On my (edit: manual) 1987 740, shifting into 5th gear (is it actually an overdrive?) via the electronic button on the stick is iffy. Sometimes it works fine, usually for a whole trip or a whole day, but then next time it won't do anything (not engage) and stay that way for awhile. I am guessing it is a bad connection somewhere, but I don't know where to look. I mostly drive in the city, but it is still useful. For what should I look?

 

First I assume this is an automatic? If that is so then there is no 5th gear . It is a 4 speed auto with a lock up torque converter. The button on the shifter allows you to step down to 3rd gear when you press it. Is the down green arrow on in the instrument panel when you press the button ? Please provide more information .

 

Sorry I was unclear. No, it is manual. When it is working, pressing the button lights up a green "5" on the instrument panel

 

OK got you. That would be the overdrive unit. If it is intermittent then I would believe it is an electrical problem. Does the green light stay on when the problem happens? If you press the OD button when the unit drops out does it work again. See if you can get the wiring diagram to look at what to check. I don't have the OD unit in my service manuals.

 

The problem isn't the overdrive dropping out. Usually when I push the button, nothing happens and no light turns on. When it works, the light pops on and overdrive is definitely engaged. I will look around for a diagram. I am thinking maybe there is a loose connection in the shifting stick itself, because (according to stuff online) the top where the button is can be removed.

 

Those wires under the shifter boot that go to the transmission can get tore up rubbing on the sidewall there. undo the tabs and inspect the wires best you can, be gentle with them. but ill bet you will se a little place where the insulation has rubbed away. there is one section of wire that goes into the dash that has your OD relay, and the other section that goes to shifter, and then to the OD unit. I believe its a blue and a yellow wire.


Also the top can be removed, just pry the R-1-2-3-4 cap off from behind and the button will be staring you in the face, it also come up but be gentle.. ha hope this helps

 

Thanks! I haven't had time to properly look at it, but as soon as I can I will be--carefully--prying things apart haha.

 

FWIW, on a 1992(*) 740/940 M46, the wires to the button are brown and blue. both go through a 2-pin connector "B" near the central electric unit, and from there to the OD relay. The Blue wire is powered (via fuse 12) and the brown wire is on the other side of the button.

There is also a blue-black wire that comes from a switch on the gearbox which tells the relay that the car is in 4th so it can enable the OD on demand, this blue-black wire goes through another 2-pin connector, along with the yellow wire that goes to the actual OD solenoid/actuator on the OD unit, and the other side of that 5th gear sensor switch is to ground.



(*) I don't have prints on earlier models than 92, so the color codes /could/ be different on your 1987.

 

I poked around, and found where the two wires go through a 2-pin connector, as described, though I didn't see any noticeable degradation there. Bumblebeeman, do you mean under the rubber section? I don't know how (or if) that comes off, the wires were sort of pressed up by the side of that, under the edge of the surrounding plastic.

I haven't tried prying the cap off to look at the button itself, though pulling off the whole **** I could see the two wires appear to attach in there securely.

I haven't found a real pattern of when it works vs doesn't, though I would tend to say it works more often on warm days (could be coincidence, especially if it is just a bare wire at fault).

 

Directly under the button, on the shifter **** is usually fine IME, was everything connected in there? Then its fine.. Yes the ripper shift boot comes up, it is held in by 4 tabs on the corners. feel under the carpet and pull it back with a pick one corner at a time and the boot will pull right up and the tranny and wires will be staring you in the face. the lip on the shifter hole can be a little sharp and the constant rubbing from shifting gears for 30 years had in my harness worn some small cracks and holes in the insulation. this could be your problem.

It could also be a faulty relay, pierce or the bently manual may have the testing sequence for that relay. as i think that are kinda pricey.

 

When I looked into removing the rubber shift boot, it looked to be attached by some bolts (see the pictures). Is that what I need to remove? I didn't see what you described with the tabs and such.

I hope it isn't a relay issue...

 

in my case all I had to do was find out which fuse handled the overdrive make sure the fuse and connections are clean and I literally have to twist the fuse sometimes and then it will drop into overdrive

 

you replied to a 6 year old post...

That said, 240 ceramic fuses should all be replaced with the updated Flosser brand copper/brass real ceramic replacements... the tin ones corrode and make intermittent connections. clean the contacts of each fuse holder using a soft pink pencil eraser.