can a switchover aka 3/2 valve be bench tested? - PeachParts Mercedes-Benz Forum

sleepstar · #1 03-22-2012, 11:08 PM

still working out vacuum problems to get the trans to shift right. i have the setup that has 3 of those 3/2 valves on top of the motor. i've deleted the EGR lines (since this car will be used for... offroad purposes) but i need to test the function of the one remaining valve that will be used to control the downshift.. is there a way to test it with a mityvac? since there's three valves, i'm hoping at least one is good, if all three are good i might have one to offer up to someone that needs it

this is basically the vacuum system i'm using (from a 78-80 300SD)
http://www.peterschmidtransmission.com/images/616_617_91.jpg

charmalu · #2 03-23-2012, 11:55 PM

Bump.

Charlie

JamesDean · #3 03-24-2012, 03:12 AM

From my experience and what other members have said you can pretty much eliminate the 3/2 valves and maintain proper shifts. The 3/2 valves mainly govern EGR functions.

What shifting problems is the car having?

If you're having flares inbetween shifts, the first thing you should do is disconnect vacuum to the transmission and drive the car. If you observe firm solid non-flaring shifts then the problem is vacuum related. If you still have flaring then the problem is internal to the transmission and the first thing I'd do is replace the K1/K2 accumulator packages with the parts from the Superior Shift Kit.

I did the above on my 300SD, that along with a new B2 piston helped its shifts immensely.

sleepstar · #4 03-24-2012, 03:44 AM

yes, usually you can eliminate those valves, but on a certain subset of trans, one of the valves controls the downshift (you can see it in the vac diagram i posted. on the models where the valves can be completely removed, two of the holes on the plug are already open.) the trans is a 722.120, 4 bolt pan, so the shift kits aren't applicable. there's a more detailed vac. diagram in the factory manual that shows how the valves operate the trans, but i couldnt find an image online to post (manual is in pdf)

i dont remember offhand which models used a setup like this one. certainly 78-80 w116, maybe 80-81 300D?

i should note that after finalizing the egr removal yesterday the transmission shifts uhh.. 1000% better, so at this point i want to test the valves for full functionality so i can keep a backup and let someone here who needs one have the other

JamesDean · #5 03-24-2012, 02:34 PM

Interesting, I wasn't aware of that. Most of my experience is from the 722.3/4 series transmissions. I've never dealt with 722.1's.

I know most 722.1's were used in pre 1981 cars. I think by 81/82 the .3's were more prevalent.

tyl604 · #6 03-24-2012, 06:29 PM

Confused. At least in the 81 300SD the switchover valve is purely for pick up in acceleration. Has to do with the turbo. Do not think it has anything to do with the tranny. I changed mine and got the hoses switched. Could hardly get over 30MPH but no change in the shifts. Switched the hoses and have power and pickup again. No change in tranny.

sleepstar · #7 03-24-2012, 07:49 PM

yeah... it only applies to model years 78-80 on the SD, with the 722.120 trans , and another year or two on the 300D, maybe even the 240D, though i don't remember exactly which. the factory diagram shows exactly which two valves are for the EGR (one for cutout at idle, another for cutout at full throttle) and the last one is for the auto trans.

tyl604 · #8 03-24-2012, 09:02 PM

Good info. Thx for the reply.