QuestionQUESTION: Howard
You've helped me in the past so I hope you can again. I recently woke up a 1980 MGB that had been slumbering for about 10 years. Got the motor running etc and am working my way through the electrics. The fuel gauge was not working at all. Checked voltage stabilizer and it is ok at around 10V out, temp gauge seems to be about correct. Removed wire at sending unit and grounded it. Gauge shows full deflection to "full" when grounded and "empty" when disconnected. Removed sending unit and tested for resistance. Found float arm to be stuck as expected. Was able to free it up but must be damaged inside rheostat because resistance is infinite in any arm position. Purchased new sending unit. Tested resistance and per other sites on web adjusted for 220 ohms when arm is in empty position and 22 ohms when full. Installed in tank (careful to make sure it was oriented correctly on the locating tabs) and filled with fuel, gauge never reads above about 1/4. Suspected bad ground at sending unit so installed a jumper from known good ground to sending unit housing. No change in gauge reading.
The only thing I can think of is to remove the sending unit again and bond the housing to ground and operate it from empty to full and check gauge indication. I have this same problem on a 1972 MGB except it will only indicate up to 3/4 of a tank and never full.
When I did the "out of tank movement test" on the 1972, it would only yield 3/4 deflection on the gauge over full travel of the float arm so I'm not optimistic that the results will be very telling on the 1980 either but I'll try.
My question to you is -- can you confirm the 220 ohms at empty and 22 ohms at full for the sending unit?
If those are correct, then what am I doing wrong? The 1980 I have owned for nearly 30 years and never had a fuel gauge issue and the wiring is original and unmolested. The 1972 had numerous wiring issues that I sorted out back to original and believe it is correct.
It seems like an unlikely coincidence that two of my cars would have a similar problem from something as simple as a sending unit replacement so I'm concerned that I might be doing something obviously wrong.
Your thoughts?
Dave
ANSWER: Hi Dave,
It sounds like you are doing everything we did in the dealerships and I have done in the last 48 years on the "B" for a fuel gauge problem. I lost count on the number of sending units I have replaced in dealerships.
Our standard procedure was to first test the charge rate at the battery (13.8 to 14.5) then test the stabilizer for the 10v. (using a analog meter) reading. We then turned the key on and removed the sending unit wire and watched the gauge needle deflect and then grounded the wire on the sending unit ground and the needle was suppose to deflect in the opposite direction. This was a rough test of the gauge and wiring. If all that was ok we just replaced the sending unit. We never had the ohms resistance readings to test a sending unit. So I can't confirm your statement of the readings. With today's reproduction parts I don't know if that is correct or not.
I can tell you that I worked for MG dealers for most all of the MGB's life span and worked in Jax FL where we worked on from 2 or 3 to 5 or 10 MGB's a day and I replaced hundreds of sending units over the years and only replaced a small hand full of gauges in all that time and I have maintained a shop to work on my own MG's ever since. The procedure you used is exactly as I have always done and never had a problem although I have not used any lately in the last ten years.
I do have a sending unit out of my "B" that I am restoring that I know was working correctly so if you have a little time I will measure it today to confirm the resistance for you.
The only item we were firm about and I know does affect gauge readings is Alternator charge rate which you did not quote. You check that and I will measure my sending unit today and send you the results.
Howard
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QUESTION: Howard
I checked the output from the voltage stabilizer. I think my analog voltmeter is reading a little low. I don't use it much any more and need to either calibrate it or replace it. It showed the output voltage switching from about 7 to 9 volts. I checked the battery voltage with this meter and it showed 12V during charging and about 11 when not running. Again I don't use this meter much anymore and don't trust it. I checked the battery voltage with my digital meter and it was charging at about 14.5V or so. I think the voltage regulator is ok and the alternator is putting out sufficient charge. Consequently I think the voltage stabilizer is working and probably switching between 9 and 11 volts as I would expect.
I think the problem is likely to be oxidation or corrosion of the bullet connector in the wire from the sender to the gauge or maybe in the multi pin connector to the dash harness. It might even be at the connection to the gauge but I'm not counting on finding it there.
I think the next test is to measure resistance in the wire from the connector at the sending unit to the sender side connection at the gauge. If there is significant resistance, say 100 ohms or so, that would account for the faulty reading. Then it would be a matter of determining which connection(s) were at fault.
Am I headed the right way here or do you have a different suggestion?
Thanks for the sanity check.
Dave
ANSWER: The Lucas bullet connectors are noted for poor connections after aging and I have had to take many apart and clean them. I use a worn out 22 cal. bore brush to clean the female connectors. A wire brush in a Dremal kit works for the male end.
I still need to dig out my sending unit and measure it for you. Got tied up in other business today. But I have not forgotten.
Howard
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QUESTION: PROBLEM SOLVED!!!!
There are a series of bullet connectors under the bonnet located near the fuse block that join the rear wiring harness to the front. I noticed a problem with a couple of these connectors being oxidized and that kept the passenger side marker, tail, and license plate lights from working. I found that the fuel sender wire also has a joint in this area (green with black stripe). I cleaned the bullets, replaced the barrel and the gauge works fine now. I'll try the same repair on my 1972 in the near future. Thanks for the help!
AnswerGlad you got it fixed. I found my sending unit and measured it and got 22 ohms in full position and 7.69 K ohms in empty. The sending unit looks like new inside and I couldn't tell if it was an aftermarket unit or not. It had to be 10 + years old as I tore the car down 10 years ago. But I do remember the fuel gauge working.
I hope to have it ready for our British car show in early Oct.
Howard
