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MG MGB Technical - Voltage Stabiliser ?

On a couple of occasions my fuel gauge fails to lift off empty. I know I have just over 1/2 tank full. I have checked the contact of the green black wire at the sender. (My sender is the later type with no separate earth lead).I am suspicious of the voltage stabiliser altougth the temperature gauge appears to be working OK. The stabiliser is a pig to get at being high up,behind the dash & partly masked by the stem of the windscreen wiper motor. It seems a little loose in the holder/bracket that is mounted by one screw to the captive nut just below the large grommet that gives access to the clutch master cylinder. Does the stabiliser just pull out of the holder which I can then hopefully clean/tidy up/tighten the fixing ssrew to improve the earthing which I assume is necessary to enable the core to heat up & then go off & on to then average out the voltage for the gauges at 10 volts. Is it easier to get at by removing the grommet & feeling / adusting by hand (with no vision)by accessing from the engine side of the bulkhead. (I suspect I may have disturbed the mounting when replacing the grommet after my recent under bonnet refurbishment ).
When I last drove the car,on start off there was no reading. A couple of miles later it suddenly started working so I suspect a poor connection somewhere. (Curious that it first occurred after the recent MOT) !
Cheers
Charles
Charles9

As a 1980 with electric temp and fuel gauges, if the temp gauge continues to function when the fuel gauge fails then it can't be the stabiliser itself. The diagram shows the stabilised supply daisy-chaining to the fuel gauge first and then the temp gauge, so it can't be that either. However this can only be determined for sure if you switch on while the engine is still warm enough to move the temp gauge but the fuel gauge doesn't move, or the fuel gauge fails while you are driving.

The stabiliser should be screwed to the bulkhead through a tab, not pushed into a clip like the indicator flasher. It needs a good earth to function correctly, but if that earth fails the gauges read high, not low.

I'm surprised yours is right up there, it is on my 73, but on my 75 it is on the lower edge of the bulkhead and much more accessible, as in the attached. The indicator flasher can just be seen to the left of the wiper motor.

Try giving the gauge a rap with a knuckle and see if that wakes it up. If that doesn't work you need to monitor the voltage on the green/black on the back of the gauge. If that is zero then there is no voltage coming from the stabiliser through the gauge. If it shows either 12v switching on and off (original stabiliser) or a steady 10v (modern electronic replacement) then the circuit from there through the sender to earth is faulty.

With the 77 and later sender there is no wired earth as you say, it depends on the mechanical fixings of sender to tank to body. If you get the same intermittent 12v or 10v reading on the green/black at the sender as you did at the gauge then the problem is with the sender or tank. If no voltage there then it's the wire between gauge and sender. If you get the intermittent 12v or 10v on the sender body, then it's the tank earth that is missing.



paulh4

Hi Paul,
FYI i have taken a picture of the Voltage Stabiliser which is above both the Hazard & Indicator flashers & as you will see is not easily accessible!
I will follow through the checks you suggest (I did track down this issue in the archives last night).
Is there a separate grounding wire for the tank itself somewhere or does earthing rely on the mounting screws & bolts? - i think that earlier versions of senders had an earthing wire to the boot locking bar mounting.
My Stabiliser appears to be in a clip which is then screwed with a single screw on the top side (totally unreachable of course).
I have looked again at the screw/bolt just below the large grommet. This has nothing to do with the stabiliser & is immediately above one of the mounting screws of the windscreen wiper motor ,but seems to serve no purpose as i can't see any wires to it. Was it a quick fix at the factory to fill a mounting hole of an earlier fixing for a master cylinder of a previous generation car or different market?
(I will attach a picture of this as well).
Cheers
Charles
P.S. i will send to pictures separately due to file size.
Charles9

2 Pictures relating to Voltage Stabiliser? post.


Charles9

Sent second photo again as seems to have dropped off post!


Charles9

No earthing wire for the tank, it just relies on its mounting bolts, which can cause problems in a restoration if the paint-job is too good! I had the same problem with the number-plate lights in the over-riders and had to provide earth wires before they would work.

All the stabilisers I have seen for the MGB have a mounting tab, which is screwed to the bulkhead, never a clip, and yours looks like that to me, as in the attached

Bulkhead holes to follow.


paulh4

I don't recall any unused tappings on my 73 or 75, just two plain holes for the pedal frame either side of and towards the lower edge of the large blanking plug - arrowed in the attached, and one tapped hole for the wiper motor U-clamp bolts lower down, the other tapped hole is inside the double-skin. This looks identical to yours.


paulh4

Thanks Paul,
Without being able to see how the U camps hold the wiper motor to the bulkhead due to location I must be mis understanding what I am looking at.
Your picture of the stabiliser looks like mine so I won't try and wrench it out.
If it does eventually turn out to be faulty I will mount a replacement one elsewhere as I have seen others do.
Many thanks
Charles
Charles9

Charles. As Paul has said, if the temp works then the stabiliser is fine. Check the operation of the sender unit by earthing out the sender live to a good earth. The gauge should then read full. If this is so, make an earthed lead up with dia 5/16" (8mm) eye and use a tank securing bolt, cleaned of paint and underseal, earth the tank, again to a good earth.
If shorting out the sender live doesn't affect the problem, examine/clean the bullet connector which is not too far removed from the tank itself.
Allan Reeling

There is a bullet connector where the rear harness joins the main harness in the mass near the master cylinders, but I don't think there is another one in the green/black lead other than that, apart from a multi-way plug and socket behind the dash.
paulh4

Thanks Paul & Allan for your responses.
Last night I just turned on the ignition and the fuel gauge started to move albeit to just below 1/2 when I know the reading should be just over 1/2. Earlier I checked the green black bullet on the inside wing where the main harness joins the body harness & for good measure used my bullet closure tool on it. (I had already cleaned all the bullets & replaced all the sleaves as part of my underbonnet refurbishment).
I have checked the wiring diagram in the repair manual & the late UK MARKET diagram from Haynes which both show there is not an additional bullet at the rear of the car before the wire gets to the sender.
I have just been out for a run.On start up the fuel gauge started to move,but by the time I was off the drive it had dropped to empty. I drove for about 15 miles during which the temp gauge functioned as normal & after about 5 miles the fuel gauge started working showing what I believe to be the correct reading.
I shall focus my attention at the sender end and let you know the outcome or if the problem goes away. These intermittent faults are always the most difficult to sort!
Cheers, Charles
Charles9

When the fuel gauge has dropped see what voltage you get with respect to a good earth on that bullet connector, the spade on the sender, and the sender body.
paulh4

Hi Paul & Allan,
Having checked the earthing was OK using a test light with one end connected to the brown on the fuse box & the other to the body of the sender i was confident that the problem was on the green black feed side.
Given how difficult it is to both see and get access behind the dash i had another look at the female connector at the sender end.I clearly hadn't cleaned it well enough.
I pulled back the plasic sleave & did the job properly.A quick check with ignition on,but without starting showed the fuel guage pointer to be rapidly rising as it should. Thia was confirmed by a subsequent test drive.
Thanks for your help.
Charles
Charles9

This thread was discussed between 17/05/2018 and 20/05/2018

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