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Overdrive electical issues

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RonR

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I’ve recently burned out my second overdrive solenoid. The original solenoid lasted approximately 5 hours of constant driving time after a complete restoration of my 1960 BT7. Thinking that after 45 years of life, it had just seen enough, I replace it with a new Lucas solenoid that lasted approximately 30 minutes. According to Norman Nock’s book (page 54), the solenoid should only draw 1-1/2 amps during normal operation. I assume that the overdrive relay is responsible for reducing the amperage to 1-1/2 amps. If this is true, and is my problem, how does one test the relay to determine if it is operating properly?

Ron
 
Hi Ron,
Welcome to the forum, a really fun & informative place.

The OD relay does NOT reduce the amperage.

The solenoid has two windings. A high current "pull in" winding, around 20 amps, & a low current "holding" winding, around 1.5 amps. When the solenoid is not energized, there is a contact on it's top, under the rubber cover, that puts the pull in winding into the circuit in addition to the holding winding. As soon as the solenoid is energized, it pulls in, this takes fractions of a second, & opens the contact on top for the pull in winding. The holding winding remains energized & holds the solenoid in the operated position. It's a good idea to remove the rubber cover & carefully clean this contact.

If the OD operating arm is not adjusted so that the solenoid has full operated travel, the pull in winding will not be deenergized by the top contact opening. If this happens, the pull in winding will burn up in a short time.

A good operate winding should have a resistance of about 1/2 Ohm. The holding winding should be around 10 Ohms.

The shop manual says to move the OD checking arm, near the right front, to the operated position & insert a pin through the arm & into a hole on the OD body. Then adjust the position of the solenoid arm on the LH side so that the solenoid makes a full stroke or bottoms (actually tops) in the operated position. This method of adjustment works sometimes, but not always, due to wear in the various parts.

I prefer this method for precise setting:

Loosen the OD operating arm clamp on the LH side of the OD. The arm that the solenoid operates.

On the RH side of the OD near the front is a brass hex cap. Under this cap is a spring & a ball valve.

Set the now loosely clamped arm control so that the valve ball lifts from .030" to .040". Measure with dial indicator, cap & spring removed. Tighten the operating arm. Verify ball lift by moving the solenoid plunger, not the arm.

Set solenoid plunger to .160" drop. Adjust the stop or add a stop under the plunger if needed. (Arm drop is not the same as plunger drop due to slack in lifter.) If the plunger drops too far, the solenoid may not have enough power to pull the plunger up when it is energized.

You can see a little more about the OD control circuits here:
https://www.britishcarforum.com/lore/attachment.php?id=13
D
 
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