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#1
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Vacuum amplifier, is it really needed?
My 1991 350sdl with a transplant 3.0 OM603 exhibits strange shifting through the gears. So I put a vac gauge on the black line going to the trans modulator. When I floored the acel pedal, the vac gauge would go from 8in to 0 slowly and in spurts. I then hooked up the vac gauge to the bleeder on the IP and it went form 8 to 0 smoothly and quickly. so I'm guessing some where between the bleeder and the trans modulator there is most likely clogged vac line. Might even be in the vac amp.
So why do I need a vac amp? EGR has been deleted by PO. Door locks operate via vac pump in trunk. (If they work at all I don't know) So the only accessory for the vac amp is the trans modulator (and shutoff).
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#2
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Check the line to the ALDA. The "Vac amp" as its called varies the transmission modulator vacuum based on boost to provide smoother shifting.
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2004 F150 4.6L -My Daily 2007 Volvo XC70 -Wife's Daily 1998 Ford F150 -Rear ended 1989 J-spec 420SEL -passed onto its new keeper 1982 BMW 733i -fixed and traded for the 420SEL 2003 Volvo V70 5 Speed -scrapped 1997 E290 Turbo Diesel Wagon -traded for above 1992 BMW 525i -traded in 1990 Silver 300TE -hated the M103 1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one 1979 Silver 300D "The Silver Slug" -Sold |
#3
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Can you be more specific on the erratic behavior you're having? It may have nothing to do with the vacuum signal.
You can get away without the vac amp, but on a turbocharged engine it helps maintain smooth shifting across a variable boost range. Without it, the transmission will tend to shift firmer than it would otherwise since the VCV is not progressively operated. You likely have a vacuum leak somewhere, something that isn't plumbed correctly, or a leaking ALDA. If the ALDA doesn't hold vacuum (indicating leaking seals), you will have some strange behavior since the boost signal affects the final vacuum sent to the transmission modulator. So that you don't think you're going crazy, the vacuum signal to the modulator will not always be the same as the vacuum signal from the VCV. The boost level will control the final output to the modulator. Think of the vac amp as a summing amplifier, it takes 2 signals and compares them and sends the result down the line to the transmission. Your input vacuum level does seem a bit low. Typically you should be somewhere around 12" at idle and without any throttle. Read the FSM on the setup procedure for the VCV and make sure it is adjusted correctly. |
#4
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I just replaced my VCV to get back to smooth shifting. I find the vacuum amplifier and surrounding plumbing to be a wonder of nature. When everything is working right, the transmission will always shift smoothly. Make a change to any component, and you get jerky shifts. It responds to two signals: boost and variable vacuum from the VCV. When the turbo kicks in, the output would be zero regardless of the reading on the VCV side. So what you are measuring may well be normal.
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#5
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here are two examples of what I'm talking about. You can tell when I floor it from the tach,,, https://youtu.be/zGIdQfg54V8
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#6
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I think you're confusing the operation with the older OM617 cars. The ultimate vacuum to the modulator should follow boost pressure, not solely the VCV signal. Yours seems to be dropping as boost builds as it should.
You still haven't told us what the problem is you're having that you're trying to solve. |
#7
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There are also two vacuum dashpots which prevent the vacuum from building instantly. Let's back up a minute and ask what the shifting problems are.
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#8
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Erratic shifting, sometimes shifts fine, sometimes flares, sometimes hard shifting. The vacuum change to the transmission, should be instantaneous. For example, When the accel pedal is floored, do you want to wait for the vacuum to drop to the trans, (2-5 seconds as is happening) or would you prefer immediate vac drop to trans so trans knows that you just floored it?
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#9
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Is it possible to clean out the vac amp with brake cleaner? Don't know the inner workings of the amp, but I suspect something somewhere is clogged in it.
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#10
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You're still focusing on the wrong things. The vac amplifier is not going to follow the VCV. The vacuum modulation on the turbocharged OM603 is a radical departure from the crude system of switches used on the 617 and 616. What you are expecting and what really happens aren't the same thing. The vacuum sent to the modulator is biased by boost. If you aren't building boost, the vacuum will still be high despite the VCV bleeding off. You need to make sure that all of your inputs to the vacuum amplifier are correct before proceeding further. Don't use brake cleaner, you'll ruin the rubber diaphragm in the amplifier.
Erratic shifting sounds more like transmission problems (wear), misadjusted bowden cable, or broken springs in the accumulator springs positions. |
#11
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Thanks, I was concerned about ruining the vac amp by trying to clean it. I have boost, hooked up my vac/psi gauge and verified. (10 psi max on the gauge so I quickly disconnected it. )
Earlier it was stated that the vac amp is like a summing amp. So I gather that if VCV drops to 0 that output to trans should be 0 ( or lowered), as well as if there is turbo boost, the vac amp output should lower. In my case, the vacuum drop to the trans modulator is delayed, sometimes more than others. And it isn't occurring immediately, there is a 2-5 second delay. I will check vac line routing and the like. OOh, when the vac gauge was hooked up, and car shut down. The vacuum slowly (over the course of several minutes) bleed down to 0. Both when hooked up to the trans mod (from vac amp) and from VCV to vac amp.
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#12
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Flares and hard shifts may be symptoms of a K1 or K2 accumulator spring being broken. Does this happen in all gears?
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#13
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Found a vacuum leak,,, red/green stripe line going into the cabin.
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Closing the store Benzbonz.biz on your smart phone or tablet. Last edited by mespe; 09-20-2017 at 01:53 PM. |
#14
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I'm pretty sure it's a vacuum thing. sometimes the car shifts perfectly, other times not
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#15
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On the W126, the Red w Green stripe goes to the Automatic Climate Control. Specifically, the bank of switchover valves for the vacuum actuator pods.
I'm not sure how much changed in those years but that may be a place to start looking.
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