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OldMerc2 12-27-2011 10:17 PM

Delayed upshift on E420
I've recently purchased a '94 E420 and have spent the past couple weeks fixing minor problems and getting used to it. Since I've become more comfortable with flooring the throttle more frequently an odd shifting issue has cropped up. Shifting at anything other than WOT has been fine, under WOT fuel cutoff seems to kick in at 3500 rpm on the 2-3 shift and at 4000 rpm on the 3-4 shift. Under WOT upshifts do not take place unless I back off the pedal momentarily. I did do some searching and my problem sounds a lot like the one outlined here:

A few good ideas there but no definitive conclusion. Thus far I'm not sure where I should start looking but I want to say it's less of a transmission issue and more of a premature fuel cutoff problem. The rpm points of 3500 and 4000 would seem far too early for a full-throttle upshift, shouldn't normal full-throttle behavior be more like this:

That type of thing above simply does not happen with my car as is. I don't want to blindly throw parts at the problem, I'd be more interested in getting to the root of the issue and knowing if anyone else has dealt with this type of thing.

I should mention two things that might be related. I noticed the accelerator cable was slack and I tightened it up a bit, the ignition and engine were off but could this have messed with the ETA (refer to E500 thread above)?

The second thing had to do with the starter cutoff switch (or so I thought), an occasional no start in park turned out to be two dissolved shifter bushings. I got the idea from another thread that the early fuel cutoff may have been connected with a maladjusted starter cutoff switch, (ECU would not recognize that the shifter was in drive). I ended up replacing the bushings, adjusting the linkage length and not touching the starter cutoff switch; the intermittent no start problem has gone away so I would assume that the ECU is not being misinformed about shifter position, is there a way to confirm?

OldMerc2 12-31-2011 11:43 AM

Yesterday I built a DTC reader and pulled codes to see what would turn up, here are the results:

Pin 6 (ABS) - code 13 (brake lamp switch)

Pin 7 (CC/ISC) - code 5 (stop lamp switch), 6 (n/a to US cars) and 11 (fuel safety shutoff)

Pin 19 (DM) - no response

All other modules checked out ok (1 blink), I found it odd that pin 19 did nothing at all.

Not sure why codes 13 and 5 are coming up, the brake lights seem to work fine.

Code 11 pretty much confirmed my suspicions about what was happening, under WOT the car pretty much falls on it's face at either 3500 or 4000rpm and will not upshift until one backs off the pedal.

But now what? This document ( makes reference to "23 - 18.0" in the "Test step/Remedy" column regarding code 11; I have no idea if that document comes from the manufacturer or what so I'm not sure where to go from this point. Now would be a good time for some input from someone familiar with MB service literature/trouble-shooting procedures.

macdrone 12-31-2011 08:44 PM

Do you know the 38 pin connector is oriented upside down when looking down at it? If so The codes you pulled maybe wrong.

Good Luck.

Clarkz71 01-01-2012 06:11 AM

Note the locating slots for the diag connector, use those as a guide
to the pin location.

OldMerc2 01-02-2012 12:49 PM

Location of the indexing notches on the connector confirms that the pin sockets I'm looking at are indeed the ones I think they are. I went ahead and erased all codes and put on a few drive cycles, the only code to come back is 6 from pin 7. A few other DTC documents I've looked at say that a code 6 may actually be valid for a U.S. car, it corresponds to the starter lockout switch. The fuel cutout issue is still present as I've run the engine up to 4000 rpm a few times with the same "fall on face" results; how many times would that have to happen to trigger a code 11?

The return of code 6 has me concerned about the state of the neutral safety switch, I have a extra used one that I can toss in with little effort. The intermittent no-start problem went away when I replaced the shifter bushings, common sense tells me that if the car starts in park and neutral only then the neutral safety switch is not sending bad information anywhere else, could I be wrong?

Clarkz71 01-02-2012 03:23 PM


Originally Posted by OldMerc2 (Post 5041941)
common sense tells me that if the car starts in park and neutral only then the neutral safety switch is not sending bad information anywhere else, could I be wrong?

The NSS/reverse light switch sends a signal from "each" gear position
to the LH module and has 2 extra pins as well for idle increase/decrease.

The problem you describe could very well be a bad switch.

It's not just P + N start any more to diagnose.

OldMerc2 01-02-2012 08:47 PM

Swapped out and adjusted the NSS, no change. I haven't yet checked if code 6 is back but it seems likely. I'd like to test the current NSS in situ as well as the wiring between it and the LH module, is there some sort of document outlining the NSS in detail so I can figure out what to test for?

OldMerc2 01-05-2012 01:52 PM

Last night I took apart my original NSS (a used one is currently installed). Though pics of the NSS's innards are relatively easy to find there don't seem to be any of the circuit board that also resides inside:

W124 :: NSScircuitboard.jpg picture by evermod - Photobucket

Aside from functioning as a starter lockout and reverse light switch the NSS also serves as a gear position indicator as pointed out by Clarkz71. Withing the NSS are two sets of contact points, one set is closed only when the shifter is in park or neutral (starter lockout), the other set is closed only when the shifter is in reverse (you guessed it).

There are two pins on the NSS that are smaller than the other four, these lead to the gear position circuit, the circuit runs through one of the smaller pins, through a slider and then through one of six resistors depending of shifter position and finally back out of the second small pin. Here are the measured values for each resistor (ohms):

P - 1,407
R - 296
N - 28,000
D - 11,340
3 - 5,900
2 - 3,100

I also decoded the color bands to make sure the measured values agreed with the actual ratings:

P - 1,400 +/- 1%
R - 294 +/- 1%
N - 23,000 +/- 1% (I checked this one multiple times)
D - 11,300 +/- 1%
3 - 5,900 +/- 1%
2 - 3,090 +/- 1%

It appears all the resistors from the disassembled switch with exception of the one for "N" were within the rated tolerance. The two small pins on the NSS are connected to pins 3 and 4 on the CC/ISC module as shown in this diagram:

The resistor circuit portion of the NSS is shown as "S 16/3". My guess is that some sort of reference voltage is fed through the resistor circuit and whatever is returned to the CC/ISC module indicates the transmissions gear position. When I get some time I will confirm what is coming from the module; I did have time today to test the currently installed switch at the CC/ISC module plug, here are the results of that:

P - 1,429
N - 29,100
D - 11,440
3 - 5,930
2 - OPEN

So yeah, the used switch I installed appears to be a mess, the original switch could've been just as bad as the measurements shown for those resistors were taken directly off the removed circuit board.

The results for the "D" resistor are not as far off as I would like, they are just beyond the 1% rated tolerance but I can't say if they fall outside an acceptable value for the CC/ISC module. It goes without saying that the results for the "R", "N" and "2" resistors show that a new switch is in order regardless. I really want to simulate a proper "D" position value, feed that to the CC/ISC module then test drive, but I have a feeling that if any weirdness is detected in any of the gear positions the system will simply default to a fuel safety shut-off mode.

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