'80 300SD (W116) -- vacuum pump question

[flainn]

Hi PeachParts folks, it's been a long time.

I got an '80 300SD a few months ago that has been neglected and is pretty "ghetto'd" together. I took it for a drive and the transmission is being very strange -- starts off in 2nd and then, at about 23 mph, downshifts into 1st and sticks there. The only way I can get it to shift up through the gears is to start off in "L" and then carefully nurse it through the gear shifts, which kind of defeats the purpose of having an automatic.

Watched some Kent Bergsma videos and so now I'm troubleshooting the vacuum system. I isolated the vacuum pump from the rest of the system and put a gauge on it. The vacuum is not only reading much lower than it should (-5 to -10 mm Hg) but the vacuum rapidly fluctuates between the two. Revving up the engine results in no change.

I thought it might be the check valve on the vacuum pump, so I pulled it off and it seems to be working fine (I can get air to go through it only one direction, although it seems to have a bit of leakage even in the "checked" direction).

Does it seem like replacing the valve might still be worthwhile, or should I simply try to find a used or reman vacuum pump? It doesn't seem that rebuild kits can be found anymore for love or money.

(anybody got a good single-port vacuum pump for an OM617 they want to sell?)

[multiphase]

I'm in the same position as you with the vacuum pump in my 85 300sd.
Did you find a solution?

[flainn]

I found a used vacuum pump (in Latvia!) on eBay and replaced the pump.

The replacement pump gives much better and non-fluctuating vacuum, however it did not solve my problems with the transmission.

I've ended up relegating the '80 300SD to parts car status and using what parts I can to donate to my '83 300SD, which has fewer mechanical issues.

Good luck to you. I was at the point where I was thinking the issue was likely either the Bowden rod adjustment or a faulty VCV -- unfortuntately the latter are becoming nearly impossible to find.

[multiphase]

I think I'm going to go to an electric vacuum pump from a Ford Super Duty.
I also think I'll pull the original pump & remove the actuator so that the roller bearing won't ride on the cam anymore.

[flainn]

That sounds like a solid plan. You'll get a little bit of power back from not having to drive that vacuum pump, and no worries about it sending ball bearings through your engine when it fails.

[multiphase]

I have since found out that an electric vacuum pump is not a good idea on these because the CVC valve for the transmission is design to bleed off vacuum in proportion to the throttle position. In other words, an electric pump would run all the time & likely burn up.

[flainn]

Originally Posted by multiphase

I have since found out that an electric vacuum pump is not a good idea on these because the CVC valve for the transmission is design to bleed off vacuum in proportion to the throttle position. In other words, an electric pump would run all the time & likely burn up.

I didn't know that electric pumps *didn't* run all the time. TIL.

[BillGrissom]

Many people here worry about an electric vac pump dying early from the constant VCV draw, but haven't read a report yet. I have one from an Audi to use when needed, but haven't found a cheap vac switch yet. In the Audi, it just supplies the brake booster.