Vacuum Test: Can I use rubber hose - PeachParts Mercedes-Benz Forum

RML · #1 05-10-2008, 12:01 AM

I have read quite a few good posts here about troubleshooting the vacuum system. I found a few bad rubber connectors and replaced them, which vastly improved my shifting from 2-3, but 1-2 is still abrupt, and I can feel a clunk coming back from 2-1. I put my Mityvac on the line going to the tranny and it does not hold vacuum. As recommended, I am going to measure what vacuum I am starting with coming off the vacuum pump. If I run a line into the cabin to my guage while I am driving, will I be able to use a length of vacuum hose from the auto parts store, or do I need to use a stiffer plastic line? For that matter, should I pull and use the line going down to the tranny for this purpose? It looks like it would probably be long enough.

bustedbenz · #2 05-10-2008, 12:13 AM

Quote:

Originally Posted by RML

I have read quite a few good posts here about troubleshooting the vacuum system. I found a few bad rubber connectors and replaced them, which vastly improved my shifting from 2-3, but 1-2 is still abrupt, and I can feel a clunk coming back from 2-1. I put my Mityvac on the line going to the tranny and it does not hold vacuum. As recommended, I am going to measure what vacuum I am starting with coming off the vacuum pump. If I run a line into the cabin to my guage while I am driving, will I be able to use a length of vacuum hose from the auto parts store, or do I need to use a stiffer plastic line? For that matter, should I pull and use the line going down to the tranny for this purpose? It looks like it would probably be long enough.

Plain old rubber hose would work fine. There's nothing magical about hard lines, as far as I know. I'll put it this way, last time I was reading my gauge inside the car, I used rubber hose to get it in there. Seemed accurate.

That line to the transmission, i believe, should be holding vacuum. That it does not in itself could be causing your problems.

RML · #3 05-10-2008, 12:19 AM

Quote:

Originally Posted by bustedbenz

That line to the transmission, i believe, should be holding vacuum. That it does not in itself could be causing your problems.

I still need to go back and inspect the rubber connector at the end of the line, going into the modulator on the tranny, and also inspect the line itself for cracks. The last time I was under the car I ran out of time to do those steps. I would love to find that replacing the rubber connector hose at the end of the line fixes the problem. We'll see.

dmorrison · #4 05-10-2008, 09:27 PM

The black line going from the IP shift control valve to the tranny modulator should hold vacuum. The line should be solid from the rubber connection at the rubber connection at the modulator valve in the transmission. Apply vacuum at both the line in the engine compartment and at the modulator. To do it at the modulator you will need a short piece of hart plastic line, like those use in the engine compartment. You can buy it by the foot at the dealer or mercedesshop.com.
If the modulator does not hold vacuum then you must replace the modulator. Be very careful doing this. You don't want to get any dirt in the tranny or strip the bolts going into the aluminum transmission case. Use a torque wrench.

Dave

RML · #5 05-12-2008, 08:44 PM

Quote:

Originally Posted by dmorrison

The black line going from the IP shift control valve to the tranny modulator should hold vacuum. The line should be solid from the rubber connection at the rubber connection at the modulator valve in the transmission. Apply vacuum at both the line in the engine compartment and at the modulator. To do it at the modulator you will need a short piece of hart plastic line, like those use in the engine compartment. You can buy it by the foot at the dealer or mercedesshop.com.
If the modulator does not hold vacuum then you must replace the modulator. Be very careful doing this. You don't want to get any dirt in the tranny or strip the bolts going into the aluminum transmission case. Use a torque wrench.

Dave

Dave: I think I follow you. Having these parts described, like the IP shift control valve, is helping me to understand what is going on in this complex vacuum system. I take it that the IP shift control valve intervenes at some point in the process to apply its own level of vacuum when called for, to control a certain shift.

I think what you are saying above is that I need to isolate whether my leak is in the black line or in the modulator. But I do not understand why a short piece of flexible rubber vacuum tubing connected to the modulator will not work. It sounds like you are recommending I use a piece of hard plastic line.

Thanks for the input.