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Overdrive electrical diagnostics question

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nevets

nevets

Jedi Knight
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I am attempting to conduct some basic diagnostics of the Overdrive electrical system and am consulting a tech brief from a 2007 issue of Austin Healey Magazine. There appears to be what I think is a typo in the article, but I am not sure. I attached a pic of the page in question with two lines highlighted. The discussion starts by describing how to diagnose the OD transmission switch, but then refers to the throttle switch. Which is it? Hopefully someone can help.
Thanks
 

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Looks like a typo to me. The 'throttle position switch' is only there to keep the O/D activated until the throttles are opened a certain amount; i.e. the drivetrain is being driven by the engine and not the rear wheels. If it's not working correctly the O/D will drop out as soon as the panel switch is moved to non O/D mode, which can result in reverse torque being applied to the drivetrain and possible damage to the O/D. Note the O/D relay can prevent the O/D from engaging, or cause it to cut in and out of O/D.

The shop manual has good O/D diagnostics; if in doubt check there (this writeup looks like just a rephrasing of the shop manual diagnostics).
 
Thanks for the reply. I'm venturing into unknown territory here. I am attempting to troubleshoot the electrical components with a jumper and a newly purchased multimeter. My hunch is that the solenoid is bad, but thought I would eliminate all the other, more accessible electrical components before removing the transmission tunnel (ugh).
 
nevets said:
I am attempting to conduct some basic diagnostics of the Overdrive electrical system and am consulting a tech brief from a 2007 issue of Austin Healey Magazine. There appears to be what I think is a typo in the article, but I am not sure. I attached a pic of the page in question with two lines highlighted. The discussion starts by describing how to diagnose the OD transmission switch, but then refers to the throttle switch. Which is it? Hopefully someone can help.
Thanks
Click to expand...

It is a typo, because the instructions are attempting to check the gearshift switch by bypassing the overdrive relay and connecting the gearshift switch and solenoid directly to 12 volts. However, there is another mistake in the instructions. The ground wire at the overdrive relay is connected to terminal W1, not terminal W2 as the instructions indicate. I would have said to pull off the wire at terminal C2 and connect the jumper wire between it and the fuse box terminal.

If you hear the solenoid clicking when you follow the procedure, then both the gearshift switch and solenoid are working. If you don't hear the click, it could be either the switch or the solenoid at fault (more likely the solenoid) and further investigation would be required.
 
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nevets said:
Another typo...that's a worry. Here's a link the the entire article, written by some Healey heavyweights.

https://www.healey6.com/Technical/Tech-OverdriveTroubleshooting.pdf
Click to expand...

Even the Healey heavyweights can make mistakes. Two of those heavyweights wrote a Healey reference book and it has some errors, too.
The BMC shop manual shows terminal W1 of the overdrive relay connected to ground, not terminal W2 as shown in the article's diagram. I imagine that the labels on their diagram got reversed, and that led them to get it backwards in the text. Since W1 and W2 are the two ends of a small solenoid coil inside the relay, it shouldn't matter which is connected to ground, but make sure you know which one is on your car. Still, the BMC shop manual should be the standard.

I have another version of the overdrive electrics wiring diagram if you want it. Contact me at sbyers@ec.rr.com if you do.
 
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