Last week after I got the oil changed, I noticed the transmission slipping in 4th when climbing hills. I called my local Mercedes guru Brent Groth and asked what he thought it could be. He had this worried sound to his voice and said, "that is not good." I had the transmission oil changed and a new filter fitted in Burbank just about 6 weeks ago so I was a bit worried.
I see from records that the previous owner provided that he installed a rebuilt 722.109 automatic transmission from Sun Valley Mercedes Transmissions in December of 2008. That is the same transmission that was used in the 300D from 1957-1981. Checking the records of when the car was built using a European VIN Decoder, it looks like the car was originally fitted with a 720.109 4-speed automatic transmission.
Since I need to have the front and back master cylinder boots rebuilt anyway, I decided to have the transmission looked at. I took the car to a place in SLC called Transmission Man (I'm not making that up), gave Ben all the info I had and asked them to take a look at it. He had his folks look the car over and while they are good at what they know, he asked me to come down and go over it with him.
We started by looking over the vacuum system and tested the Vacuum Modulator with a hand pump and gauge. With our setup, it would not hold pressure indicating a ruptured diaphragm. The "red" Vacuum Modulator used in the 722.1xx transmissions seems to be hard to find, but we found one at Ecklers and ordered it.
Once the new Vacuum Modulator arrived, I removed the old modulator and about a quart of transmission fluid drained out from the hole. Once the modulator was out, I pressure tested it again and it held pressure perfectly. My guess was the test rig performed by Ben and me had a leak. Since I had a new modulator, I pulled the pin from the old modulator and adjusted it to have approximately the same length.
Once the new modulator was in place and I added back the quart of transmission fluid, I used the supplied key to adjust the shift points. Note: The correct way to adjust the modulator is to measure the pressure from the transmission pressure port just next to the modulator, but I did not have the equipment to do that.
I then noticed the vacuum modulator was connected to the VCV vent valve. The VCV vent should vent into the car cabin and not be connected to the modulator. I installed a new T to the hose at the top of the VCV and connected it to the Vacuum Modulator. I purchased an OTC 5613 Vacuum/Pressure Gauge Kit and tested the vacuum coming from the top of the VCV and it was within spec at 15 in Hg and dropped to 0 in Hg at full throttle.
The result, shifting much smoother but still slipping in 4th. With all of the pressure points going to the transmission good, I've done all I can do without pulling the transmission.
I put in a call to Sun Valley Mercedes Transmissions and Marc said he knew exactly what the problem was. Apparently, there is a valve in the transmission that dramatically reduces the pressure when the transmission shifts to 4th. His guess was valve needs to be replaced and said because it has been slipping that a rebuild is probably in order. Brent at Transmission Man in SLC said he could rebuild it but since I'm going to LA this week thought it would be best to let a Mercedes Transmission expert do the work. Brent pulled the transmission and loaded it in the back of my car for the trip to LA.
When I dropped off the transmission at Sun Valley Mercedes Transmissions, Marc said he had never had a transmission he rebuilt come back before. He took a look and as he suspected the valve and spring needed to be replaced. After about three days, I came to see him test the transmission. They basically rebuilt it again, cleaned it up, gave me the correct dipstick, washers for the banjo on the dipstick tube and a new Bowden cable. I had to admit the transmission they loaded up in my car looked new. All this for a very fair price!
I loaded the transmission back into my car on my next trip to Park City where the folks at Transmission Man put it back into the car. The guys at the shop did not really know how to adjust the modulator or Bowden cable, but the process of testing the vacuum and throttle taught me how to perform the adjustments.
Once I adjusted the vacuum modulator and Bowden cable, all is well again. Well except, I'm out about $1000.
We started by looking over the vacuum system and tested the Vacuum Modulator with a hand pump and gauge. With our setup, it would not hold pressure indicating a ruptured diaphragm. The "red" Vacuum Modulator used in the 722.1xx transmissions seems to be hard to find, but we found one at Ecklers and ordered it.
Once the new modulator was in place and I added back the quart of transmission fluid, I used the supplied key to adjust the shift points. Note: The correct way to adjust the modulator is to measure the pressure from the transmission pressure port just next to the modulator, but I did not have the equipment to do that.
I then noticed the vacuum modulator was connected to the VCV vent valve. The VCV vent should vent into the car cabin and not be connected to the modulator. I installed a new T to the hose at the top of the VCV and connected it to the Vacuum Modulator. I purchased an OTC 5613 Vacuum/Pressure Gauge Kit and tested the vacuum coming from the top of the VCV and it was within spec at 15 in Hg and dropped to 0 in Hg at full throttle.
The result, shifting much smoother but still slipping in 4th. With all of the pressure points going to the transmission good, I've done all I can do without pulling the transmission.
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| Ready to Travel to LA |
I put in a call to Sun Valley Mercedes Transmissions and Marc said he knew exactly what the problem was. Apparently, there is a valve in the transmission that dramatically reduces the pressure when the transmission shifts to 4th. His guess was valve needs to be replaced and said because it has been slipping that a rebuild is probably in order. Brent at Transmission Man in SLC said he could rebuild it but since I'm going to LA this week thought it would be best to let a Mercedes Transmission expert do the work. Brent pulled the transmission and loaded it in the back of my car for the trip to LA.
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| My Rebuilt 722.109 Transmission |
I loaded the transmission back into my car on my next trip to Park City where the folks at Transmission Man put it back into the car. The guys at the shop did not really know how to adjust the modulator or Bowden cable, but the process of testing the vacuum and throttle taught me how to perform the adjustments.
Once I adjusted the vacuum modulator and Bowden cable, all is well again. Well except, I'm out about $1000.
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