91 300D 2.5T Cold Shifting Problem - PeachParts Mercedes-Benz Forum

mcaro · #1 12-10-2003, 10:26 AM

When leaving in the AM and the car stone cold, it will not shift out of 1st gear until around 4000 rpm. 2nd the same way maybe not as high in rpms, and I count my self lucky if I see 3rd. I will not get to 4th until the car is fully warmed and been running for say 20 minutes or so. This is new behavior since the weather has turned colder.

I have not checked the AT fluid level but have no worries there as I've never seen any fluid under the car. I have checked as many vaccum lines under the hood as I could see and nothing looks disconnected and all connections look tight. I'm thinking this is a vaccum related problem but don't know where to look next...

TIA, Mark

Grdenko · #2 12-10-2003, 05:11 PM

i dont know how cold it is wherever you are, but it would probably be best to let your car warm up before you start driving, at least for 3-4 minutes. but i think your problem may be related to a vacuum leak. the system can still hold vacuum, but it may take a long time for it to build up because of small leaks. warming your car up would probably solve that too.

gsxr · #3 12-10-2003, 05:38 PM

The shift firmness is controlled by vacuum. The shift point (MPH/RPM) is determined by the cable, and the internal valve body & fluid etc. You need to check the fluid level - absence of visible leaks means nothing. Next you need to change the fluid & filter if it hasn't been done in the last 25kmi. You need to do this every 30kmi or sooner (I can tell after about 20k that the shifting gets noticeably less smooth, especially when cold). Finally, while you're chaning the fluid, use synthetic like Mobil-1 or Amsoil ATF. That makes a big difference in cold shift properties.

mcaro · #4 12-11-2003, 08:36 AM

I haven't been driving the car lately the wife has and two nights ago the battery went dead so I went to replace it. The car had been sitting about 8 hours, ambient temp 35, and when driving home is when I first noticed this not wanting to shift problem.

Last night I went a little further and not only rechecked vacuum lines but also cleaned the boost fitting on the intake manifold as well as the electrical fitting just above it. They were both pretty mungged up with soot. The warm performance did improve, but I will not be able to drive the car from stone cold until the weekend.

I always thought shifting was controlled by both vacuum and the bowden cable. That's why I was thinking vacuum in my case because of the inherent maintenance problems this system has as well as the strange things that happen on an older car when the weather does get colder. I did check the AT fluid level when Janice got home last night and it's OK, actully a little overfilled. The color didn't look all that good and it's about 10K overdue as it stands now for a service, so I will plan to change this weekend and hope cold shifting peformance improves.

- Mark

gsxr · #5 12-11-2003, 01:30 PM

Good idea. Remember you need to drain the pan AND the converter, AND change the filter. Much better to underfill than to overfill. And I do highly recommend using synthetic ATF, many people who have switched say it's a really noticeable improvement compared to the dino fluid.

mcaro · #6 12-15-2003, 09:20 AM

Dave,

I changed AT Filter and Fluid with Mobil-1 but the cold-shifting condition has not changed. The car does not want to shift properly under low/moderate speeds when cold. Yesterday(Sunday), I went out in the evening and the car had been sitting since Saturday afternoon after the AT service. Cold shifting again was prolonged and it would not shift into 4th gear for the 5mi drive over to a friends house. Went out maybe 30 minutes later up to store and the car shifted fine (low/moderate speeds shifted through all 4 gears). It is only when the car is stone cold it seems that this problem shows up.

After the service on Saturday, I did drive the car around to warm the fluid and get the proper fluid level set. The car had already been driven earlier in the day and shifiting was for the most part normal, got better again when the car was fully warmed up.

Could this still somehow be vacuum related? Are the any 'most common' areas to look? I have already cleaned the boost fitting on the intake. I need to look at the overboost valve (that one gave me a problem on the wagon) and check for restriction. I'm open to any and all other ideas before sending it to the shop.

TIA, Mark

gsxr · #7 12-15-2003, 01:01 PM

Wow - that is really strange! I'm stumped. Maybe the kickdown soleniod circuit is acting up (inside the tranny)? I'd post at a couple other MB forums, and the MBZ.org email lists, before resorting to a shop... they're quite likely to try and sell you a rebuilt tranny for ~$2000 installed.

mcaro · #8 12-15-2003, 01:27 PM

Thanks Dave,

I did search over at the mbz.org list and found someone with a similar but not the same problem. Their tranny had just been re-built and after 25K it would not shift out of 1st. The problem turned out to an internal ball/spring check valve of some kind. Since mine will shift but requires full warm up, I'm hoping there are no internal problems...

I know you are knowledgeable regarding these cars and have another question for you regarding the vacuum systems on the OM602/603, is there any type of vacuum metering/contol going on based on coolent temp when the engine is cold? What might you look at in the vacuum system since fluid/filter didn't change anything? I still want to pursue the vacuum angle and plan to bypass the overboost valve/switch tonight to see if that helps any.

- Mark

gsxr · #9 12-15-2003, 01:37 PM

Hi Mark,

Yeah, it's strange about your '91 tranny - I believe it's something internal, but I don't know what.

About the coolant temp thing. On 86/87 cars with a 603, there is a temp switch that affects the tranny vacuum system. It makes the shifts *softer* when coolant temp is below 50C. I disconnected this on my cars because I found it made the shifts flare when cold, and I didn't find them harsh with the system disabled. It has nothing to do with shift points though, or not letting the trans upshift. Things to look for in the vacuum system are broken or cracked hoses & plastic pipe, or other leaking items. You'd want to apply vacuum to the tranny modulator with a Mity-Vac, it should hold vacuum - if not, the modulator is messed up. There are some other things too, read this article for more info (mostly applies to pre-90 cars, but the basics are the same: )

http://www.continentalimports.com/ser_ic20242.html

CSchmidt · #10 12-15-2003, 09:23 PM

Dave,

I have an 87 SDL that shifts soft when it is cold also. I'd like to disconnect this feature. I have a long hill in my neighboorhood and the car flares a bit on this first 2-3 shift every morning. Is it a simple plug somewhere?

Thanks,
Chuck

gsxr · #11 12-15-2003, 09:35 PM

Chuck,

Yes, thankfully, this is easy to disable. It's a round sensor with one wire attached, directly in front of and a little above the thermostat housing. It's on the passenger side, front of the cylinder head. Simply unplug the wire and make sure it won't get snagged by anything. Your cold shifts should be nice and firm now.

CSchmidt · #12 12-17-2003, 10:15 PM

Dave,

Thanks for the pointer. I'll give it a try in the next few days. Got too much ho ho hoing to do and too many rascally computers not behaving.

BTW - my buddy is bringing his '03 F250 with the 6.0 powerstroke over tomorrow to haul enough treated lumber to resurface my dock. ( The "green" arsenic treated lumber is not for sale after 1/1/04). We'll use his car hauling trailer see how it hauls this load of lumber - I'm guessing around 4-5000# worth. My friend has turned into a real diesel convert over the past year.

Chuck

CSchmidt · #13 12-22-2003, 10:35 PM

Dave,

Thanks for the tip. It does shift more quickly with the sensor disconnected. It still seems to want to rev the motor - no 2-3 shift until 3000 rpm while cold (25* this morning). The flaring was better/gone going up the hill near my house, so I'm leaving it that way.

FYI - I rode in my buddy's F250 with 6.0 L diesel last week. It was towing a 1700# car carrier trailer and about 6000# or so of pressure treated lumber. Man that truck REALLY pulls. He scooted up onto I-77 and 70mph in no time.

OK - another thought. He rode over to Charlotte and we loaded the lumber. It took about 2 hours in 40* weather. When he started the truck he didn't wait for the glow time ( "wait to start" on the Ford). It started in just 1 or 2 revolutions. I think it also has what we would call an afterglow type of glowplug. So my question... still being a newbie, I always at least wait for the glowplug light to go out even when hot. Do others? After a couple of hours?

Chuck

mcaro · #14 02-03-2004, 04:29 PM

I just wanted to follow up on this issue as I have corrected the problem without replaceing any major components, namely the transmission.

While I was having the shifting anomoly, the car's glow light began to signal that a glow plug(s) had gone bad (glow light wouldn't come on when key was turned and would come on after the car started). I decided to replace all 5 glow plugs. In doing so, I removed the intake manifold, injector spider lines, and all associated shift linkage (ball sockets etc) in this area. When putting everything back together, I took the time to lubricate all the throttle linkage ball/sockets. There is a part of the throttle linkage assembly where the boden cable conncts to (whick is on the passenger side on a 2.5DT) that seemed stuck. When actuated it would not return smoothly. I cleaned and lubricated this mechanism along with all the other throttle linkage ball/sockets and shifting retruned to what it had been.

So to return to my original post, the lesson learned is that in following the KISS principle, I should have looked at an even more basic mechanism (throttle linkage) prior to condeming the vacuum system or transmission altogether.

Mark