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MG MGB Technical - Problem starting a 77

Well I got my junk 77 home last week. So far I have got more than the $100 I paid for it on minor replacement parts for my 74. The engine does turn over by hand and I have a fresh full charged battery hooked up.

The only reason why I bought this was to use it as a "toy" in learning more about these cars so as an aid in trouble shooting on my 74, changing parts etc.

The 77 will not turn over and its dead when I put the key in the ignition. No sound and the only light which appears to work is the map/door light. The car's clock in the dash works and thats about it.

I know that the main leads from the battery to the starter/soleniod, and trouble shoot from there, but when I put the voltage tester on the soleniod I cannot get a reading. It is hard to get the tester down there and I cannot see the different colours of wires etc because of the crud etc.

Basically is there any other way to test and rule out the soleniod as the main culpret?

GG Ginty

Remove the White with Red tracer wire from the small terminal on the solinoid. Using a piece of wire jumper from the large terminal on the solenoid, the one the battery cable is connected to, to the small terminal. If the battery cables have a good connection to the battery and the ground cable has a good clean connection to the body, the starter should spin.
John H

John,

Thanks. One of the issues is that it is hard to reach down to the starter/solenoid area and what makes matters worse is that the cables are so dirty one cannot see the colourings!
GG Ginty

Do you get an ignition warning light? Do any of the ignition-powered electrics like fuel gauge, heater fan, wipers, washers, turn signals work? If the answer to these is all 'no' then there may be no 12v reaching the ignition switch. A 77 should have an ignition relay and if that isn't operating (or the green circuit fuse is broken) then that would stop all the ignition powered stuff above working, but it should still crank the engine when turned to 'start' if there is power to the switch and the switch is OK. It's easier to look for 12v coming out of the ignition switch on the white (ignition) and white/red (start) first, and if there is *then* get down and dirty. You really need to get at the starter conenctions from below, not above.

A 77 starter also should have two small terminals on the solenoid and while connecting 12v to one of them should cause the starter to crank connecting it to the other one won't. Originally these spades were different sizes (standard size on the first and smaller than normal on the other) but rebuilt starters I have seen have them both the same size.
Paul Hunt

Paul,

Thanks for the input. I friend of mine who is into electrics is coming tomorrow afternoon to take a look. I'll pass this info onto him.

GG Ginty

GG. Clean the battery terminals and the inside of the battery cable clamps. While doing so, see how the battery cable clamps are attached to the cables. Some of the molded on clamps have been replaced, over the years, with the type having a U shaped groove into which the bare end of the wire is secured with a cross strap held to the clamp with two small bolts. Corrosion builds up between the bare wire and the clamp in this area, limiting conductivity.

Also, check that the end of the positive cable is clean and securely attached to the large terminal of the starter.

Measure the voltage between the two battery terminals and record it. Measure the voltage between the two battery clamps (attached to the battery) and record it. If the two readings are not the same, there is something causing a bad connection between the clamps and the terminals. Inspect and correct.

Measure the voltage at the starter's large terminal. It should be the same as at the battery. If not, there is internal corrosion within the long cable running from the battery to the starter. The "clamp on" style of battery terminal clamp frequently demonstrates this. If you have good system voltage at the starter terminal, check the lower fuse (number four), front side of the fuse box. That is the input for the brown wire circuit which is constantly hot. You should see the same voltage as at the starter. If not, check the connections of the brown wires at the large starter terminal and clean the connections at the fuse box.

The 77 is a somewhat unique model year. My daughter's car is best described by the "78-80 Model" wiring diagram rather than the "1976 with 77 Supplement" which is supposed to be correct.

Print out a copy of the wiring diagram, available on the internet, in colors, and have it expanded at a print/copy shop. I have mine laminated so that I can draw on them. You will find this a useful tool over the years which pays for itself quickly by its convenience.

When you have more information, post again and we can provide more direct comments about specific problems.

Les
Les Bengtson

Les,

Thanks... I have taken a look at both the wiring diagrams for "Late US market MGB" from Haynes and the one for a 77 from Bentley's. Haynes shows an ignition relay, Bentley does not, according to the diagrams on advanceautowire.com Which is correct?
GG Ginty

There is an ignition relay. Its a black cylinder near the fusebox that looks like a 35mm film cannister.
Ross Kelly

GG. As Ross notes, there is an ignition relay and it is located immediately in front of the fuse box. The factory one does look like an old 35mm film can, but it may have been replaced with a later, aftermarket version which can be either round or rectangular.

Again, I find the 1978-80 wiring diagram the most descriptive of the wiring in my daughter's 77 MGB.

Les
Les Bengtson

It could well be a 76-built 77-registered in which case it won't have a relay, the delay between build and registration often causes a problem, sometimes the date on the American chassis number plate on the B-post is wrong. The chassis number change-point was 410001 for roadsters and 410351 for GTs.

However it is irrelevant for cranking problems, the circuit for that is the same on both models, it only affects ignition-powered components.
Paul Hunt

Thanks for all your input.

Yes it does have the relay and it appears to look much "newer" than the rest of electric components. I didn't have much time last eve to do more trouble shooting. One thing I did notice as well was that the whole fuse area is clean in comparion to the rest of the stuff and makes me think that the PO had visited this same sort of problem in the past. Unfortunatley he is not very helpfull when I have approached him.
GG Ginty

As I say neither fusebox nor *ignition* relay affects cranking, but could explain why fuel gauge, wipers, turn signals etc. don't work when the ignition is turned on, if indeed that is the case, in which case you have at least two problems.

The problem with no cranking could be caused by the *starter* relay, which is in the same location. If this isn't even clicking when you turn the key to crank then you need to be looking at the output from the ignition switch on the white/red, it's arrival at the starter relay, and the ground at the starter relay.
Paul Hunt

This thread was discussed between 16/06/2008 and 19/06/2008

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