Starter isn't getting battery power intermittently.

[Offgrid]

This problem has been annoying me for weeks now, and I can't work out the cause of the failure.

The problem - The car starts easily most of the time. However, sometimes there is some hesitancy before the starter spins after turning the key. About every two days, the starter refuses to engage, and there is no power dip. If I connect a battery bank to the installed battery, and set it to boost, then the engine fires up instantly.

The equipment - The car is a 1990 740 estate with a manual B200E engine. I've tried 3 good batteries, which made no difference. The current one is a 2 year old Yuasa 5000 battery rated at 85 Ah. Charge level is 12.8 volts. The alternator kicks out 14.7 volts. The starter and solenoid look to be the original ones with surface rust and peeled paint.

What I've tried - I've connected the solenoid to the negative pole on the battery. I've cleaned all the obvious connection, and sprayed with switch cleaner. I've cleaned the battery terminals. I've banged the starter with a hammer. All of these actions seem to have no effect when the starter won't activate.

My thoughts. I suspected the ignition switch, but rejected that, as the engine fires when the battery is boosted. I doubt that all three apparently good batteries would cause the same problem. I can only assume that there is an extra resistance somewhere, and adding the booster pack overcomes this. I can't think where this would be, unless it is in the solenoid or the starter.

My next option is to remove the starter unit from the saloon, and try that in the estate. This would give me a chance to examine the starter and solenoid, and clean any internal connections. Has anybody got a better idea?