I am tracking down a vacuum leak that seems to have recently developed (or got worse) in my '73 450SE. I traced the vacuum line from manifold to transmission modulator - stainless steel all the way, banjo fittings at both ends. I cracked open the banjo fitting at the TM, and AT fluid leaked out - ruptured diaphragm it seems.

So... would someone be kind enough to tell me where in the service manual I can find the instructions for changing the transmission modulator? I have looked through all the sub-function groups for the engine manual, the maintenance manual, and the body & chassis manual - can't find the instructions! I also checked Haynes and Chilton (and searched this forum!). I found several references to the transmission modulator, but no detailed instructions on swapping it out. I am concerned about this business of "adjusting the pin" that I have seen mentioned.

I have confirmed that the part pictured below is the correct part for my transmission, I just need confirmation that the part I have shown is the transmission modulator, and that if AT fluid comes out when I crack the vacuum line seal to this part, then this part is what I should replace. Would someone be able to confirm this, please?

Many thanks,
Joe

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Yes that is a transmission modulator.

If you cracked the vacuum side of the modulator and got ATF - it's dead.

I never adjusted the pin when I did mine (just make sure it's in there) and all was well.

Replacement is just winding it out and the new one in, then remake vacuum lines. New sealing washers are always a good idea too.

I have a paper version of the technical manual and the procedure recommended for testing etc is akin to brain surgery and requires an assortment of interesting MB devices and guages to carry out - to re-iterate, I did mine in 15 minutes without all that and it worked a treat but yours maybe be different (doubt it though).

Good luck

J

P.S. were you getting ATF inducted into the engine up the pipe ?

Thanks very much for your help Jengasan! This is just what I needed to know. I will go ahead and replace the modulator. I believe I have been getting AT fluid sucked into the manifold - smoke issues.

I am guessing that I just allow the AT fluid to pour out when I take out the old modulator, then replace the lost AT fluid (with engine running, etc, according to the manual) after I have swapped the new modulator in?

Could you tell me where to look for this information in the technical manual - I looked (on the CD) and can't find anything.

Joe

I believe CDs for the newer models do not contain the transmission data. (None of mine do, and I have virtually all of them) Some of the early model CDs have data for the old transmissions, but not the new stuff. The only things covered are the simple maintenance items...fluid changes, etc. I may be wrong and have just not found the info.

The Haynes manuals may be of more help if you cannot locate a hard copy of what you need from the original factory manual.

230/8

Sorry Joe but I don't use the CDs, I bought a paper copy of the w108 technical manuals off ebay some time ago and use these. It would appear that they may contain more/different information.

As for the ATF, I replaced my modulator during a standard tranny service so it was near enough empty anyway. But I don't see the problem with just topping up post replacement, I don't reckon you'll lose much anyway.

cheers

J

Joe, where did you find a modulator valve in stock (I need one for my 74 450sl which uses the same valve)?

Thanks,
Hal

Thanks very much 230/8 and Jengasan - I feel "set to go now". Halw540, I found the modulator in three places - my local dealership - too expensive; a place in NY - diyautoparts.com; and the place I ordered from (because I have dealt with them before) *************** (backordered, but due this week). The good people who support this forum, unfortunately didn't have the part available. The Mercedes-Benz part number is A 000 270 01 79.

Joe

Joe - I am curious to know what were the symptoms of your original Vacuum leak? Did the vacuum reduce to the extent that it upset the FI system?

I am trying to pinpoint a loss of vacuum in my inlet manifold and hadn't considered the modulator until I read your post.

Peter
73 450 sl
Melbourne Australia

Hi Peter,

Yes, the vacuum is low enough to make the car run very rich, and it is smoky. I am working through the possible sources of leaks. I have replaced all the rubber vacuum lines under the hood. I found the transmission modulator to be faulty as described. After I replace that, if there is still an apparent leak, I will have to look at the intake manifold gaskets. I have already run propane around the injectors and intake manifold with the engine running, listening for idle changes (there were none), but I am not comfortable with this procedure, so I might not have done it for long enough.

Good luck with yours,
Joe

"I have already run propane around the injectors and intake manifold with the engine running, listening for idle changes (there were none), but I am not comfortable with this procedure, so I might not have done it for long enough"

Most of the seals used between the manifold halves are very hard to get to with the propane. I put a fuel line hose extension on the torch tube, treaded it into the manifold halves, then snaked it around the seals multiple times before finally stumbling onto a seal that would cause the idle speed to drop - that was with a leak bad enough to cause a 9 psi manifold vacuum. I never got a idle speed change when checking around the injectors but when I tore into the motor the injector seals were so hard they pulled apart as I removed them (mine is a 74 450SL w/92K miles).

Good luck,
Hal

Thanks Joe for the description, & I will watch for your modulator replacement report.
On my 450 I found that the vacuum take-off for the brake booster was a bit loose. This was at the bottom of the tube on the lower part of the intake manifold. On a DJet motor its very hard to get a normal spanner down there, so its not really surprising that a PO did not do it up tight. I used a crow foot spanner to tighten it, and suggest that you check out yours.

However I don't think I have totally solved my vacuum problem yet. Its the middle of winter here & very miserable outside, so I haven't been able to really test out my repair. ( I am also gathering parts for a timing-chain replacement.) Regards Peter in Melbourne.

Thanks for the "heads up" Peter & Hal. I'll check the brake booster take-off, and I guess I had better go back and look again at the manifold seals. But I am hoping that swapping out the transmission modulator for a new one will fix the problem . I'll let you know.

Joe