender and gauge wiring. Thats my guess i??

cncsparky · April 29, 2014, 06:34 PM

Reading past full indicates an open circuit through the sending unit wire. At the sending unit wire where it connects to the tank, short it to a good ground, against the chassis with the wire disconnected from the sending unit. If all is good, the gauge will read empty. If it does not, most likely a bad wire between the gauge and sending unit. Mouse chewed it, etc. Also, I have seen where PO's tried fixing with a butt splice that gets corroded and not making connection.

Also, not sure how our rv's are done, but have seen where the wire harness connector on the back of the gauge carrier can make a bad connection to the PC board on the back of the gauges. Would be another place to check.

circleD · April 29, 2014, 06:47 PM

So you're saying to unhook the wire that is on the threaded bolt in the middle of the sending unit and touch it to a ground? I've tried hooking a ground to the tab on the sending unit but still no luck.

DaveVA78Chieftain · April 29, 2014, 08:36 PM

What he is saying is check the gauge first AT the gauge without the sender connected to it. If that checks out, you can then work towards the sending unit.

Dave

cncsparky · April 29, 2014, 10:17 PM

Quote from: circleD on April 29, 2014, 06:47 PM
So you're saying to unhook the wire that is on the threaded bolt in the middle of the sending unit and touch it to a ground? I've tried hooking a ground to the tab on the sending unit but still no luck.

Yes, disconnect the wire from the sending unit and touch it to a ground.

TripleJ · April 29, 2014, 10:43 PM

Quote from: circleD on April 29, 2014, 06:47 PM
So you're saying to unhook the wire that is on the threaded bolt in the middle of the sending unit and touch it to a ground? I've tried hooking a ground to the tab on the sending unit but still no luck.

Yes, since you already have access to the sending unit as-is, this is an easy place to start. As said, when you touch the wire to GOOD ground, the gauge should go to Empty.

Keep in mind these setups are very sensitive to bad/corroded connections. Contact points need to be very good and clean

circleD · May 08, 2014, 02:50 PM

I finally got to messing with this thing and hooked the ground to the sensor wire going to the gauge. It went to Empty so that's good to know I guess. So now I get to drop the tank and do some OVERHAULIN while I'm under there. It'll be a few weeks before I do it but I'll keep you posted and let you know if the float is stuck in the middle or loose connection. Now to find out what kind of sending unit D:oH!