Anyone know how to check to see if the transmission vacuum valve mounted on the injector pump is good or not? My wife's 300D shifts rather erratically and I'm thinking this might be the problem. I put a new modulator valve on the transmission but that didn't help. Vacuum gets to the valve okay but I'm not sure how the linkage works from that point.
300D transmission vacuum valve
Apr 30, 2005
3 Replies
The valve allows maximum vacuum at engine idle and reduces that as the throttle is opened until there's no vacuum at full throttle. The valve is not repairable. Suggest the vacuum line to the valve be checked for consistent vacuum.
You may have a vacuum leak that is preventing the correct volume of vacuum from getting to the tranny modulator. To determine this, with the engine hodd up, stand in front and look at the vac pump. Notice the "H" shaped vacuum fitting that the vacuum hose goes to and remove the left two hoses, block the holes in the fitting where they plugged in and experience a huge improvement in handling, shifting, mileage, etc. Tou can order an "F" shaped fitting at any MB dealer. it is part #601-078-06-45. P.
The entire vacuum system holds good vacuum overnight to operate door locks, etc. However, as I recall there is a check valve in the system going to the firewall and that may not have an effect on the remaining system under the hood. I'll try the ole Mity Vac on it and see if there's any leaking. Thanks guys.
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