On my new toy, a 1992 500E (124-036) I’m getting a “code 8” on the “distributor ignition” computer, which translates into “transmission overload protection switch does not close”. I understand the function of this system (to retard engine timing during full throttle upshifts) but I am trying to diagnose the cause of the switch malfunction, as well as understand how the switch itself works.

A shop with a good history on MBZ transmissions apparently rebuilt the transmission two years ago; I don’t know if this problem is new, or if it has existed since the transmission was rebuilt – thus my concern whether it may be something internal vs. the switch itself. In the transmission diagrams, there is a pin and a “reaction valve” behind the switch - my concern is if either of these parts could be defective. My questions:

Is the switch itself a common failure point?
Can the switch be tested (and how)?
Is the reaction valve or pin a possible failure point?


Thanks in advance for your help…

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

I have a '92 300SE, and just changed this very expensive (for it's size, and apparent lack of complexity) part - maybe my experience will help. I think the wholesale cost on my part was about $160.

Is the switch a common failure point? Mine didn't "fail" - at least no codes, but it was leaking tranny fluid. I had just gone through the pain of pulling the transmission to replace the leaking rear main seal on the engine and had buttoned-up all of the other leaking/seeping points on the transmission.

Can the switch be tested - don't know the answer to this either. Mine has a threaded socket on it that mates to 2 plugs on the switch. Since the switch appears to be electrically isolated from the housing (the center portion is all plastic from front to back), I'd guess that 12v+ to one pin and 12v- to the other should cause the switch to cycle.

When I had it out, it appears to be a solenoid that actuates a pin which slides into a bore inside the transmission. I'm guessing that when it's actuated, it restricts fluid flow. The actuation occurs at gear-change at the same point when the computer should be retarding engine timing to reduce torque. Maybe the pin actuates an actual valve? Don't know this either. If there is a valve, and it is the problem, it's probably something inside the valve body, so maybe the tranny doesn't have to come out.

I can't see how the pin would be a failure point without there being other serious issues inside the transmission. For instance, if there's something really wrong and there's lots of metal and/or clutch material circulating in the oil, I guess it's conceivable that the pin could get stuck.

If your switch is like mine, it screws into the side of the transmission on the driver's side. (Your tranny is different for a V8, so maybe it's elsewhere). It takes a monster socket to get it out - I don't remember the size, but it's in excess of 30mm. There's a small o-ring that seals the unit at the tip - the switch housing itself has no sealing point. BE SURE to get this part when you buy the switch (if that's your problem) - I almost didn't and the results would have not been good.

The pin referred to snaps into a socket (kind of a ball & socket affair), and will be troublesome for a re-installation. When you snap the pin into the new switch, it wants to dangle and not stick straight out, which will cause it to not align with the bore when you install it. My trick was to click it into the socket, and then pull it out ever so slightly to create some friction so it would stay in line.

Maybe this is no help - maybe it is.

Jump the connector and try to drive the car, If it feels like you're dragging an anchor it's the switch.
These switches don't leak, the o-rings for reaction valve #1 and the o-ring at the switch leak (much cheaper than the switch)
Mike

wbrian63: I believe that the switch does not control the transmission; it’s the other way around. The switch closes when the transmission initiates a gearshift; this signals the EZL to retard timing for 400ms thereby reducing torque output.

Mike Freeman: That’s a great idea – it will confirm that 1.) The wiring is O.K., and 2.) That the retard function is working. Plus, it should throw the diagnostic code that the switch “does not open” versus the “does not close” code that I’m currently encountering. Good point on the o-rings – if they are leaking, then perhaps the switch isn’t seeing the proper hydraulic pressure. (?)

I’ll crawl under there tomorrow and try it out…

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

The switch sending a signal to the engine makes more sense. Thanks for setting me straight. My perspective is from a US car perspective where the engine and transmission are controlled by the same computer.

Therefore as I understand the concept - the plunger in the transmission is actuated by pressure associated with impending up-shift, bridging the poles on the switch (closing the switch), allowing a signal to pass to the engine management computer to cause the timing to retard.... what a mouthful.

As for "these switches don't leak" - not in my experience. The switch on my car was leaking between the plastic center and the metal housing - no leak at the attachment point to the tranny.

New switch with small o-ring and 10 minutes to R&R and no more leaks.