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MG MGB Technical - 1980 MGB/LE - Engine won't turnover

Hello to all,
I hope I can get help with this one. I recently installed a new turn signal/dipper switch unit to my B. I also added a volt meter and a stereo. To my knowledge everything is wired correctly since they all work. Now my problem. The motor will not turn over at all. It's like there is no power to the starter. When I turn the key I can hear the relay click. I made a jumper to go from the #7 fuse and the white connector on the relay. It clicks when it contacts, with the key off but doesn't when the ignition is on. That is not to the start position. Is this normal? The battery is showing 12.4 volts on my hand held multimeter. The B is on 4 jackstands because of replacing seals and universals and exhaust. I noticed that coming from a wiring loom a yellow and a white/brown wire that looks like it should go somewhere. I also see on the left side of the tranny near the top a plug with two male connectors. Could this be where these wires go? The car did run before I elevated it. I hope this all makes sense to someone because I am STUMPED. I would appreciate help on this one please!!!

Thanks much,
Barry Crum
B.R. Crum

Hi Barry. I am not very familiar with the extra equipment fitted to US market cars but the yellow connects to the overdrive switch on the gearbox top. I have a haynes wiring diagram that shows an inertia switch which carries a white and brown wire which is also connected to the O/D circuit, A sparkcontrol switch and spark advance solenoid are also connected to the yellow wire between the trans interlock switch and the O/D on/off switch which may account for your non start problem . If you cannot find this wiring diagram let me know and Iwill post a copy for you Hope this is of some help Jim
j soutar

<I made a jumper to go from the #7 fuse and the white connector on the relay. It clicks when it contacts, with the key off but doesn't when the ignition is on.>

I assume by #7 fuse you mean the fuse with brown wires attached to to it. #7 refers to the fusebox terminal number. The relay you applied the jumper to is the ignition relay and what found is normal because the relay is on when the ignition switch is on, the relay will be energized with ignition on.

There is another relay under the hood of the 80 model MGB, it's the start relay, it's near the fusebox on early cars I'm not sure of the location on later cars. It will have brown, brown/white, white/red and black wires on it. It isn't clear to me if you are hearing the start relay or the start solenoind when you attempt to start the car. If it's from the relay you need to determine if there is 12 volts on the brown/white wire at the starter solenoid when the key is turned to the start position.

The yellow wire near the gearbox is for overdrive, it's usually on cars that do not have overdrive. I think the switch yor are asking about is the transmission interlock switch. Do you have overdrive?

If you don't have a good wiring diagram you can find easy to follow diagrams here.
On the left find stock schematics, then MGB to load a PDF, you want the late US market MGB from Haynes diagram, it's around page 33.

Clifton Gordon

Barry. As Clifton notes, the relay forwards of the fuse box is the ignition relay, a part of the "white wire" circuit and is energised when the ignition switch is turned to the "run" position.

The starter relay is a similar looking "can" type relay located to the rear of the fuse box on the right hand inner fender.

But, let us start from the beginning. First, definitions which, in the past, have caused problems. "Engine turning over" means that the engine is physically capable of being turned over. "Starting" means that the starter is capable of turning the engine over and the ignition circuit is capable of causing the engine to fire and run.

When starting out, if the starter will not turn the engine over, the first thing to do is to make sure the engine is capable of rotating through a full 360 degrees of movement. A 1 5/16" wrench or socket can be used to turn the engine crankshaft when it is applied to the crankshaft pulley nut. Again, make sure you can turn the engine over a full 360 degrees.

Assuming the engine is free to turn over, the next step is the battery. Check for voltage. Your 12.4 volts should be fine. Have an assistant try to start the engine--move the ignition switch to the start position, while you hold the probes of your multi-meter in contact with the terminals of the battery. If the battery voltage drops below 11.5 volts, you have a battery problem. If battery voltage is good, test the voltage reading on the outer surface of the battery cable clamps. It should read battery voltage. If not, you have a problem between the battery terminals and the clamps. Remove and clean the clamps, the terminals, the connection of the ground cable to the chassis, then reassemble them using a little grease to prevent future corrosion. If the measurement at the clamps was the same as on the battery, you need to begin checking under the car.

Since the car is up on jackstands, get under it, a large piece of cardboard, laid on the floor, helps keep you clean. Check for the ground strap between the right hand side of the transmission mount and the transmission crossmember. It should be present and in good condition with tight connections. If not, the system will try and ground through other means which often are not capable of carrying the current. If the ground strap is good, move to the starter motor's main terminal where the battery cable connects. Touch the positive probe of the multimeter to the end of the terminal, touch the negative probe to any ground point and have your assistant turn the ignition switch to the start position. You should see 11.5 to 12.5 volts present. If not, you have a problem with either the front end of the cable connection or the cable itself. I have seen both problems present over the years. There can also be a problem internal to the starter. The "quick check" for a cable problem is to attach a jumper cable from the positive terminal of the battery to the main terminal of the starter and try to turn the starter over using the ignition switch. If it works then, bad cable. If you have one of the replacement battery clamps, the ones with the strap and two bolts, these will frequently corrode where the cable is attached and cause a voltage loss. Cleaning, greasing, and reassembling is required.

While you are still under the car, make sure the wires from the starter relay and to the coil are properly connected. The wire from the starter relay goes to the lower spade connection and the wire to the coil goes to the upper spade connection.

If you have a good voltage reading present at the starter, it is time to begin work in the engine compartment. Go back to the starter relay and check for power at the brown wire. This should be full time power and will always be present. If not, you have a problem with the brown wire circuit and need to trace it. If there is power, disconnect the wire from the ignition switch, connect it to the positive terminal of the volt meter and ground the negative probe of the volt meter. Have an assistant turn the ignition switch to the start position and make sure you are getting power to the relay. If not, check backwards from there. If so, turn off the ignition switch and reconnect the wire. Touch the positive probe of the volt meter to the wire going to the starter, have the ignition switch turned to the start position and check that there is voltage flowing through the relay. If not, replace the relay. If there is, turn off the ignition switch, climb under the car again, and repeat the test at the starter, seeing if the line from the relay to the starter solenoid is bad. If it is bad, replace the wire. If it is good, you have a starter problem.

Les Bengtson

Thankyou very much for such a detailed check list. I went through all the steps you outlined and found out that it is the starter solenoid that is the culprit. I would never have guessed! Thanks also to Clifton for your information, It's always valuble, and to Jim for the info on the OD gearbox.
Thanks very much guys,
B.R. Crum

Barry. Glad to be able to help. Bought my first LBC (Little British Car) in 1968. I am still learning about them and there is a great deal that I do not know about them. Thus, keep your sense of humor and do not worry about having to know everything. We are all students here, helping each other out.

Les Bengtson

This thread was discussed between 08/09/2007 and 16/09/2007

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