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MG MGB Technical - Electrical fault

Driving the B today and have lost the electrics....no taco, temp, fuel, stop/rear lights, wipers etc....but the headlights work, as does the electric radiator fan.
But, if the car is driven over a bump it will all start again, before stopping. I have checked the fuses....all ok. If I jiggle the wires aroung the fuse box I can get it to work, then once I drive it will all stop again.
Any ideas?
Matt

Matt. You are not perfectly clear about what all is lost. Does the engine stop running when power to the electrics is lost? If so, that indicates a problem in a different place than if just the dash gauges are lost.

It would be good to know what year your car is because they were wired differently over the 18 year production run.

Get a copy of the wiring diagram and find each instrument/system that fails. Then, trace them back to the common point where they all come together. Somewhere between that point and the battery is your problem.

On my 79, I lost all the dash gauges, brake lights and turn signals. Checking the wiring diagram I found all of these were "green wire circuit" systems which receive power from the third fuse down from the top of the four fuse box. Power comes into the front of the box from the "white wire circuit" which is supplied by the ignition relay. Since the engine kept running, I knew the white wire circuit was working and the possible trouble points were the white wire connection (power input) at the front of the fuse, the connections that hold the fuse, the fuse itself and the connection that the green wires connect to at the rear of the box (power output to the items which were not working).

At that point, it was easy to test the various portions of the circuit using a test light. Power from the white wire to the connector and from the connector to the clamp holding the fuse. No power at the front metal cap on the fuse--cleaned the connection and power was restored. You, probably, have something similar and can check it out the same way.

Les
Les Bengtson

Replace the fuse first. I have had fuses like this break the internal connection inside the cap of the fuse. You can't tell by looking at it that its broke but you can go nuts trying to find it
John H

The previous posts are good advice, and you might also want to remove the fusebox and clean and solder the back rivet to the spades on the back of the box to eliminate possible corrosion problems between the rivet and spade.
Mwhitt

John. Yes, you can "go nuts" trying to trace down a fuse with the wire broken at one of the caps. Hence, the use of the test light. I had used one, on a casual basis, for years. Many of Paul Hunt's postings mentioned the use of one. After following Paul's advice I realized just how valuable a test instrument it was. Cheap ones are available for about $2 at the car parts stores. I keep a good quality light in the shop and one of the cheap ones in the tool box of each car.

Les
Les Bengtson

I agree with Les and Paul. The test light is an awsome tool.

When I removed, checked, cleaned, replaced bullets and connectors on my 68 wire harness, the test light was my main tool to ensure all was working well. I had the harness out of the car and tested each wire using a trickle charger and the light. It helped find loose bullets and bad wires for replacement. I cleaned up each bullet and replaced poorly soldered ones. Then put all new female connectors on the system. It works great. The cost was very low but I did spend a lot of time on it. Like MWhit said, soldering the connections on the back of the fuse box in a necessity in my opinion. The mechanical crimps on my new fuse box left much to be desired.

Cris
Cris DeYoung

Thanks so much for your advice.
Les....The car runs perfectly otherwise....just a loss of dash electrics/turn signals and stop lights only.
I'll have a few beers and a look this weekend.
Matt

Matt. Green wire circuit. On the later model cars, this is the third fuse from the top and the power comes out the back connector of the fuse box. I have had this problem twice--once bad fuse and once bad connection. Since the power comes back if you hit a bump, the bad connection is the most probable cause. Good luck. My daughter returns from Oz on Thursday.

Les
Les Bengtson

i had the same probem a few years ago ,after investergation i found a rusty earth in the engine on the left hand side . i cleaned it up problem solved
daz

Ok....I'll have a look there to start.
Its a 77 model (with 69 engine/box) .... I have the workshop book with the electic plans in it. I'll grab the multimeter/test light, the esky, a few beers, put on a few tunes and have a tinker.
Thanks everyone,
Matt.
Matt

Matt,

Instead of squinting at those hard to read diagrams in the manual, why not download a redrawn, enlarged, and color coded diagram? Go to http://www.advanceautowire.com and click on the stock schematics button. This'll make your trouble shooting much easier.
Dan Masters

Test lights are good, but you can do a lot more with a multi-tester. Checking fuses off the car is just one example.
Dan

Matt. That should work. By the way, my daughter, in an e-mail, decribed the portion of Australia she was visiting as "West Texas with more exotic road kill-kangaroos and wallabys". She gets home tomorrow and I am looking at a more exact description.

Les
Les Bengtson

Whilst I agree with Dan that you can do more with a multi-tester than a test-lamp, testing fuses off-car isn't the best example. Resistance measurement is notoriously unreliable where bad connections are involved. By there vary nature these are variable and will change according to the load on the circuit. A multi-meter typically only passes a microscopic current to measure resistance, and this can easily indicate a low or zero resistance, but a higher current in normal use will cause the resistance to increase dramatically. Paradoxically when measuring the contact resistance of switches with silvered contacts the opposite can occur - on an ohm-meter you can get an apparently unacceptable resistance, but in circuit the current burns off the insulating film that can develop in storage and work perfectly. The only accurate and reliable test of connections is what voltage is present either side of the connection when carrying its design load current. Whilst a multi-meter will give you the actual voltage reading and so indicate very small volt-drops, if a test-lamp glows with equal brightness as judged by eye both sides of the connection then any volt-drop that *is* occurring in the connection is pretty minimal, and irrelevant in cases like Matt's.

But Matt has already found where his problem is - by jiggling the wires round the fusebox he can get it to work. Obviously he needs to clean the fuses, holders, spades and wiring connectors, and check the rivetted connection is sound on the back of the fuseblock for each fuse holder. Could still be a weak connection between the fuse wire and the fuse end cap, which can be diagnosed by shifting the fuses around if you are tight like me and only want to have to change one, or simply chucking them all and treating the car to a new set. Mind you Hunt's Fifth Law may well trip you up then: "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals."
Paul Hunt 2

Thanks again.
Les....funny stuff.
The wildlife down here has wrecked a few of my cars...the latest being my subaru's front bumper from a kangaroo. I hit an emu in an old ute once and it totalled the rear guard. Then there was the time I hit a wild pig in the same ute and it tore the engine from its mounts.
The kangaroos are more of a problem when they get on the trains and buses......
...plus the drop bears can be dangerous when the fall from trees and bite your head and scratch your eyes out.....
Matt

Matt. I am due to pick her up an a few hours. Am sure I will have plenty of time to hear about all her adventures.

Les
Les Bengtson

Hi all.

You don't have to go to Australia to have motoring 'adventures' with animals.

A large number of wild (?) boar were released here (allegedly by animal rights campaigners) and have been causing 'incidents'.

(are they really wild boar if they are in captivity ?)

Don
Don

If they were released they were no longer in captivity ... :o)

'Wild' as opposed to 'domesticated'. You can have a wild animal in captivity, but it will probably be pretty cross.
Paul Hunt 2

This thread was discussed between 10/01/2006 and 13/01/2006

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