Code #26 Check engine light - PeachParts Mercedes-Benz Forum

#1 01-24-2004, 10:32 PM

Hello again everyone on the forum, Ok here goes again, I have checked all the vac. lines great vacuum, I have purchased a new upshift delay switch over valve, which everyone said was the problem. Now instead of the light coming on every 3 day's as it did before, it now comes on everyday, since I have replaced the valve. before it used to be every 3 day's, still a code 26. Now I need real good help on this one, does anyone have any surefire way's how to repair this ugly problem. I just threw $34.00 into the trash it looks like. I can sure use everyone's head on this one. I have no place else to look or what else to look for. Help everyone, please. Plenty of vacuum at the rear of switch over valve. Thanks everyone for any and all the help you can give me. Before the snow starts flying here in Colorado again. Ben

stevebfl · #2 01-25-2004, 12:13 AM

Does the vacuum element hold vacuum? If it holdsvacuum does it change the shift point when the vacuum is applied? Does it actually work under the conditions required?

#3 01-25-2004, 11:19 AM

Not sure just what you mean. When I disconnect the vacuum line from the rear of up shift valve I do hear the vacuum release.?? So hard to tell just what the timing is on the 2-3 shif of the automobile, Temp in the garage is 55 Deg. when I or I should say when my wife leaves to go to work. Should I just leave the car outside and then try to drive it a few hours later and see how long it takes to shift from 2nd to 3rd?? Really need help on this one I am completely stumped.

stevebfl · #4 01-25-2004, 11:44 AM

Testing is very precise here and specifications are necessay.

When cold the controller switches the switchover valve to direct vacuum to the vacuum element. This must be tested in atleast three ways. First take the vacuum line to the element and pull a vacuum on it. I usually do this by sticking it in my mouth and suck. Does it hold vacuum (this is the most common problem - a bad diaphram).

Second, does the vacuum appear at the element side of the switchover valve when it is supposed to. I'm not sure of all the conditions for all cars so I look up that criteria. I then hook in a gauge to the line in question and see if vacuum appears when starting the car cold. If it has vacuum then and not later when warm it is probably working properly.

Third, does the shift point change when vacuum is applied. Hook a line to the line going to the trans vacuum element and apply vacuum. Check the shift point, release the vacuum recheck. Are the shift points the same or different? Just being different is not enough, the vacuum effect is adjustable and there is a criteria for the rpm at the shift point. It's not likely that an old unit would be out of adjustment but it is a problem after installation of a new vacuum element.

Everything is exactly measurable so the answer is as easy as 1,2,3.

#5 01-25-2004, 12:00 PM

looks like I have a lot more checking to do, I am going to have to find someone with a vacuum guage and find out how much vac. should be there at all times when starting and driving I guess. I should never learned to work on Cars?? especially mine, I am getting to old for this stuff, getting harder to bend over. I will see just what I can do, and thank you very much (I hope) Ben

stevebfl · #6 01-25-2004, 12:05 PM

In this situation vacuum is yes or no, not how much.

#7 01-25-2004, 12:10 PM

ok, hope I fully understand, Thanks very much, I will let everyone know. Ben

stevebfl · #8 01-25-2004, 12:13 PM

BTW, if you tire of this they have guys who do this for a living (bg).

suginami · #9 01-25-2004, 09:22 PM

It's funny that most guys on this site always point to the switch over valve as the problem when responding to an owner with a code #26 upshift delay valve problem. I guess it's hopeful thinking as the part is very cheap.

However, my MB mechanic, the famous Enrique of MB Motors, who has worked at MB delearships on 3 continents for about 40 years, and has owned his own shop for the past five years, tells me that he rarely, if ever, replaces these switch over valves.

95% of the time, the problem is the vacum diaphragm on the transmission no longer holds vacum. It is not a cheap part - about $250-ish, if I remember correctly.

I watched Enrique diagnose mine. He pulled the vacum hose off of the switch over vavle, applied vacum with a vacum pump, and we could watch it immediately lose vacum.