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MG MGB Technical - Electrical problem

Who doesn't have one, right?

Anyhow, I'm convinced that all (or at least most) of my electrical problems are caused by a single loose wire. I've studied wiring diagrams, and everything that doesn't work appears to have a few points in common. The following things do not work:

Windshield wipers
Windshield washers
Hazard Lights
Brake Lights (tail lights work -- just don't brighten when I hit the brakes)
Heater (or maybe just heater fan)
Brake warning light

Probably a few other things, too, that I either haven't discovered or have and forgotten. I've spent quite a bit of time with a voltmeter staring at the mess under the dash and inventing new words to call the DPO, but none of that has solved the problem. Can anyone suggest where my single point of failure might be? (Physical location within the car would be helpful, too -- i.e., "rear wall of the driver's side under-dash" means more to me than "between the induction heater and the flux capacitor.")

Chris St. Pierre

I think every one of those things runs from the white wire on the ignition to the 2nd fuse from the bottom of the fuse block then out to their respective homes.

Start there, and look here: and click on "spanners" on the left side menu.


That's already been one of my major reference sites.

The fuse is good, and I don't think it could be the connection directly from the fuse. While all the things I listed are on the same circuit, it looks like temperature and fuel gauges and turn signals are too, and all those are working. To tell you the truth, I'm confused as hell that the turn signals work while the hazards don't; it seems that the usual situation is exactly the opposite.

The fuse does give me a good starting point for tracing the two green wires off it, though.
Chris St. Pierre

I spent the better part of two hours under the dash of my car today, looking for the elusive dangling green wire, to no avail. Part of this was due to the fact that my B has as many wires disconnected as connected, and there was certainly some confusion until I figured out that the PO did a lot of his electrical work with green wire. But there were no obvious dangling green wires, which, according to every wiring diagram I've seen, is the color I should be looking for. Does anyone have any ideas of common problem spots? How likely is it that one of the molex connectors is bad? (I've never seen one go bad before, but there's a first time for everything.)

There is a largish green wire in the engine compartment, though, that's not connected. It departs from the harness right before the brown wires go to the alternator, and it has the female end of a spade connector on it. Someone once suggested to me that this wire was probably to the carb induction heater, so I haven't paid much attention to it. It also seems to be going the wrong direction -- i.e., towards the front of the car, not towards the cabin.

A lot of this is just random information that I think might be relevant, but I'm at my wit's end here. Any suggestions at all are appreciated; if you need pictures of anything, I'm happy to oblige.
Chris St. Pierre

Chris. What year is your car? That makes a great deal of difference. You may have posted such information before, on another thread. But, us "old folks" have "senior moments" and do not remember who owns what.

Do you have a wiring diagram for your car? If so, get it blown up, I had mine expanded 300%, then laminated. That allows me to trace all of the problem circuits with a Vis a Vis marker (water erasable) to see where the common points are.

If your car has been re-wired by a DPO, you need to trace each circuit to find out if it is hooked up correctly. An ohm meter, with a long jumper lead, can be used to trace the continuity of various wires. You need to find the common point for all the circuits that are affected, then trace from where there is no power to where there is good power. Somewhere between the two is where your problem lies. More information would allow us to provide more accurate information to you in response. Les
Les Bengtson

Chris, forgive me if you've already done it but I'd hit all the connections on the fuse box, control box if fitted, and ignition switch. If it is a single power connection affecting all those items it will be on one of those. (You'd need multiple earth faults to cause that lot and a power fault previous to that, on the solenoid or battery, would affect everything. Rich.



It's a '79 roadster.

That's a great suggestion for the wiring diagram, especially since I know this won't be the last time I have to deal with the PO's wiring job.

Well, three day weekend coming up. Hope I won't need it all! :)
Chris St. Pierre

Chris. The wire going to the front, if it is the green/blue one goes to the cylinder head temperature sending unit.

After laying out the system on my wiring diagram, the first common point is the fuse box, the rear section of the third fuse from the top. Check for power into the clip, power through the fuse, power to the clip, then power to the spade connectors. Yesterday, I found the fuse below it had 12.6 V going in and 9.3 V coming out due to bad connections. Cleaned them and full power through the system now. Check there first. After that, there are a number of connectors and you have to keep moving rearwards, checking all the wires from each connection, to see where the loss is. Les
Les Bengtson

I won't put up with DPO wiring messes. Just replace the harness and start fresh. Here's a link for an awesome harness for an MGB.

Most of the major items are run through relays, and all the fuses are the modern type agc, and they're all together on one board! You can even mount the board INSIDE the car on the passenger side of the vehicle so you can clean up the engine bay significantly. It's the same cost as buying an OEM mgb harness, and the wire color codes are all the same too. It's a great way to eliminate most of the wiring issues you'll face when dealing with a DPO



Can't help you in locating the problem but have come to the view that any appreciable time spent under the dash chasing wires etc is a lot more comfortable with the seat/s out and a cushion against the bulkhead or (especially) the pedals.
Roger T


I've considered that, being as the wiring in the car is in horrible shape, but was under the impression that it was very difficult and time consuming. Can you (or anyone else) comment on that? If you've used that particular harness (which certainly looks nice), how good are the instructions?


Thanks! I've been cramping the hell out of my back trying to get to this problem.
Chris St. Pierre

I bought my existing wiring harness from a company named British Wiring in Illinois and had it installed in under a day. The harness from Advance Autowire may take a little longer, but it is way more suited to todays automotive environment. Advanced's harness comes with GREAT instructions, and it's really not all that difficult. Just strip out all the existing wiring (except the dash harness) and start all over again. Your engine bay will be so pretty without all the junk going down the passenger side!


Les is right on. No substitute for getting exact diagram color if possible. Also agree with Justin, particularly with a DPO that laid you land mines like this. I had some similar probs and probably spent 200 hours screwing with electrical crap until I went to British Wiring and got new harness. Bliss is naving NO recurring electrical problems and having time to spend doing other things now. Couldn't get my mind around the money but reached frustration level with meltdown which I finally traced to 3 wires rubbed to wire down near the clutch slave cylinder and went new. VERY important to have exact diagram when doing harness. Mine turned out to be #8 or something in Bentley's. Wasn't color but used it when something got confusing on my color one which was approx. correct.
J.T. Bamford

<Sheepish>Well, it turned out to be (mostly) a blown fuse. I feel pretty dumb.</sheepish> I had just been checking the voltage way down the line, and had only inspected the fuses visually; they all looked just fine. (I'd actually had the car into the shop once or twice, too, and they'd inspected the fuses without catching it as well.)

Now it's a much less daunting task of hunting down the individual connection failures that make my windshield wipers, tach, and ignition and hazard idiot lights not work.

My hazard flashers also don't work, but for a completely different reason: the DPO, using every neuron in his tiny little pea brain, managed to wire the hazard flasher to the drivers' side speaker when he installed the radio, so instead of all four lights going, the speakers just goes: "Thump! Thump! Thump!..."

Anyhow, thanks for everyone's help, even if the (partial) solution was a little anti-climactic.
Chris St. Pierre

If the DPO wired the hazard lights to the drivers side speaker, then you need to cut your losses on that harness and start over fresh. There's nothing worse then trying to figure out if the switch is bad, or the wiring going to or from the switch is bad. Replacing the harness will eliminate MANY headaches.

From a safety standpoint, do you really want to be driving and have your headlights die on you because connections have corroded and are no longer making good contact?

Doesn't sound like fun to me...


You win. I resolved today to buy a new harness after discovering a loose connection under the dash that, when connected, made the starter turn ceaselessly. I tightened it while troubleshooting my windshield wipers and, much to my surprise, the car started, apparently on its own.

I've had enough DPO wiring! Gaaah!

Anyhow, more on my windshield wipers. The voltages seem to be correct -- i.e., when I have the wipers on low, the red and green and solid green wires are both at 12.6V -- but no wiping action. Instead, the motor gets hot to the touch. I thought maybe the wipers were stuck, so I tried turning them by hand, smacking the motor casing with a wrench, etc., but that didn't work. Ideas?
Chris St. Pierre

Get hold of me off the BBS and I'lltalk with you about this. I might have a solution. I had similar problems and spent many months trying to disvover the solution. (I already knew the problem) I finally paid someone to spend the time and he discovered a resistor wire that was incorrectly connected providing only 7 volts to the circuits that you mention. The person who sold me my 77 B did not understand the electrical system and I was having extreme problems trying to find his connections. The $500 that I paid a qualified mechanic allowed me a complete summer of superb driving. So decide what you want to do, drive that beautiful car of tinker with it.
Let me know if I can help.

Bob Anderson

This thread was discussed between 26/05/2005 and 02/06/2005

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