Fuel gauge

[Terry_Geiger]

I'm helping a friend with a 3000. The car was put together by the previous owner and is a combination of BJ7 and BJ8 components. The fuel gauge is a BJ7 gauge (silver face) and goes to full when the sending unit is grounded. It goes empty when no ground is applied to the sending unit. The sending unit appears to work backwards, it shows a dead short to ground when empty and shows resistance when full. Is it possible we have a mismatch between the sending unit and the gauge? Do BJ8 and BJ7 sending units work backwards from each other? Thanks...

 

[ThomP]

I can confirm that BJ8 senders show ground when empty and about 80 ohms when in the full position.I can also verify that the patrs book shows two different part numbers for the BJ7 and BJ8 sensors ( 1B2736 for BJ7 & BHA4435 for BJ8 sensor)and gauges (1b9061 BJ7 and BHA4432 for BJ8 gauge). From the schematic in the shop manual, I would guess that they work in the same way.

Has the car been converted to negative ground and this gauge was overlooked?

 

[Terry_Geiger]

Yes, the car has been converted to negative ground. Do the wires on the back of the gauge need to be swapped? The factory wiring has the "T" terminal connected to the fuel tank sender and the "B" terminal connected to the battery (12+). Do we need to connect the "T" terminal to the 12+ supply and the "B" to the fuel sender to make it work correctly?

 

[Dave Russell]

Hi Terry,
To my knowledge, the gage should read full when there is about 70 ohms connected between the gage terminal on the dash unit & ground. A quick check is with nothing connected to this terminal, the gage should read over full. Conversely, with the gage terminal on the dash unit grounded, the gage should read empty.

If the dash unit does work this way, but doesn't with the tank unit connected, there is something wrong with the wire between gage & sending unit.

The gage is not polarity sensitive. It does however, rely on good grounds at the dash unit AND at the sending unit.

It's possible for the magnetic adjustments in the dash unit to get out of adjustment or be defective in a manner that might make the unit read backward. Ie. a coil open circuited or shorted.

Here is a very detailed how to on testing & repairing the gage. It is for the MGA, but Healey used the same type of gage. I'm nearly certain that the BJ7 gage works in this manner. To my knowledge, the BJ8 gage works in a similar manner, but I have never verified such.
https://mgaguru.com/mgtech/electric/fg_01.htm

Your sender appears to be consistent with the BJ7 gage which you apparently have.

If the BJ8 gage works backward to the earlier ones, somebody that knows please say so.
D

 

[dklawson]

I'm not a Healey owner so please give me a little info and correct my mistakes.

Wasn't the BJ7 produced up until 1963 or so and the BJ8 after that? This was the exact period when BMC was phasing in the use of gauge systems requiring the Smiths voltage stabilizer. The link Dave posted above for the MGA guru site is an excellent source of info on the earlier (pre-stabilizer) gauges and gauge system. Those gauges used a sender that was near zero ohms to ground on empty and about 80 ohms to ground when full. It's important that the gauge itself has a good ground to work properly. The pre-stabilizer gauges worked with balanced magnets and have nearly instant response.

The gauges using the voltage stabilizer will be SLOW responding as they work by the heating of a bimetallic element attached to the gauge's needle. Those gauges typically use a sending unit reading from 230 (or so) ohm when empty to about 10 ohms when full (at least for most LBCs).

You can't mix the two gauge systems. Their resistance range and direction of change are not compatible. Again, I'm not a Healey owner so please excuse my errors and inform me if I've got my history wrong.

 

[Dave Russell]

Hi Doug,
To my knowledge, none of the "Big Healeys", right up to the last BJ8, used an IVR or instrument voltage regulator. There was "some" kind of change in the fuel gage part numbers for the BJ8 but I don't think it was major.

As I said, it should be easy to check the gage itself by open circuiting the gage wire to the sender (I believe this would be the right hand terminal as viewed from the FRONT of the gage - the terminal with a single wire on it) to see if it reads full or empty & likewise by grounding the same terminal. Then, knowing which way the gage works, if it works at all, It's easy enough to check the senders resistance to ground with it in the full & empty positions, provided that the sender works. If the sender's operation corresponds to what was found that the gage needed, the only thing left is the connecting wire between the two.
D

 

[Keoke]

Well DK it must be the otter one because the big Healey guages dance around until you run out of petro.--Keoke- /ubbthreads/images/graemlins/grin.gif

Hey DK ! the Big Healey fuel guage is kinda like a Smiths clock.They are only accurate twice: 1] when the tank is plum full and 2] when its plum empty.- /ubbthreads/images/graemlins/laugh.gif

 

[dklawson]

Those dancing gauges would certainly be the pre-stabilizer type. Thanks for the history.

I do remember from working on the pre-stabilizer systems that having a good ground for the case of the gauge itself was important. If nothing else corrects your gauge problems, at leat try running a dedicated ground wire to the fuel gauge.

Happy 4th !

 

[Terry_Geiger]

Thanks for all the suggestions. I had another chance to look at it again last night and it appears a faulty ground at the gauge was the root problem.

 

[dklawson]

I'm glad you got it sorted. Those fast acting gauges need the ground connection while later (post-stabilizer ones) do not... except when you consider both types need a ground for illumination.