300SD Trunk Vacuum Pump Question
 

All right guys, please don’t beat me up too bad. I have searched the forum and have not found an answer.

1981 300SD 97,700 miles (New to me)—The vacuum pump in the trunk is not working. Would this affect the tranny?. Or does the pump in the trunk only provide vacuum for the doors? My transmission flairs between 2nd and 3rd and I am trying to figure out if the problems are related.

Once I figure that part out I can start with the vacuum system under the hood and then move on to the B-2 if need be.

Thanks!

:D Sounds like me!

Anyway... everything is vac related on these cars. For the trans... I bet it has something to do with the underhood business.

The lil diaphragm (sp? it's late) on the trunk could be leaking/torn or the hose loose.

IMO - I'd start under hood and work the opposite way. As I understand stuff works in that direction on these vehicles.

The pump on my 84 SD is only to perform locking and unlocking. It locks the doors, the gas door and the trunk.

If you are having a problem with the tranny, sounds like the tubes up under the hood would be the best place to start.

Fuzzball's right - the door locks on the SD are controlled by the trunk-mounted pump and it has nothing to do with the transmission.

Do a search for flaring, transmissions, etc and you should find lots of info.

Seems most SD's flare on the 2-3 shift. If you have a nice firm 1-2 and 3-4, there's probably not much you can do without hardening up the whole thing, which will make your 1-2 pretty hard.

If the flare is bad, some people have been known to disconnect the vacuum from the trans in order to get firm (really firm) shifts that they feel prolong the life of the trans.

That's really low mileage for an '81!

My 83 SD use to do this a lot, but after following Marshall Booth's advice, I put synthetic in the Tranny. After about 10-15K it reduced dramatically. My son doesn't even notice it as the car is now assigned to him. This probably is not a vacuum problem.

[QUOTE=tenknots]...some people have been known to disconnect the vacuum from the trans in order to get firm (really firm) shifts that they feel prolong the life of the trans.[QUOTE]

Yes, it WILL prolong the life of the tranny. But it WILL kill your flexdiscs in short order (Mine self-distructed less than a year after I disconnected vacuum.)

Yeah it runs full line pressure at low throttle... not what you want. That gives you bumps/excessively firm shifts at low throttle.

You want line pressure to increase as throttle does (vac drops) to prevent slippage of the clutches under load.