722.315 transmission vacuum modulator question - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 05-18-2017, 02:26 AM

I have a 1984 300TD with 292k. The transmission was rebuilt 2 years (20k ago). I've been having an issue with a really bad flaring 2-3 shift. I replaced the K-2 Accumulator Spring and it still does it. I replaced the vacuum modulator and while trying to adjust it notice something odd. When I adjust it to the factory spec of 48 psi, at idle it stays like that. As soon as I give it throttle the pressure increases. When driving the only time it will read at 48 psi is when I'm at idle. Is it suppose to stay at 48 all the time or should it fluctuate?

Diseasel300 · #2 05-18-2017, 09:33 AM

It will fluctuate with road speed and engine loading. That's the whole point of having the vacuum modulator and the VCV setup.

If the modulator were bad, the transmission would have the same behavior in most if not all gears, not just one. The 2-3 shift is usually resolved by replacing the K1 accumulator spring, not the K2.

It's worth taking the time to make sure all of your VCV settings are correct before touching anything on the modulator. Unless it was leaking or wouldn't hold vacuum, the old modulator was probably fine.

joshuajeeper · #3 05-18-2017, 09:58 AM

3.3bar (or 48psi) is the modulator pressure spec only if you have a vacuum amplifier (blue saucer - 1985 models). All other 722.315's should be set to 2.9bar (42psi). The modulator pressure should be read out and adjusted after the engine is at proper operating temperature, vacuum line to modulator disconnected, and driving the vehicle at approximately 50km/h (31mph).

If the flare is not present with the vacuum line disconnected, then the problem is with the vacuum supply to the modulator and not the transmission. Otherwise you may have a leaking K1 piston seal, K1 valve body spring, or worn clutches (K2 is for 3 to 4 shift - applied in four gear). A lot of "rebuilds" are bare-bones minimum repairs that very rarely involve replacement clutches or bands. They will simply install a thicker snap ring or longer pin (for the bands) to compensate for wear.

Hope this helps!

vwnate1 · #4 05-18-2017, 10:23 AM

Dang ! this detailed info is priceless .

Hit Man X · #5 05-18-2017, 03:32 PM

I did not read all this, but K2 is for the direct drum. Do you always have a flared 2-3? My 300SEL has it only when cold and very slightly. I attribute this to 317k on the stock internal trans and figure the orings are weak in the K1 drum.

The 2-3 upshift keeps the B2 band applied, drops B1, and applies K1 all in the same motion. You would do yourself a good benefit to buy and fully install that Superior shift kit.

Do you have a detailed receipt of what exactly the shop did on the rebuild?

turbozeke418 · #6 05-19-2017, 03:06 AM

It was the K1 I replaced not the K2 (my error). If i drive with the vacuum line disconnected. It still flares (BADLY) from 2nd into 3rd gear. The others are all OK.

joshuajeeper · #7 05-19-2017, 09:06 AM

Unfortunately, to me, it sounds like you have a leaking K1 piston seal rings or worn clutch discs. What is happening is more fluid is required on that circuit to take up the extra needed volume. You can test to integrity of the piston seal by removing the valve body and using compressed air to that circuit. Its somewhat of a mute point since whether its a seal or clutches, a disassembly of the transmission is in order.

722.315 transmission vacuum modulator question-k1.jpg

136 is the K1 drum assembly
147 & 150 are the K1 piston seals

177 on the far right of the picture is the snap ring that's available in different thicknesses that repair facilities will sometimes use to compensate for worn clutches instead of replacing them.

722.315 transmission vacuum modulator question-trans-cutaway.jpg

turbozeke418 · #8 05-20-2017, 06:02 AM

Quote:

Its somewhat of a mute point since whether its a seal or clutches, a disassembly of the transmission is in order.

That's what I was afraid of. I know someone that is willing to sell me a working trans for $200.
Hopefully I can handle the removal and replacement myself. I want to sell the car, but won't get much for it with it not driving properly.

turbozeke418 · #9 05-21-2017, 11:48 PM

I am going crazy trying to figure this out. I have a gauge hooked into my trans now so I can read it in the car. I adjusted the trans to the proper psi. If I give the car throttle the pressure goes way up then when it levels out it is less than what I adjusted the modulator at. If I rev it again, it goes way out and when it levels out it is again even lower than before. When I drive it, the gauge is all over the place and appears to be no rhyme or reason. The modulator holds pressure. Is there any way I can disconnect everything on the vacuum system and just have a line to the modulator for testing purposes so I can see if it is an issue with the tranny or the vacuum system?

tyl604 · #10 05-22-2017, 09:11 AM

Sure. All you need to do is look under the hood and unhook the plastic line from the VCV to the trans modulator. Put a plug in the VCV . Hook the Mityvac into the line to the trans modulator. When you drive, if the tranny is OK, you should get smooth upshifts. After reaching fourth gear, pump up vac in the Mityvac; when you come to a stop you should have smooth downshifts.

With no vac to the trans modulator you get smooth upshifts and clunky downshifts. That is if the tranny is OK. This is based on my 1981 300SD and I assume yours works the same.

If you are worried about K1 and B2 and all those valves, the first thing you need to do is make sure it is not just a simple vac problem.

Diseasel300 · #11 05-22-2017, 09:45 AM

When you're setting the modulator pressure, you do it with the vacuum source disconnected and travelling at a speed specified in the service manual in a certain gear. If you're setting it at any other point, the setting is wrong.

joshuajeeper · #12 05-22-2017, 06:07 PM

I may be reading into it incorrectly, but it almost sounds like you are trying to adjust modulator pressure by measuring vacuum to it, or if you are in fact measuring hydraulic pressure, it may be connected to the wrong port. A hydraulic pressure gauge capable of reading up to 10bar should be connected to port M when making your adjustments. Port R is for governor pressure (~1.2bar at 30km/h to ~2.8bar at 90km/h) and port A is for working pressure (11.5-13.5bar up to 1984, 13.3-14.3 for 1985). If you are reading the modulator pressure (and yes, its with no vacuum line connected at all to the modulator valve) and you are getting a fluctuating reading, there is a good chance that the valve body intermediate plate gasket is leaking, the incorrect gasket was installed, incorrectly installed modulator valve, or incorrect valve pressure pin based upon the design variant of the valve.

722.315 transmission vacuum modulator question-pressure-port.jpg

722.315 transmission vacuum modulator question-pressures.jpg

I hope this helps and doesn't make it more confusing.