I've been trying to fix my overdrive. After driving for a 30 minutes it starts to go in and out of OD. I took off the shift boot and got rid of all the plastic connectors so the wiring should be good, The switch is new and the fluid is topped off. I don't think that there is a relay on the 1980. What's next?
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1980 240 324k 20 years.

After reading ODGuru's OD diagnosis I've determined that my problem is more than likely the 4th gear switch. The OD comes off when I rest my hand on the shift lever and has been doing this for years. It has got yo the point where it goes out on its own though ususlly on bumpy highway. Is the 4th gear switch easy?
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1980 240 324k 20 years.

Easy?? Maybe. Depends on how many joints you have between your elbow and finger tips. If you have the standard issue from birth, possibly not enough. There sometimes is just enough space for a mid length socket on a wobble or stiff universal, but the task is doable.

A trick I use to open up access: Place a fine control jack under the tranny. One at a time, remove then reinsert three full turns the left side trans cross support bolts. Remove both right side bolts. Slowwly lower trans until support hist left bolts and rests level. Check access to switch. If needed, lower jack a bit more which rolls the trans to the right opening up the space quite quickly so drop only small amounts and only enough to access switch.

When finished, as you jack everything back up, use a large Phillips driver or drift to guide the right support back to the correct hole alignment. Make sure the bolts on both sides of the support are tight.

Duane

Sorry for a late response. Regular job had a few items to prep for a long week out of shop ahead.

How OD goes in and out is key.

First make sure the tranny is full of fluid. Low fluid when hot will cause problems that make tracing while moving impossible by giving false information.

Off and on with less than a second for the action and a solid "contact" of the unit is most likely electrical, even though you think you have all the connections covered.

Off and on with shudder and slipping is fluid related. This requires new O-rings and removal of the OD unit if the tranny is full of fluid.

If the dash light flickers when the OD is going off and on, the problem is in the wiring to or the slide switch in the shifter. If the dash light stays steady, problem is in the wiring under the auto or the 4th gear switch.

Electrical problems you may have missed. 4th gear switch, internal, which is not repairable. It needs repelacement. Test is while driving, if traffic allows this safely, when the off-on starts, pull the shift lever back and to the right, which pushes the rod further onto the 4th gear switch. If off-on stops, and if SAFE in traffic and at a lower speed, push the lever forward a bit with out taking out of gear and hold it there. If OD turns off and stays off, culprit is 4th gear switch. Or do this while standing still: see test below.

Contacts at the 4th gear switch dirty or loose. Above test will do nothing to Off-on sequence.

Terminals on the solenoid. The wires at the crimp love to break inside the insulation and remain hidden by the insulation. Also the wire from the 4th gear switch to the solenoid can break internal. Getting pused around by the air flow under the auto causes intermittant e4lectrical contact.

Test is engine off, ignition on, in 4th gear. Removing wire at solenoid should click solenoid. This means you have power through the 4th gear switch. If no click, hae someone pull shift lever to right and rear. If you can get the solenoid to click by wiggling the shift lever around, 4th gear switch.

Wiggling the wire while you hold the terminal should not click the solenoid.

Your power flow for the OD solenoid is: Fuse, shifter switch, dash light AND 4th gear switch, solenoid.

Educated guess from afar. 4th gear switch first then O-rings internal.

Duane

Thanks for the great post. I'll go through the items and find out what the problem is. I read this after emailing you so I'll hold off on the O rings for now. I have a new shifter switch and re-did the wiring in the shifter boot. There were four plastic connectors which I reduced to two. Fluid is topped. the symptom is on and off with less than a second between the shift so it does sound electrical. I've never had an OD light. If there is one on an 80 then mine has never worked.
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1980 240 324k 20 years.

I had same problem with my 450k(KM) 240GL m46 b23E After eliminating all
possibility of electrical problems i changed the tranny fluid from ATF to
80W90 synthetic gearbox oil.Problem gone.I am guessing the thicker
oil is compensating for wear in the pump(the Atf is much thinner at warmer
temp)
Ps MY 81 has push button od so it has relay.Have 79 with switch and no
relay. 80 dont know
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Rene

My '80 doesn't have a relay. Slide switch for overdrive rather than a push button.
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.

Seals.

I'm assuming that the solanoid is either on or off. Does the OD have to come off to replace seals? It's one of the only things I couldn't do was getting the OD off when I replaced it a few years back. That was before I was a member of this board.
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1980 240 324k 20 years.

ODGuru, (if he doesn't see your post), has a good write-up in the FAQ (manual tran rebuild) section. Also, as that is his specialty, search his posts.

I was having trouble with mine but as I had a spare, I ended up switching rather than fixing. Will probably get around to fixing ...one day if the need arises.
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.

I used to instruct at the Brampton Flying Club. How did you get the top two bolts off the OD?
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1980 240 324k 20 years.