81 300d 2nd gear non existent - PeachParts Mercedes-Benz Forum

nr5p · #1 01-17-2017, 06:18 PM

Got another Mercedes diesel. This is an 81 300d. Very very nice, my last one was worn out and this one has 125k miles and I think it's accurate due to condition.

I've felt with these before so I'm fairly knowledgeable about the typical issues. The 81 300d has no Bowden cable. Only uses vacuum. Previous owner had transmission shop install new modulator valve and it IS holding vacuum. I checked vacuum after the vacuum valve on top of valve cover that's controlled by throttle. Well it's about 20hg and drops to 10 with throttle full. I think it's supposed to be 15 to 0 with full throttle. I just got it 2 days ago so I haven't had a chance to change transmission filter try.

Now here's what it's doing. With vacuum hooked up it flares in all gears and slips very bad after 1st and I let off and goes into 3rd.

With vacuum disconnected (0 hg) flare is better from 3rd to 4th but 2nd is still pretty much non existent. I remember some spring kits for the valve bodies on these. Seemed like the were some issues with some springs breaking. Assuming the changed filter doesn't help am I missing anything? If I disconnect vacuum it should be like I'm accelerating full throttle from a stop so this should eliminate the vacuum valve on valve cover from causing the problems. Any other ideas?

tyl604 · #2 01-17-2017, 07:42 PM

Correct about vac. It should bleed down to zero vac when you hit the accelerator and shift smoothly with zero vac. Only needs vac for the downshifts which will be clunky otherwise.

Your problems do not seem to be vac related.

nr5p · #3 01-18-2017, 09:20 AM

2nd day with vacuum disconnected from transmission and at least when cold it's using second gear for like two seconds then flares on shift into 3rd. When manually putting into second it stays in 3rd not second. Maybe valve body has crud in it from sitting

Frank Reiner · #4 01-18-2017, 10:05 AM

Quote:

Originally Posted by nr5p

Got another Mercedes diesel. This is an 81 300d. Very very nice, my last one was worn out and this one has 125k miles and I think it's accurate due to condition.

I've felt with these before so I'm fairly knowledgeable about the typical issues. The 81 300d has no Bowden cable. Only uses vacuum. Previous owner had transmission shop install new modulator valve and it IS holding vacuum. I checked vacuum after the vacuum valve on top of valve cover that's controlled by throttle. Well it's about 20hg and drops to 10 with throttle full. I think it's supposed to be 15 to 0 with full throttle. I just got it 2 days ago so I haven't had a chance to change transmission filter try.

Now here's what it's doing. With vacuum hooked up it flares in all gears and slips very bad after 1st and I let off and goes into 3rd.

With vacuum disconnected (0 hg) flare is better from 3rd to 4th but 2nd is still pretty much non existent. I remember some spring kits for the valve bodies on these. Seemed like the were some issues with some springs breaking. Assuming the changed filter doesn't help am I missing anything? If I disconnect vacuum it should be like I'm accelerating full throttle from a stop so this should eliminate the vacuum valve on valve cover from causing the problems. Any other ideas?

Quote:

Originally Posted by nr5p

2nd day with vacuum disconnected from transmission and at least when cold it's using second gear for like two seconds then flares on shift into 3rd. When manually putting into second it stays in 3rd not second. Maybe valve body has crud in it from sitting

In the 722.1xx, 2nd gear is the only use of band B1. Look to the condition of the band friction material (lining), or the servo piston and its seals.

Prior to opening the servo, change the filter & fluid, and increase the modulating pressure (which increases the working pressure). The modulating pressure is adjusted with a screw inside the vacuum fitting opening of the modulator. 2-3 turns of the screw to begin. It that is too much, the flares will be replaced by hard shifts.