'85 300D trans problem? Any ideas? - PeachParts Mercedes-Benz Forum

1985-300D · #1 08-26-2017, 01:20 PM

I got an 85 300D a few months ago. 230,000 miles, nice shape, runs great. Needed typical minor work like tune-up and such. Been extra careful inspecting everything seeing as I got it from a little old lady and judging by the repair paperwork she gave me, she'd been getting taken advantage of by her mechanic. (Billed for 5 Bosch glow plugs, got 4 autolites and they didn't try replacing the rear one. Billed for 4 calipers and all pads, got 2 new calipers but brake pads were almost metal on metal, etc). Also, the vacuum lines to the locks and such seem to have been cut and plugged off, but that doesn't bother me. It's been running, driving and shifting perfectly up until a few days ago.

So I've changed all fluids, glow plugs, got everything working that didn't and have been driving it about a month. Everything was excellent. Trans fluid was at the correct level, looked nice, didn't smell burnt, so I left it alone for the time being. The other day while driving I noticed a bit of a vibration like a wheel out of balance and a sensation of being "held back" like my brakes were hanging up and the car was fighting it. Almost like tapping the brake pedal while driving because it would come and go.

Acceleration is no good now, it goes into 3rd fine, but acts as if I'm holding the brakes. It gets worse as I drive, it will rev to 3500 and stay in 3rd. I can let off the gas and then it will clunk into 4th and that will feel like driving a manual in the highest gear at 20mph. Can't increase or maintain speed. The smallest press of the accelerator then drops it back into 3rd. Oddly it doesn't feel like it's slipping. It's even like the car is actively slowing down when I try to accelerate. Like it's fighting something, or trying to pull something far too heavy. Like I have a semi tied to my back bumper. I can put it in neutral and it rolls along just fine though. Everything spins freely, nothing is dragging or hanging up. Every now and then it would act like it might snap out of it and would start picking up speed, only to start slowing again and worse.

I was on my way back from a 50 mile trip and by the time I got home I could barely do 35mph. Trans fluid looked great still, no shavings or discoloration but I drained the pan and torque converter, and changed the fluid and filter and it had no effect. Acceleration is slow, acts like something is actively holding the car back and slowing it down, if I let off the gas it shifts to 4th but immediately drops back to third if I push the gas as if I'm going up a steep hill. Engine runs like a dream, all fluids and filters are new, everything greaseable is greased. This is a sudden thing that happened during one drive and now seems like it's here to stay. Has anyone had anything like this happen? I've looked but it seems everyone else mostly has normal slipping or typical vacuum issues.

I have two parts cars with good transmissions, and while I'd rather not, I could replace it. Problem is the other cars are an '83 and an '84 an I've heard they won't interchange. Any help or advice would be definitely appreciated, because I'm all out of ideas!

dieseldan44 · #2 08-28-2017, 08:53 PM

hey - 85 300D -

1. try removing the wire from the kick down switch if that has an effect.
2. remove all vacuum signals from the transmission - put it off at the tee
3. how long does it take to go from P to R or D?
4. check the bowden cable for signs of wear / tear / loosness / cracks. if this is off it could create all sort of issues.
5. try shifting between 1, 2, 3 manually like you would on a standard

where are you located? I have a lead on a parts car 85 for $200 with a good trans in the Boston area im looking for an excuse to go get.

try those out and report back...ill help you through this :-)

Diseasel300 · #3 08-28-2017, 09:35 PM

Sounds to me more like you have dragging brakes or fuelling problems to the engine. When you let off the gas, how quickly does the car slow down when you should be coasting? If you're losing speed relatively quickly when you let off the gas, you probably have brake problems physically slowing you down.

If on the other hand, you just have no power, you could have a blocked ALDA line or be in need of fuel filter service. You may or may not know, but the 80s cars don't have a lockup torque converter, it is very unlikely to be the transmission that's the issue unless you're shifted into 3rd stupidly early (like before 20mph with your foot hard down).

Check for brakes dragging first, then do a fuel system service. See where that gets you.

Volvo Diesel · #4 08-28-2017, 09:36 PM

I know you mention in your post that the fuel filters are new but...to me your issue sounds as though it could be fuel related. The behavior you describe sounds exactly how my car acted when the strainer in my fuel tank got clogged with muck after I let the car sit unused for a period. The filters looked fine on mine, by the way; the strainer was probably 95%+ clogged. Might be worth a look...

Also, the transmissions from the earlier models will work. Is your 1985 a California emissions-compliant model, or a federal model? I was under the impression that only the 85 CA models were equipped with the 722.4 and that the 85 Federal models had the 722.3. Either way, they can be swapped. I believe that forum member mach4's 617-powered SL started out with a 722.4 but was later switched to a 722.3 when the first transmission failed.

chasinthesun · #5 08-29-2017, 04:09 PM

I would make a ck of all conrods to confirm their postion is correct,espicially the cruz ,a miss step here wont allow the vac modulator for trans to function properly.
I had sold a 300cd back last yr. and Mentioned a mechanic to the buyer ,this mechanic tells me later the car I onced owned had a bad torq converter .It would allow the car into 2nd only ,no shift after that.