a bad modulator valve can cause flare in older 722.3x - PeachParts Mercedes-Benz Forum

iandiam · #1 03-19-2009, 08:42 AM

Yes, it is true. If the spring is broken or somehow rendered useless, the case pressure will blow off the valve, making the vacuum adjustment totally useless.

In this picture you can see clearly that the stop on the end of the spring is rusted away - as in not there - as in the little adjusting screw you can turn with the 'T' handle had no effect on the spring tension at all, giving me a mushy 1/2, and badly flared 2/3 and 3/4 even with vacuum disconnected. The new mod fixed it.

Yes, I was that kid whose parents might have come home to see that I'd taken apart the cat to see how it worked and maybe even to have gotten it back together properly.

nickofoxford · #2 03-19-2009, 09:50 AM

This is exactly what my 240D has been doing, maybe I should pull the modulator..

iandiam · #3 03-19-2009, 11:24 AM

Quote:

Originally Posted by nickofoxford

This is exactly what my 240D has been doing, maybe I should pull the modulator..

Give it a shot. I'ts a $50 part and a 20min. job on your back, maybe 10 on a lift.

I could not adjust my shift quality at ALL with the bad one, just mushy and full of flare.

tangofox007 · #4 03-19-2009, 12:51 PM

Any one with a W123 with an original modulator valve (the youngest of whch are nearly 25 years old) can be reasonably assured that the spring has been severely compromised by rust at the point.

rrgrassi · #5 03-19-2009, 01:28 PM

How does rust get in there? Through a bad rubber cap?

Mesalm · #6 03-19-2009, 01:58 PM

Where is the modulator valve located?

rrgrassi · #7 03-19-2009, 02:08 PM

On the tranny, on the left-driver's side.

nickofoxford · #8 03-19-2009, 02:14 PM

Quote:

Originally Posted by iandiam

Give it a shot. I'ts a $50 part and a 20min. job on your back, maybe 10 on a lift.

I could not adjust my shift quality at ALL with the bad one, just mushy and full of flare.

Im getting the same results, I can try EVERY notch on the little key and it's always the same, I've went over all the vacuum components and got my numbers right to no avail.

Would this make the kickdown stop working? I know it used to work a month ago and it still does click, just no drop in gears.

Thanks for posting this, great timing for me!

gsxr · #9 03-19-2009, 05:51 PM

Quote:

Originally Posted by nickofoxford

Would this make the kickdown stop working? I know it used to work a month ago and it still does click, just no drop in gears.

Shouldn't affect kickdown, that is a separate circuit, with several different possible causes... fuse, kickdown relay (if applicable to your chassis), switch under the pedal, wire to the tranny, and finally the solenoid at the tranny itself. This failed on my 1987 300D last year, turned out to be the solenoid - installed a new one and voila! All fixed.

nickofoxford · #10 03-19-2009, 11:06 PM

Quote:

Originally Posted by gsxr

Shouldn't affect kickdown, that is a separate circuit, with several different possible causes... fuse, kickdown relay (if applicable to your chassis), switch under the pedal, wire to the tranny, and finally the solenoid at the tranny itself. This failed on my 1987 300D last year, turned out to be the solenoid - installed a new one and voila! All fixed.

Weird, I got under the car today with my multimeter and I get 11 or so volts (car off - autozone battery

) so I get power to the solenoid and it clicks.. I'll have to look into this..

nickofoxford · #11 03-19-2009, 11:07 PM

Quote:

Originally Posted by rrgrassi

How does rust get in there? Through a bad rubber cap?

Im going to go out a limb and agree with your guess, maybe if it had a bad rubber o-ring on the cap and the vacuum sucked in water while driving in the rain or something like that.. Is the spring actually in the transmission or is the assembly isolated to just the modulator?

iandiam · #12 03-26-2009, 06:01 PM

Quote:

Originally Posted by nickofoxford

Im going to go out a limb and agree with your guess, maybe if it had a bad rubber o-ring on the cap and the vacuum sucked in water while driving in the rain or something like that.. Is the spring actually in the transmission or is the assembly isolated to just the modulator?

I'll agree, for the win. Who knows how long the cap was cracked when I bought it from teenager who drove it hard for 20k...Nice kid who admitted it was more car than he could handle...

You can see the spring in the center of the right half in the pic.

Note the rust that settled in the bottom of the green half...

Cr from Texas · #13 03-26-2009, 09:03 PM

Where's that thread that tells you how to adjust the modulator?
IIRC, you attach a banjo bolt and adjust using transmission fluid column height. Wouldn't this be a good way to check function of the modulator? If the fluid column height doesn't change - the modulator is bad????

I just can't find the link. Did I imagine that thread?

iandiam · #14 03-27-2009, 08:21 AM

Quote:

Originally Posted by Cr from Texas

Where's that thread that tells you how to adjust the modulator?
IIRC, you attach a banjo bolt and adjust using transmission fluid column height. Wouldn't this be a good way to check function of the modulator? If the fluid column height doesn't change - the modulator is bad????

I just can't find the link. Did I imagine that thread?

The banjo bolt is to adjust case pressure at the modulator, but in a used (read as: worn, broken-in) transmission, lots of people say to just do your best without. Clockwise is firm, counter-clockwise is mushy.

Make your modulator adjustments with vacuum disconnected (and drive it hot) so that the shifts are pretty harsh, then start on your vacuum adjustments. If you adjust the VCV all the way up and set teh linkages properly, you can fine tune from there. The vacuum should drop from 10-15" at idle to 0-2" at full throttle. If you're at the 15"->2" end, you may want to dial the modulator down a little bit (CCW)

good luck

gsxr · #15 03-27-2009, 11:29 AM

Quote:

Originally Posted by iandiam

... Make your modulator adjustments with vacuum disconnected (and drive it hot) so that the shifts are pretty harsh, then start on your vacuum adjustments.

The modulator should be adjusted so that WOT shifts are firm, with no flare or slippage during the WOT shift. You can adjust it either way as necessary to tweak the part-throttle shifts, as long as the WOT shifts remain firm & crisp. But there's no need to drive with the modulator disconnected (zero vacuum) since that will provide a full-harsh shift at all times.