Transmission control unit
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- Posts: 47
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Transmission control unit
The transmission on my auto 1989 635Csi worked fine with no problems whatsoever...smooth shifts every time. However with 120,000 miles on it I decided to do a little preventive maintenance and change the trans fluid and filter. Put it all back together very carefully (three times) but no matter what I did the car would not shift into reverse. The trans now also will pop into neutral in first and second gears. There was no sound whatsoever when I shifted the lever into reverse. Just complete silence. I took the pan off again and took the valve body out and had it disassembled, cleaned and checked very carefully and then pressure tested at a local transmission shop. All good. The mechanic said the valve body looked brand new. Reinstalled the valve body and still no reverse... not even a click. I noticed that there is something called the transmission control unit in the left hand drivers kick panel area. (Page 7000-4 in the Electrical Repair Manual. Is it possible the no reverse problem is being caused by a faulty transmission control unit? I have a hard time believing the reverse is bad when there was never even a hint of slippage or hesitation in all forward gears and reverse.
Re: Transmission control unit
Are you absolutely sure you have re-filled the gearbox to the correct level?
Have you checked that the cable link from gear lever to the box is responding to all lever inputs?
Does the rotary mode change knob on the trans tunnel register the different settings?
I'm clutching at straws really, but thought it might at least be worth asking the questions.
I know exactly what you mean about there being a clunk when reverse is engaged, but is that indicative of reverse gear actually being selected or it is the taking up of drive once selected - if you see what I mean?
Have you checked that the cable link from gear lever to the box is responding to all lever inputs?
Does the rotary mode change knob on the trans tunnel register the different settings?
I'm clutching at straws really, but thought it might at least be worth asking the questions.
I know exactly what you mean about there being a clunk when reverse is engaged, but is that indicative of reverse gear actually being selected or it is the taking up of drive once selected - if you see what I mean?
Phil
__________________________
1988 Highline 635 CSi
__________________________
1988 Highline 635 CSi
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- Posts: 47
- Joined: Mon Mar 11, 2013 12:06 am
Re: Transmission control unit
That's the thing. There is no clunk or any sound whatsoever when the lever is moved to the R position. Shouldn't there be a noise of some kind if the transmission parts including the valve body and clutch plates etc. are being told to activate reverse? The lack of any noise makes me think the problem is electronic. A signal is not getting sent to the trans to activate reverse. ?????
Re: Transmission control unit
This might be worth a quick read.
viewtopic.php?f=2&t=29420&p=212324&hili ... it#p212324
It seems to suggest that a problem wit the gearbox control unit would result in a dash warning light. Also that the gearbox would/might go into a limp mode with only 3rd and reverse available.
By the way you describe your problem neither of these things is happening, which tends to suggest to me that your particular issue is more likely mechanical rather than electronic.
Are you certain the lever on the side of the gearbox is moving through it's full range in sync with the cockpit selector handle?
viewtopic.php?f=2&t=29420&p=212324&hili ... it#p212324
It seems to suggest that a problem wit the gearbox control unit would result in a dash warning light. Also that the gearbox would/might go into a limp mode with only 3rd and reverse available.
By the way you describe your problem neither of these things is happening, which tends to suggest to me that your particular issue is more likely mechanical rather than electronic.
Are you certain the lever on the side of the gearbox is moving through it's full range in sync with the cockpit selector handle?
Phil
__________________________
1988 Highline 635 CSi
__________________________
1988 Highline 635 CSi
Re: Transmission control unit
Hello,
Everything worked fine before,
suggests you do not have enough fluid in the system, during refill, you have to shift thru gear’s
Slowly, with out any load, You do not want to run fluid foamy, best to have the rear wheels up In the air, make it easy to engage, and see if there are turning, check dipstick with engine running, in park or neutral,
Good luck
Franke
Everything worked fine before,
suggests you do not have enough fluid in the system, during refill, you have to shift thru gear’s
Slowly, with out any load, You do not want to run fluid foamy, best to have the rear wheels up In the air, make it easy to engage, and see if there are turning, check dipstick with engine running, in park or neutral,
Good luck
Franke