Help interpreting OM603/300TD vacuum readings from VCV to BFS - PeachParts Mercedes-Benz Forum

240dddd · #1 07-25-2020, 01:33 AM

I'm getting weak vacuum (around 2-3 hg) to the transmission in my 87 300TD. I've taken some vacuum readings at different points but I'm not sure how to interpret them. I'm hoping someone might be able to advise where the issue might be.

I numbered this image with the round blue dots at points where I measured vacuum:

1. ~20 HG going into VCV. My vacuum pump is working.

2. ~12 HG coming out of VCV. I think this is about correct and the vacuum increases a bit if I twist the VCV

3. ~12 HG going into the 3 way connector that goes to Y29 switchover and the VCV input on the BFS

4. This measured very low initially (around 1-2 HG), however after unplugging 5 from the system and plugging it with my finger temporarily the vacuum rose back to ~12 HG

5. ~12 HG

6. ~12 HG

At this point it looks like the problem is at the BFS because the vacuum at #4 drops when #6 is plugged into the BFS. When #6 is taken out of the BFS and plugged with a finger or golf tee then vacuum at #4 comes back up to ~12 HG.

That said, I don't really understand the BFS so I don't know if maybe this is expected?

7. As stated before, the signal going to the transmission is very low, ~1-2 HG

8. ~7 HG - the vacuum pump signal coming into the BFS that is shared with the VCV is lower than the output of the VCV. Not sure if this expected or not.

Thanks for any advice anyone can provide here. I'm leaning towards an internal link in my BFS, but I'm not sure how to validate that.

BONUS QUESTION - on my car, unlike the diagram, at #8 there is a Y connector with one of the outputs going to vacuum line that leads towards the dash. Anyone know what that additional line is leading to?

Diseasel300 · #2 07-25-2020, 09:08 AM

The BFS is a vacuum amplifier. It sums the inputs from the VCV and PRE inputs to give a modulated vacuum signal out on the TRA output.

Your vacuum input from the VCV is spot on.

The switchover valve Y29 is likely stuck. MB deleted that funky setup on the 1990+ cars. Supposedly it was to soften the shifting by applying vacuum to both VCV and PRE inputs when the car was cold. I removed the fully functional valve from my SDL and noticed no difference. My 350SD shifts perfectly and never had that system and I can't say I see any need for it. Before condemning the BFS try tying the bottom and side lines together on that switchover valve and see if you have an improvement.

Bonus answer: MB forgot to show the vacuum reservoir in the drawing. The T (or Y) connector on #8 running up by the firewall goes to the vacuum reservoir. On the W126 it's in the driver's side fender, not certain on the W124.

240dddd · #3 07-25-2020, 01:15 PM

Great feedback, thanks

Quote:

Before condemning the BFS try tying the bottom and side lines together on that switchover valve and see if you have an improvement.

To clarify, when you say this do you mean feeding the line from the ALDA directly into the bottom center of the BFS?

Diseasel300 · #4 07-25-2020, 02:02 PM

Something I missed in your earlier post: The vacuum at #8 is wrong, it should be full system vacuum (whatever your vacuum pump produces) or very close to it. Plug the line going to the vacuum reservoir. See if you get full system vacuum at #8 in your drawing. If you do, the vacuum reservoir or its line may be leaking or have a rotten rubber connector.

In regards to your question about the switchover valve lines: Yes. For added sanity, pull off the VCV vacuum line from the top of the switchover valve and park a golf tee or screw in it so the switchover valve is totally removed from the equation.

If the problem is solved, the switchover valve is probably bad. If it still exists, the BFS may have internal problems.

240dddd · #5 07-25-2020, 11:26 PM

The problem with low vacuum at #8 goes back to Y connector on top of the VCV, #1 based on my marking, also labeled 61a in the original chart.

Basically, as soon as I plug that Y connector into the VCV the output going to the BFS vacuum connection drops to ~1-2 HG.

I've tested the Y connector for leaks and it is fine. I.e. i checked it with a few golf tees and my mightyvac. I also tested it while connected to the just the vacuum pump input and it held at 20 HG all around. But as soon as it is connected to the VCV the shared output to the top of the BFS drops to very low. The output of the VCV that goes Y29 stays healthy at ~12G HG.

I also tried a different VCV I had on hand with the same results:

1. Input vacuum to VCV is normal at ~20 HG
2. Output of VCV is normal @ ~12 HG
3. Shared vacuum signal to BFS is very low

Can you confirm that the vacuum going from VCV to the vacuum input of the BFS (top center of BFS) is supposed to around 20 HG? Since both VCVs i have produce the same results I'm wondering if I just have two bad VCVs or is something else going on?

Mxfrank · #6 07-25-2020, 11:35 PM

Think of the BUFO as a vacuum transistor. 8 should always be full system vacuum, this is the "power source." The VCV is a sort of throttle position sensor, which feeds a variable vacuum signal (6) to the bufo, which decreases as throttle pressure increases. The BUFO uses this signal to send modulated vacuum to the transmission (7). In your case, the feed between 1 and 8 seems to have a leak.

The switchover valve is temperature controlled. It changes the shift pattern when the engine is cold, and also affects Alda enrichment. It may be leaking a bit, but i don't think it's the problem.

240dddd · #7 07-25-2020, 11:55 PM

Got it. So for some reason the vacuum going into the BUFO is far less than "full system vacuum". The vacuum drops when I connect the VCV (even though the output of the vcv looks healthy). Would this likely indicate i have a leak internally in the VCV?

Diseasel300 · #8 07-26-2020, 12:33 AM

If you don't start eliminating variables, you're chasing your tail. Plug up the line going to the vacuum reservoir and bypass the switchover valve. See where it gets you. If you have a leaking switchover valve, you can basically be dumping your VCV signal straight to the manifold, which would appear to the BFS as no vacuum from the VCV. The corresponding output to the modulator would also be low as a result.

The BFS is a bit more complicated than just amplifying the vacuum from the VCV. It also "sums" the input from the intake manifold pressure to adjust the vacuum signal based on boost pressure. As boost rises in the manifold, horsepower goes up for a given throttle position. The BFS reduces vacuum to the modulator as manifold pressure rises, essentially defeating the VCV to make the transmission shift firmer for the higher power level. It's clever, but it can make you pull your hair out if you leave all the variables in place, they all affect each other.

240dddd · #9 07-26-2020, 02:05 AM

Quote:

Plug up the line going to the vacuum reservoir and bypass the switchover valve. See where it gets you.

Forgot to mention I did both those things in my latest round of testing without seeing any change to the low vacuum coming into the the BUFO.

I'll keep trying some things but I'm not sure what yet.

If I plug the output of the VCV (#2 in my graphic) I should expect to see full vacuum at the Y connector vacuum input of the VCV, correct? If I did not see full vacuum would that indicate the VCV has an internal leak?

Diseasel300 · #10 07-26-2020, 10:31 AM

#2 will never be greater than 12-14" of vacuum. It is a "controlled" vacuum leak. #1 should have full system vacuum as well as #8. #2, #3, #5, and #6 should all be at VCV vacuum level.

It's important that the orifice #61a and the dampers #72 are installed in the system.

240dddd · #11 07-26-2020, 05:05 PM

After some more tests I *think* I've isolated the issue to the BUFO:

Things I blocked/Bypassed
1. I've bypassed the Y29 switchover valve
2. I blocked off the feed to the vacuum reservoir
3. At the vacuum pump I blocked everything off except the feed to the VCV

After doing all these things:
1. I have good input into the VCV (~20HG)
2. I have good output from the VCV (#2 on my chart, ~12HG)
3. I have weak vacuum going from the VCV to BUFO (#8 on my chart, 1-2HG).

As a next step:
1. I put my mighty-vac on the vacuum signal to the BUFO (#8 on my chart)
2. I unplugged VCV output between the VCV and the BUFO (#5 on my chart) and plugged the line connected to the VCV with my finger.
3. I observed that when I plugged the VCV output with my finger the mighty-vac measuring input to BUFO immediately climbed to normal operating vacuum of ~20 HG
4. When I remove my finger or reconnect the VCV output line back to the BUFO VCV connection the vacuum at #8 drops back down to 1-2HG

This makes me think the leak is in the BUFO somewhere since having the VCV output unplugged and leaking produces the same result as having it connected to the BUFO.

Diseasel300 · #12 07-26-2020, 05:34 PM

#8 is a feed from the vacuum pump, it is merely Tee'd into the vacuum supply to the VCV. It is not a VCV signal. Regardless of what the VCV is doing, #8 should ALWAYS have system vacuum present. Make sure the VCV is hooked up properly, it's very important that the vacuum lines are plugged into the correct ports and the various orifices and dampers are installed. If the green orifice is missing, the VCV will drag down the other lines connected to it.

240dddd · #13 07-27-2020, 04:53 PM

Ack. Agree with what you say and confirm that I have everything connected as per the diagram (including the green orifices).

Is there any way to 'bench test' the BUFO with the mighty-vac? I.e. if I supply normal vacuum to the #8 and cap off the other BUFO connections are there specific vacuum measurements i should be able to measure at each input?

240dddd · #14 08-28-2020, 01:09 PM

To wrap this up, I got a used BFS and the problem resolved itself. I have the correct vacuum readings all around now