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MG MGB Technical - Overdrive current

I have the LH Overdive. It has an intermittant fault that causes it to drop in and out. With an ammeter in circuit I get 780mA as it engages stationary and cold. Running and warm the current varies beteween 680 and 720mA as it engages. This seems a bit low. I cannot (yet) replicate the fault with the ammeter in circuit. What is the lowest current that will keep the solenoid engaged? My logic is that if the lowest current to keep in engaged is close to 680/720, then I have probably found the fault in the solenoid/ it's earth or the final bullet connector on the "tail" to the solenoid. If on the other hand, the a current of say 400 mA will keep it engaged, I am looking for a more fundamental ( and intermittant) fault. Any ideas gratefully received. Thanks
Michael Beswick

After a long battle, last Summer, I was able to trace my similar problem to an intermittent short in the solenoid. The normal operating current of the LH OD solenoid is 800ma. A reading lower than that is suspect. My solenoid was 40 years old. I can't complain that it didn't last forever. RAY
rjm RAY

A short would cause high current draw; intermittent open would cause variable low/no current draw. Low current draw is a result of low voltage. Check voltage drop from B+ to the OD solenoid feed with the car running and all IGN controlled things ON; should be 0.2V max. Then subdivide the circuit to find out where the loss is. Most common worst places are N feed to IGN switch, and W from IGN switch, since these are the most heavily loaded. By experience, any and all double bullet sleeves are prime suspects, but any connection or switch in the circuit may be the problem(s).

FRM
FR Millmore

Thanks to you both. Presently having "fun" trying to fish enough slack in the gear box loom to reach the final bullet on the solenoid tail. Pulling it apart is one thing-getting it back (cleaned/changed) is another!! At worst I can introduce another short tail as I could reach the loom connection from the gear stick "hole" and the tail from below. However thats two more connections....

Off to Duxford (Ex RAF airbase now a museum) to see a flying show so another chance to test with ammeter in circuit.
Michael Beswick

This thread was discussed between 29/04/2011 and 01/05/2011

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