‘89 doesn’t shift into 4th gear (overdrive)

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75r6q3
Posts: 14
Joined: Sun Mar 01, 2020 11:05 pm
Location: Massachusetts, USA

‘89 doesn’t shift into 4th gear (overdrive)

Post by 75r6q3 »

Hi guys,

My ‘89 has been having this issue when I first acquired it but it’s been an on and off problem. Sometimes when I put the shifter in “D”, the gear indicator won’t light up in the gauge cluster and the car doesn’t shift into 4th gear. I’d be doing 70 mph at 4000 rpm on the freeway whenever that happens. But sometimes it fixes itself when I go over a bump or something (doing this could potentially reintroduce the problem when it’s working fine, very unlikely but it has happened). I’ve read another post from a while back with the similar issue but he solved it by replacing a fuse, I tried that but it didn’t work. I’m assuming it’s probably a loose connection at the gear selector but can’t be completely sure, has anyone had similar issues?

So here is a list of things I’ve done that have fixed it in the past:
Simply waiting while driving using only the first three gears for 5-15 minutes
Banging hard on the shifter
Going over a bump
Serving the transmission

Here are the list of things that have introduced the problem in the past:
Going over a bump
Shifting too fast from R to D
Battery dying and me having to jump start it
And sometimes it just decides to stop working for no apparent causes

Thanks a lot
75r6q3
Posts: 14
Joined: Sun Mar 01, 2020 11:05 pm
Location: Massachusetts, USA

Re: ‘89 doesn’t shift into 4th gear (overdrive)

Post by 75r6q3 »

So I just wanna add that every other gear works perfectly, I don’t think it’s permanently stuck in sport mode, but I think it thinks I put it in “3” rather than “D”…
plip1953
Posts: 400
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Location: Leeds

Re: ‘89 doesn’t shift into 4th gear (overdrive)

Post by plip1953 »

If you're in Drive and all is normal, what happens if you select Sport or 1-2-3? All normal? But if it's having a funny turn and you do the same, does nothing happen? Or does 1-2-3 operate as it should, but for Sport it's as if you are in it already?

And if your going along happily in D, and D is illuminated on the dash, does the the D light extinguish when it has a hiccup and puts you in 3rd? And the other way round, if the problem rectifies itself eg when going over a bump, does the D light illuminate?

I have a vague recollection that the shifter cable may has scope for adjustment where it attaches to the gearbox. Maybe that's worth looking into a bit more? And it there is no length adjuster, maybe the cable itself has stretched and needs replacing?

But what puzzles me most is why D illuminates only intermittently. So what you're saying is no light illuminates in those circumstances?

Have you located the gearbox control unit which I presume it somewhere behind the dash? Exercising the connectors to both that and other parts of the control unit harness might be worth a go? And swapping out the control unit is of course another option, but I guess they are either hard to come by second hand and/or expensive.
Phil
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1988 Highline 635 CSi
75r6q3
Posts: 14
Joined: Sun Mar 01, 2020 11:05 pm
Location: Massachusetts, USA

Re: ‘89 doesn’t shift into 4th gear (overdrive)

Post by 75r6q3 »

plip1953 wrote: Tue Apr 12, 2022 7:53 am If you're in Drive and all is normal, what happens if you select Sport or 1-2-3? All normal? But if it's having a funny turn and you do the same, does nothing happen? Or does 1-2-3 operate as it should, but for Sport it's as if you are in it already?

And if your going along happily in D, and D is illuminated on the dash, does the the D light extinguish when it has a hiccup and puts you in 3rd? And the other way round, if the problem rectifies itself eg when going over a bump, does the D light illuminate?

I have a vague recollection that the shifter cable may has scope for adjustment where it attaches to the gearbox. Maybe that's worth looking into a bit more? And it there is no length adjuster, maybe the cable itself has stretched and needs replacing?

But what puzzles me most is why D illuminates only intermittently. So what you're saying is no light illuminates in those circumstances?

Have you located the gearbox control unit which I presume it somewhere behind the dash? Exercising the connectors to both that and other parts of the control unit harness might be worth a go? And swapping out the control unit is of course another option, but I guess they are either hard to come by second hand and/or expensive.
Hi, thanks a lot for taking your time to answer! The situation I’m having is that the D gear only functions as intended when the D gear indicator lights up. That is if I go over a bump and triggers the problem, the indicator would go away and the car behaves as if I’ve shifted into 3. Every other gear/mode works perfectly, and there is a noticeable difference when I put it in Sport (works perfectly as well).

So the only issue, to phrase it better, is that the car would sometimes think i selected 3 instead of D and block off the 4th gear, and whenever it does that, the gear indicator for D would go away as well. The D indicator is a reliable indicator of a working 4th gear in my case. I have not personally checked out the gearbox control unit yet cuz I’ve been fiddling with the wires recently, but I’ll give that a look! Thank you so much for the detailed answer :D
plip1953
Posts: 400
Joined: Mon Aug 13, 2018 4:15 pm
Location: Leeds

Re: ‘89 doesn’t shift into 4th gear (overdrive)

Post by plip1953 »

Just to be clear - when D is selected (and illuminated as normal), but you go over a bump or whatever and it extinguishes, does 3 light up instead or is nothing illuminated?

Assuming the latter, I'd almost certainly be thinking in terms of a control unit issue, but that's only on the basis of my attempt at logical thinking rather than any real knowledge regarding such matters.

Or maybe there's a simple earth fault somewhere?
Phil
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Re: ‘89 doesn’t shift into 4th gear (overdrive)

Post by Pod »

I’d try substituting a ‘known good’ selector switch as a first attempt.
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