Vehicles

rv - Fixing Water Tank Gauge

My water tank was cracked around the sender. I replaced the tank and sender only to find that the water tank gauge would only move about 25% of the sweep. This was a bummer because that new sender was expensive. The waste tank gauge had the same problem, I simply didn't know it because I didn't think to conduct that test when I recently installed a new sender in the waste/black water tank.

I did some research and everyone agreed the GMC motorhome only ever used 0-90 ohm gauges and senders. My senders were producing 0 to 90 ohms. The gauge was therefore bad. Of course these gauges are no longer made.

I briefly considered just replacing the entire gauge cluser and senders for some modern unit. I decided not to do that though. The cost wouldn't have been too bad. What stopped me was the looks. I wanted to keep that original look with side lit analog gauges, even if it means using senders that are more likely to get contaminated and stuck than some modern external sensor.

My fix was to replace the guts of the gauge itself. I ordered a modern VDO 301-106 fuel gauge. Its rated for 0-90 ohms. I then cut it my old gauge, cut up my new gauge, reassembled everything, soldered some new leads, and viola, my gauge works again. Really the only thing I kept was the gauge face. Everything else is new gauge.

There are a couple hints that the gauges are not original. For one, the pointer is slightly different. A normal person would never notice. Second, there are two extra holes in the gauge face plate. I put some electrical tape over those holes from the back to block the transmission of light. This is more noticable than the pointer but still easy to miss.

I performed this replacement on BOTH the fresh water guage and the waste tank gauge. The voltage gauge still worked. I left the LP gauge alone. It does not work with a the new 0-90 ohm water tank sender. The readings are reversed and not full range. Regardless, the LP gauge seems to be working fine with whatever sender is currently on the propane tank. I decided to leave well enough alone until it stops working. I still wonder if in the final year of the GMC motorhome they switched to some other ohm range for these gauges.

Overall I am very happy with the results. $30 per gauge plus a couple of hours and its all working. I get to keep that vintage look and feel and also have accurage gauges again. Normally on my projects I would never put this much effort into keeping it original. I would just slap in some modern replacement. The GMC however has such a cool character that I had to maintain it, which I am happy to report, worked out for me.

Movies


1 - New fresh water sender, original gauge, not enough range.m4v
(19MB)


2 - Minimal variables, not enough range.m4v
(8.2MB)


3 - Original sender, intermittent but good range.m4v
(11.3MB)


4 - New fresh water sender attached to waste tank gauge.m4v
(11.4MB)


5 - New fresh water sender attached to LP tank gauge.m4v
(8.5MB)


6 - Fresh water tank gauge working after gauge guts replacement.m4v
(32.8MB)


Pictures

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testing the new sender before installing in the new water tank
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new sender does not cause the gauge to go full sweep
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close up of gauge, one line removed
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new sender tests out good with 0-90 ohms. Old sender was 150 to 390 ohms, presumably bad
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started removing the gauge cluster to more debugging
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I love this circuit board! You can just trace the leads to see what goes where.
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figured out what all the lines are for
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ran another test with the new sender and the gauge in the cluster
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next moved on to removing the gauge from the cluster
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Clever eletrical connections by GM
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Figured out the wiring for the guage itself
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Gauge still does not read correctly when directly connected to sender and battery
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new gauge, a VDO 301-106 fuel gauge, also 0-90 ohms
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Top of new gauge is E, old gauge is F. Don't worry, it will just work once installed
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had to cut the bezel to get it off. No big deal since we aren't keeping that part
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Getting pointer off is a pain in the butt. You have to twist pointer past its normal range of motion
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Getting the new pointer off was by far my least favorite part. Thought I was going to break it
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old gauge
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removed old pointer
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took grinding wheel to 2 rivets holding face to gauge
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rivets removed
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drilled new holes to line up with mounting holes for new gauge guts
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new gauge mounted, later came back and applied eletrical tape to cover 2 extra holes
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new gauge installed. Put pointer on very loosely for now. Adjust pointer once sender is wired up.
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soldered lines into terminals on new gauge
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all three leads have been soldered onto back of new gauge
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new water tank has space for a sender but the hole still needs to be drilled out
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I ran a wire through the cluster and soldered it
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repeat for all 3 wires the new gauge needs
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new gauge ready to plug into these three leads
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These leads were a little long. When doing the holding tank, I made the leads a couple inches shorte
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rebuilt gauge reinstalled in gauge cluster
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I gave both gauges duct tape treatment to block light from coming through the original mounting hole
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sexy
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gauges reinstalled, pointer position adjusted to they point at F when sender reads full

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