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MG MGB Technical - '72 B won't start

Recently replaced the starter brushes and solenoid, worked great for a couple weeks, now will not start turn key and nothing happens). Starter relay seemed faulty, replaced it and now get slow cranks but will not start. Tried fresh battery and jumping, nothing seems to work. Tested starter on the bench and it engages and turns, tested ignition switches and get appropriate volt readings. Any ideas?
J Rosen

Have you had the starter tested under load? If not, it may be capable of turning but bogs down when under load.

When you say you "tested ignition switches and get appropriate volt readings" what does this mean? That you have 12.5V going on the brown wire into the ignition switch under no load conditions?

What is the voltage present at the large terminal of the starter when the starter is being cranked? If it is less than 11.5V, you have a problem with either the battery, the connections, or the cable from the battery to the starter. I have seen this problem twice over the last five years. Seems that the old cables have built up internal corrosion over the years and are not capable of carrying sufficient voltage to allow the starter to work properly.

What is the condition of the ground strap between the engine and chassis? If it is not in good condition, with solid connections, it will not allow sufficient current flow for the starter to turn over. The engine will try to ground itself through the throttle cable and choke cables, overheating them.

Most auto parts stores offer free testing of the battery and starter. If the starter is still out of the car, that might be a good thing to have checked. If not, you might want to start checking the under load voltage at the starter is a first step.

More information and the results of the initial tests would help in a diagnosis.

Les Bengtson

Old starters may have worn bushings that cause the starter to bind under load. Have you replaced the bushings?

Make sure your ground connections are clean and secure. This includes the engine to chassis strap and the battery to chassis cable.

Watch some lights, like the ignition warning light, while you crank and see what they do. If they dim right dowm then either the battery is flat or there are bad connections at the battery, battery ground strap, or solenoid. If the light doesn't dim it could be the engine/transmission ground strap, solenoid contacts, or motor brushes/comm.

By putting a voltmeter across various places while cranking you can determine which. If, on the battery *posts* (not connectors) the voltage drops to below 10v then it's the battery. If that stays up but it drops on the connectors then it's one or both of the connectors. If that stays up but it drops between the 12v battery connector and the body then it's where the battery ground strap joins the body. If that stays up but on the engine instead of the body that goes up, then its the engine/transmission ground strap. If that stays up but it drops between the solenoid battery cable stud and the engine, then it's the solenoid stud. And so on.

If you find and fix a bad connection do an end-to-end check to see if you have any others on the way. Connect the meter between the 12v battery post and the solenoid battery cable stud and crank. Ideally you will see only 2 or 3 tenths of a volt. If you see a volt or more it is worth investigating further, I have seen as much as 3v lost. Then do the same between the battery ground post and the engine. With a good system and a good battery, battery voltage shouldn't drop below 10v when cranking, and you should only get 0.5v less than that at the starter.
Paul Hunt 2

Thanks for all the advice. I have been away and unable to further investigate, but I will keep you all posted of what I find. I did recently replace a bad battery under warrenty with a new one, and I suppose it is certainly possible the new one is defective also.
J Rosen

This thread was discussed between 02/10/2007 and 09/10/2007

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