1983 240D transmission issue - PeachParts Mercedes-Benz Forum

cjlipps · #1 08-02-2010, 11:57 PM

I have another thread on this but I'll try to reword the situation to see if I get more responses.
The car upshifts too early and will go into the next gear without regard to throttle position. IOW, it shifts at the same speed no matter how far the pedal is depressed. Probably the most telling symptom is that it behaves the exact same way even with the vacuum line disconnected from the tranny.
Question: What are the symptoms of a failed modulator? I always thought that the absence of vacuum would result in hard late shifting. If the modulator fails can it cause the vacuum to have no effect on shifting? Also it seems that the electronic kickdown has no effect.
Any ideas? Thanks in advance!

toomany MBZ · #2 08-03-2010, 08:18 AM

I believe you have a Bowden cable, it would be attached to the top of the cam cover and go down to the transmission on the passenger side. There should be a plastic nut on the top side, back that nut out a few turns, it supposed to delay shifting.
Yes, too little vacuum will result in hard shifts. Is there any transmission fluid in the vac line going to the modulator? If so, replace it. Also check to see if it holds vacuum, it should, for while.
Check the kickdown switch itself, it may be hung up.

cjlipps · #3 08-03-2010, 11:04 AM

No Bowden cable here. I'll check the vacuum line to modulator for fluid.
Will a failed modulator cause early soft shifting? This one shifts the same with or without vacuum.
Thanks!

glenlloyd · #4 08-03-2010, 11:36 AM

Put vacuum on the modulator and see if it bleeds down. If it does then the unit is defective. If the leak is very slow it shouldn't be an issue unless consistent vacuum cannot be maintained. Check to make sure the vacuum line to the trans is sound also.

steve a

cjlipps · #5 08-04-2010, 12:27 AM

Well, the modulator held vacuum. I'm puzzled now. Was hoping that it was clearly a bad modulator and now I'm not sure.
What would cause this? Basically the trans is shifting the same way (early) regardless of the vacuum signal. Is there a mechanical or internal issue with the transmission that would cause this?

toomany MBZ · #6 08-04-2010, 10:15 AM

Can you post pics of the linkages?

cjlipps · #7 08-04-2010, 12:33 PM

Quote:

Originally Posted by toomany MBZ

Can you post pics of the linkages?

I can shoot some pix later this evening but (and I'm not discounting anyone's assessment) I feel certain it's not a linkage problem. I inspected the linkage and nothing has changed on it. Also, the only linkage that would affect the transmission is to the VCV and this is all moving normally. The problem started happening suddenly and if it were something external it should be obvious.
The most telling thing in my estimation is the fact that the trans shifts the same whether or not the vacuum line is connected. Also the modulator does not leak down vacuum (at least not an appreciable amount). I don't just want to slap a modulator on there unless I know it's the problem.
Thank you for the replies. I'll dig into this further and would appreciate any and all ideas.

toomany MBZ · #8 08-04-2010, 01:55 PM

Okay, pics of any vac connections too, please. MB did some odd things.
On older models the modulator did both, I understand.

chomptown · #9 08-05-2010, 12:36 AM

Are your 3/2 valves connected? The ones on top of your valve cover?

Some here have said that the transmission 3/2 valve doesn't need to be connected because the modulator should receive full vacuum. But most of those cars have a bowden cable. Since our cars don't have one, I think it relies on the throttle position which is connected to the 3/2 valve. If you take the plastic cover off, you can see as you increase the throttle, it will depress the tab on the 3/2 valve which changes the amount of vacuum going to the modulator. I'm in no way an expert on this subject, but am just giving you my observations having watched the changes in vacuum with a mity vac. If I floor the gas, it will stay in gear for longer, which is what you are looking for from what you said in your post.

Since there was no change in the way it shifted when you removed the vacuum line. I would suspect there is a leak either in a vacuum line, or in the 3/2 valve(if it is even connected). If you don't have a mity vac, it is a $12 investment that has saved me a tremendous amount of pain and suffering. Harbor Freight has em.

cjlipps · #10 08-05-2010, 10:14 AM

I have reduced my vacuum system to the barest minimum. The transmission vacuum control consists of the supply line (from the main vac line) teed into the VCV with the other branch running down to the modulator. No 3-2 valve, no EGR, nothing else on that circuit. I was gonna take pix this AM but wife has camera. Will try to do so at lunch. Nothing has changed with the vac system or the linkage. I checked out all the vac lines last night and even drove the car with various things disconnected/reconnected and nothing affected the shifting. Also the electric kickdown switch is inop. It depresses and clicks but doesn't do anything. The trans shifts fine when you select gears manually.
In my mind it has to be a modulator or an internal problem with the tranny. If it helps with diagnosis, this happened suddenly, not over the course of time. It's my DD so I keep an eye on it.

Can a modulator fail in such a way that the car shifts the same no matter what the vacuum signal to it?

What is the black box on the passenger/front side of the tranny near the starter and the dipstick that has a wire coming from the inside of the car through the tunnel?

Why won't the kickdown override any early upshifting?

Thanks again. The frustration mounts.

toomany MBZ · #11 08-05-2010, 10:47 AM

I'm not sure what a failed modulator will do.

That should be either the kick down switch or NNS/backup light switch, as that's all the electrical devices I'm aware of.

I understand a failed kick down switch will not allow the transmission to shift into fourth gear.

I don't think I helped, but you're welcome anyway. I'm sure, hopefully you'll soon find the problem.

chomptown · #12 08-05-2010, 11:54 AM

If you have contact cleaner, I would spray down the kickdown switch liberally(disconnect battery to be safe). But like they said. It wouldn't go into fourth if the kickdown switch was stuck on.

From what I take from you post, you car was functioning correctly after you removed most of the vacuum components up until lately? I was asking about the 3/2 valves, because although one of them runs the egr, the other one is related to the transmission.

cjlipps · #13 08-05-2010, 12:28 PM

Yes the trans has functioned very well since I eliminated the extraneous bits about 1.5 years ago. This problem had a fairly sudden onset.

I hadn't thought about the neutral safety switch but I looked at pix online and it doesn't look like that. It seems the NSS is part of the reverse light switch which is on the linkage input on the driver's side of the trans. The black box in question is on the passenger side front corner and has one wire with a ring connection bolted to a terminal on the side covered by a rubber cap. I think it has a part # but will need cleaned to read it.

I'll play with the kickdown switch. For now it's just ornamental but in reading other threads I see where the wires near the trans can cause problems. Hopefully that's the case.

Will try to get pix at lunch, time permitting.

Thanks to all!

cjlipps · #14 08-05-2010, 03:36 PM

Pix of the vac lines and the mystery box on the passenger side of trans.

chomptown · #15 08-05-2010, 04:48 PM

Found your earlier post, which provides some more info. I was going to ask if your vcv valve was holding vacuum, but it apparently is. Have you checked all of those three-way vacuum connections to see if they have a tiny crack?

I've actually bought one of those three-ways, only to find out that there was a manufacturing error when it was made. I had to poke a hole through one of the openings so that it could actually pull a vacuum. Might want to check to see if they can hold a vacuum, and then to see if they can actually let a vacuum flow through. I'm not sure if I'm being much help, but sometimes this stuff just takes working through the system.