84 928 tranny won’t go in reverse - rebuild?
12-05-2018, 01:46 AM
#1
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84 928 tranny won’t go in reverse - rebuild?
My 1984 928S with 130,000 miles has an auto tranny which won’t go in reverse easily, sometimes have to shift back and forth with drive and gun the engine to get it to shift. Has been doing this for years, I just try not to park where I need to reverse uphill. Besides that it runs great in traffic and highway as a daily driver. Was thinking of shipping to an expert like Greg but not sure if this kind of thing is fixable or I need a new trans? Anyone else have this problem?
12-05-2018, 09:42 AM
#3
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Slippage
It keeps slipping trying to engage and when I rev enough it will sometimes finally engage and then it seems to stay engaged after that. Sometimes it literally takes several minutes of changing to drive and back to reverse with it slipping and not engaging until it engages. The local mechanic said “rebuild” but I’m wary of this just making it worse without expertise into old Porsche auto transmissions.
12-05-2018, 01:04 PM
#4
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Per page 38-14 in the WSM
Condition: No power flow in reverse gear
Correction: 1. Remove shift valve housing, check brake band B 3 and
adjustment.
2. Remove brake band piston B 3 and check seal.
3. Replace one-way clutch in gear set.
I would start with the B-3 piston, since everything else seems to be working ok. I could be wrong on this rationale, but I think the 1-way clutch can effect forward gears and if it were the band it would likely slip even once engaged.
Condition: No power flow in reverse gear
Correction: 1. Remove shift valve housing, check brake band B 3 and
adjustment.
2. Remove brake band piston B 3 and check seal.
3. Replace one-way clutch in gear set.
I would start with the B-3 piston, since everything else seems to be working ok. I could be wrong on this rationale, but I think the 1-way clutch can effect forward gears and if it were the band it would likely slip even once engaged.
12-05-2018, 01:31 PM
#5
Pro
Had a similar problem on mine, pulled the trans and found the seal blown on the B-3 piston. Replaced the worn disc pack, resealed the piston, flushed the trans and put it back into the car. That was 10 years ago...working flawlessly since then. Apparently it is a common failure on MB transmissions of that era.
12-05-2018, 02:34 PM
#6
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Yes, the B3 clutches are the first to wear out/have the friction material fall off. They will get so thin that the B3 piston will travel so far that it will put the return springs into coil bind and blow the spring retainer off of the front pump housing.....making a mess out of a lot of parts.
Fix it before it blows up...much cheaper!
BTW....The latest Mercedes B3 clutches are lined with "tougher" material, which engages harder....sometimes objectionably hard. There's a couple of things that can be done to a fresh rebuilt to soften up the initial engagement, if necessary.
Fix it before it blows up...much cheaper!
BTW....The latest Mercedes B3 clutches are lined with "tougher" material, which engages harder....sometimes objectionably hard. There's a couple of things that can be done to a fresh rebuilt to soften up the initial engagement, if necessary.
12-05-2018, 10:46 PM
#7
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Rear Gear
So I drove it around some today in reverse, trying not to hit any cars or dogs, and found that it has a lot of trouble getting into reverse. But once it’s in reverse, it’s solid and doesn’t slip at all.
