OD electrical gremlin[200/1976]

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OD electrical gremlin[200/1976] posted by Jim Smeall on
Saturday, 24 July 1999, at 2:15 p.m.

On my '76 245 the OD keeps blowing the fuse. I can sit in the driveway and flip the switch and the light will come on and not blow the fuse but on the road it will blow the fuse when I flip the switch. Bypassing the 4th gear switch caused the fuse to blow as soon as you fliped the switch. Disconnecting the solenoid didn't change anything. Can I run new wires to the solenoid from the shifter switch? --
Jim jsmeall@harbornet.com

Re: OD electrical gremlin[200/1976] posted by Don Foster on
Saturday, 24 July 1999, at 6:05 p.m.

Jim --

The electrical circuit for your OD is different than the newer cars -- it's simpler, and you do not have a relay. The OD relay was introduced about 1981, I think.

The circuit is as follows:

Current starts at the fuse panel, probably fuse #3.
Current goes through the shift-mounted slide switch.
From here, current goes to the dash light, and also to a tranny switch (turns on in 4th gear only).
After going through the tranny switch, current goes through the OD solenoid to ground, completing the circuit.

If the fuse blows only in 4th gear, three strong possibilities are:

The wiring going from the tranny switch to the solenoid is shorted to ground.
The solenoid itself is defective (this would be a first!).
The diode across the solenoid is shorted.

The diode is a clamping diode, sometimes called a "freewheeling" diode. It is connected backwards and does not conduct current when the solenoid is powered. However, when you switch the OD solenoid off, the collapsing magnetic field generates a large pulse in the reverse direction -- this would ordinarily arc and burn the slide switch contacts. But the diode conducts this reverse pulse and harmlessly dissipates it.
--

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