MG Car Repair: electrical - bendix wont disengage, starter solenoid, gear teeth

Question
I have a 1963 MG MIDGET. It is a positive ground system. The problem I am having is that when I start the motor, the bendix will not release. I pulled the starter and had my mechanic tested it and he said it is fine, bendix snaps back good. No sign of any marks on gear or binding on flywheel. I reinstalled and rotated the starter a fraction (3 different times)just to reposition any tension on flywheel gear teeth. The engagement of gears is 1/4" when engaged and release gap is 1 1/8" when not engaged. I've replaced the starter solenoid and the starter button. The bendix will only release after I pull the hot lead (large wire between the solenoid and starter motor) off the solenoid when the engine starts. I disconnected the leads off the starter button after the engine starts and the bendix still stays engaged. It must be getting power from somewhere - but where?? Any help would be greatly appreciated! Thank You. Hap

Answer
Hi Hap,

You are right to test but you are jumping around with your tests. You need to just follow the path the current takes in the starter circuit.

You have already done some of the tests necessary to ID the problem but you need to take a test light and start by disconnecting the cable off of the starter motor. (do not allow the cable to come in contact with any metal now) Now try to start the engine and you will be able to activate the starter solenoid but not the starter itself. Now that you turned the key to the start position you should hear a "Click" of the solenoid. Release the key back to the "Run" position as though it started. (Do not turn the key to the "OFF" position)

Now take your test light and confirm that that it would have kept your starter engaged by checking the power on the cable you removed from the starter motor. It should be powered. If it is, test the small terminal on the solenoid with the one wire (white w/red tracer) on it. If it is powered remove it from the solenoid and test the wire end to see if it is powered. If it is, you have either a failed ignition switch or a wiring problem in the harness to the switch. If the wire is not powered test the small terminal on the solenoid and if it is powered you have a failed solenoid. If the small terminal has no power on it and the cable to the starter that is off of the starter motor is still powered, you also have a failed solenoid.

The solenoid is nothing but a large relay. The battery cable on the one battery post (with one or more small brown wires) is the power supply. The other large post is the starter motor cable. The small terminal is the trigger terminal that works the small electromagnet inside which just pulls a large brass plate down on to the two large terminals (the battery cable terminal and the starter cable terminal)

A solenoid can fail is several different ways. The large brass plate that makes the connection between the battery terminal and the starter motor terminal can burn itself to the two terminals thus keeping the starter running after the switch is turned off. The electromagnet inside can short out causing the solenoid to stay engaged.

An ignition switch can fail, keeping the starter wire powered (white w/red tracer) and you can get a short in the wiring harness that can activate the starter all the time.

Howard