| Re: MacNeal dash issues [message #369552 is a reply to message #369549] |
Sat, 23 April 2022 18:45   |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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rjw wrote on Sat, 23 April 2022 11:49I am still fooling with my gas gauge not working. I have ruled out the fuel tank selector switch as being the problem.
First of all I unplugged the 2 pin connector by the fuel selector solenoid and found that there was a lot of corrosion. I thought that might be the problem so I cleaned it up and also cleaned up the ground wire at that location. I reconnected and it still shows over full. If the connector is not plugged together the gauge registers more over fill then if connected. If I short the pins to ground it shows empty on either tank.
If I connect a known good fuel level sensor or a number of resistors in the 30 to 50 OHM range to either of the pins that go to the front and the fuel level gauge I can get the gauge to register properly. i.e. 50 OHM shows up as ~ 1/2 full and that is regardless of which pin I connect them to at that connector.
I checked the resistance of the pins going back to the senders and found them to read 108.9 and 107.2 ohms. By the way the tanks are approximately 1/2 full as far as I know.
I would have thought that over 100 ohms resistance would show more to the empty side than over full as the gauge is 90 ohms.
If it was only one tank not working properly I might think it was a sender issue. But with both senders showing the same I have no clue. I also could not find any damage to the wiring going up to the top of either tank.
I am baffled and really don't want to drop the tanks to look at the senders.
Richard,
You just happened to dive into a pool that I know very well......
Unless Mac changed out the instruments, they are what the industry called "Air Core" gauges. They are no longer in vogue. Each instrument has two windings, both receive line power one is grounded through a resistor and the other thought the sender - Be it fuel level, engine temperature or lube oil pressure. The fields are balanced at half scale. They are configured so that an open sensor reads full scale. Full scale for most is ~90 ohms. So, your test was real good.
When you measured the senders from the connector, you should have found something like 50 Ohms.....
Did you measure that between the connector and the tank?
Or did you measure to the frame??? If it was to the frame, go back under there and now measure to the tank body.
If that comes back good (~50Ω), then drill a hole in the flange of the tank (outside the weld) and run a ground to the frame. You have just dodged a bullet.
Next, I get to tell you that the senders don't like crapahol very much.
You can (must) take the tanks down to do any meaningful repair.
You may be able to clean the connectors at the tops of the the tanks and get the sensor back.
The bad connection can also be at the bottom of that connector through-bolt. Remember that has been swimming in gasahol for 20 odd years.
You can take the senders out and polish the wipers and hit the rivets so they tighten up and it may get you by for a while. You have to do this for both tanks.
I actually took the senders out (the fourth time I had the tanks down) and found other sending units to build in there. There is a reason I didn't write about this. It is not a suggested repair and hardly cost effective.
If you can get the sending unit back with external efforts, that is great, but I would suggest that you purchase replacements and the tank o-rings as well.
When I got the replace senders working, that is when the rubber lined on top of the tanks went bad. (I had replaced them years earlier, but not with barrier hose.) So, when you have the tanks down, think about the rubber lines as well.
Did you read my bit about managing the tanks when working alone? You really should.
Was this all worth the effort? In the five trips up and down:
All the rubber fuel and vent lines, even the couplers in the fill pipe. Early coaches have more of these.
Most of the rubber has been replaced with Polyarmor line that is connected to the tank with a brass compression union.
All the remaining rubber is accessible without dropping the tanks (again)...
As said, I replaced the sending units in the original fuel pick ups.
The fill vents got the High T mod.
The little taps for the vapor vent system got resoldered with 50-50 solder and a heat gun (no flame).
The vapor vent lines were replaced with Polyarmor.
The supply line for the APU was replaced with a mix of barrier and Polyarmor.
Was it worth the effort?
The last time I worked on the fuel system was 2012. Until the vapor lock caused by old gas (it sat for nearly two years), I have had no problems.
With the High T mod, I can take on nearly 50 gallons of fuel in less than 10 minutes.
With the good senders, I frequently take on in excess of 45 gallons.
So, I think it was worth the effort.
In the works (was supposed to be a winter project (don't ask) is a rework of install two Carter 4070 pumps down at the tanks.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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