MG Car Repair: MGB solenoids, voltage drop test, volt meter

Question
Hi Howard,
I have a 1967 MGB 3 synchro. It has sometime ago been converted to an Alternator. For a good time the car has not been good at starting - sluggish at cranking over. Over the past year, I have replaced the starter motor, alternator, batteries (2 x 6v and then 1 x 12v), and recently the cables from battery to solenoid. Much of the above were done on the advice of experts! At each change it was slightly better, but soons slips back to slow cranking and has to be push started. I have just had the battery and charging checked out and it was fine. I have also attached an additional earth. with a mobile boost unit it cranked over very well, and continued doing this once the unit was removed. within 5 minutes it was back to not starting on its own.
The car has averaged 5000 miles per year, and in fact this year has done so far 6000 miles. I am thinking that of the things left to change it is only really the solenoid, as it is somehow malfunctioning.
At rest I am getting 12.9 to 13.0 volts across the battery and at the feed to the solenoid. on cranking this drops to between 4 and 7 volts.
any ideas? how do solenoids generally fail when they do fail? symptoms?

mike

Answer
Hi Mike,
If you are saying that you only have 4 to 7 v on the battery cable post on the solenoid then I would do a voltage drop test on each section of the power cable and of the grounding (engine to negative post of the battery.) (always test at the post not the cable end.)

To do the drop test take your volt meter and set it on a low volt scale and connect the positive lead to the positive post (not the cable end) of the battery and connect the negative lead to the POST on the solenoid and have someone spin the starter. Then connect the positive lead of the volt meter to the engine and the negative lead to the negative POST of the battery and have them hit the starter again.
The solenoid can have a bad connection so you can test it too by placing the positive lead on the POST that the battery cable is on and the negative lead on the other post and have someone hit the starter again. Each time you do this test you are checking how much resistance there is in that part of the circle. Current always travels in a circle. Any part of the circuit that you do this test on that has high resistance it will give you a several volt reading. Be sure to test all the way even if you find one part of the circuit having a high voltage drop.

In any of the tests if you see several volts that is the failed part of the circuit.

A mechanics quick test of cable connections is to hit the starter and hold the starter on for 10 seconds or so and quickly feel the battery posts and the solenoid posts and the engine to frame ground strap ends and the battery ground ends. If any get hot, then that is the problem. This only works when the car is in the failing mode.
Howard