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MG MGB Technical - Where is the Ground?

Car is a 72 BGT. Problem is that the left turn signal indicator is weak, and the oil pressure warning light dimly flashes along with the turn signal indicator. Right turn signal indicator is fine.

I believe that the problem is the common (?) ground for the left turn signal indicator and oil pressure warning light. But I cannot find it. Ground wire from the turn signal indicator disappears into the large loom behind the dash.

Where is the actual ground located?

Thanks

Larry
Larry Hallanger

All the inside grounds are above the wiper motor, on one of the impossible RH fender top mounting bolts. But, given that both indicators should cause the same symptoms, go after the other main ground point behind the fuse box while you are at it. And all the ground bullet connectors. And all the green circuit connectors.
BTW, what oil pressure warning light?

FRM
FR Millmore

Thinking the ground for your blinker lights are the attachment bolts (threaded studs) for light unit to body. Guessing with weak ground (high resistance) here, current will seek out another path for return.
Steve Buchina

Oil pressure warning light? There never was one on an MGB.

If the *tell-tale* is weak but the corners of the car are normal brightness, and you mean the main-beam warning, then these do share a wired ground. And the left tell-tale takes a different path to ground from the right, and it shares a 4-way bullet connector behind the dash with the main-beam tell-tale and nothing else. So check this connector and the three black wires that should be in it first. Could still be the black from that connector where it bolts to the body with several others (by the wiper motor), so if it isn't that conenctor remove the bolt and clean up all the bolt-through connectors and the body where they bolt up to.
Paul Hunt 2

Never a stock MG witha warning light. I put one on 70 BGT because the oil sender is over $200 and not accurate. I like the light.

Steve
Steve

FRM

Above the tach and speedo, from left to right:

Left turn indicator

Red warning light (oil?)

Blue high beam indicator

Right turn indicator


The red light goes out as oil pressure starts to increase from zero, and will come on at very low revs corresponding to very low oil pressure. I have assumed that it is an oil pressure light. However, could it be the alternator light?



Steve

I haven't checked the actual flashers. Will do tonight.


Paul

Car has been rebuilt in the past by a PO and the ground connections may not join together as original.



It looks like I have another session on my back with my head in the footwells looking up under the dash.


Thanks to all

Larry
Larry Hallanger

Larry-
Red is ALT/IGN
Paul's description of the ground path for that is the first place I'd look. But you undoubtedly will have to go through the whole thing - may as well do it sooner than later - save a lot of aggro!

I've posted it a million times, but the matter of the bullet sleeve connectors cracking with age cannot be overemphasized. At 30 years, the sleeves are past their sell-by date. Get a bunch of new ones (single and double, though you will find more doubles obviously bad) before you even crawl under there. The sleeves as supplied by British Wire are plated steel, rather than the crack prone hard drawn brass. Use OxGard on all connections.

FRM
FR Millmore

US cars may be different, but don't some black wires get earthed to the back of the dashboard on the tachomoeter mounting screws? They do on my 1970 GT, but I've never checked where they come from.
Neil

Neil22

Black on the rear of instruments are to connect the instruments to earth (at the wiper motor), not to ground other components to the dash.

FRM
FR Millmore

If it *is* the ignition warning light that is glowing with the left indicators then it is something else altogether. The ignition light doesn't have a ground, it runs between the white from the ignition and the brown/yellow at the alt, both of which are at 12v (nominal) with the engine running and the alt charging.

This light *can* glow as loads are applied, but that is caused by common problems such as faulty alternator diodes and bad connections in the brown and white circuits, and as such it should happen with both left and right indicators, and be even more noticeable with higher current loads such as headlights.

I have to say, Larry, that it seems a little churlish, following the question "Where is the actual ground located?" and the time and trouble taken to investigate and respond, that you then say "Car has been rebuilt in the past by a PO and the ground connections may not join together as original." That goes without saying, I would have thought, given that one could probably count the truly unmolested but in use cars on the fingers of one hand.

Neil22 - as FRM says i.e. the tach ground isn't a *source* for other components but a *destination* for the tach. Look at any schematic and you will see they all eventually go to a body ground. It varies from year to year, but sometimes the tach is at the end of a run, and sometimes one or more other components may daisy chain off the tach. The 62-64 schematic appears to show a local ground at the tach that then feeds off to several other components, but if you follow all those through again one of them ends up at a body ground, so the symbol implying a local ground at the tach is just a draughtman's Friday afternoon frolic.
Paul Hunt 2

Re Paul's point on the IGN light: This is why I suggested that the Green circuit, and the preceding N & W feeds to it, should be investigated thoroughly.
Order up 50 each of single and double bullet sleeves, and a tube of OxGard!

FRM
FR Millmore

FRM

You are absolutely right. It is the IGN/ALT indicator.


Paul

A check last night confirmed that the wiring harness grounds are connected to the second RH fender top mounting bolt above the wiper motor.


Will be gone most of the weekend but hope to make some progress in further troubleshooting the problem


Thanks to all for the help

Larry
Larry Hallanger

This thread was discussed between 10/10/2007 and 11/10/2007

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