M46 4th Gear Switch Not Working when Cold

Driving a 1986 240 Wagon. Has anybody had this issue? I can't get a signal through the 4th gear switch when testing. Drive it, and it doesn't work. I then changed the oil to 10W-30, now it works only when the transmission fluid is warmed. In 35 degree weather, I need to drive about 4 miles before it warms. In 50 degree weather, 1-2 miles will do (it hasn't warmed up yet in Michigan). I bet 5W-30 oil will be better, because it will get thrown-up to the switch quicker.

Does the warm fluid do something to the switch to make it work? How difficult is this switch to replace?

Have you checked the wiring? Its probably toast by now.

Sounds like the OD needs new O rings.

white855T said:

Have you checked the wiring? Its probably toast by now.

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First check for a poor connection. , brighten them up and use some AZCP. Same thing is ox-gard from Home Depot.

http://www.sw-em.com/anti_corrosive_paste.htm

And yes, check the wiring for poor condition.

Couldn't notice anything with the wiring to the switch. It looked good. I did re-ground the solenoid. That still had continuity, but seemed frail near the solenoid end.

Everything seems to depend on trans temp. I forgot to include this bit of information. When the trans is almost warm enough for OD to work normally, I need to push the lever slightly towards 3rd gear for the OD to engage. When fully warm, it will engage without playing with the lever.

Redwood Chair said:

Sounds like the OD needs new O rings.

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Thanks. O-rings between what components?

M46 calls for ATF type F... I was told that a long time ago by a Laycock engineer. Get some Amsoil Super Shift Type F and get that motor oil out of there as it's not supposed to be used with wet clutches. There are a lot of o-rings in the overdrive but Redwood is probably referring to the ones on the pistons. Go to this site and type overdrive in the search field, you'll get all the Greenbooks you'll need.

I think there is a 4th gear switch failure mode where it gets marginally harder and harder to make contact. Key evidence is having to force the gearshift (usually over to the side, forget which side) to over-engage the switch to get it to work.

Another important sign is whether the OD locks up correctly (once overcoming the switch's reluctance) and doesn't slip, cold or hot. That would indicate a switch problem rather than internal OD problems such as actuating piston o-rings as previously mentioned.

When warm things are expanding a bit and that probably lets the contact touch the linkage easier inside the switch. One of the fourth gear switch fail modes is having to move the shifter to make it work. It's a single contact that provides ground to the relay when in fourth gear. The wiring decays and also causes this type of issue. So, check out the wiring you may need a new pigtail spliced in. Like what Dave Barton sells.