'97 S420 Acceleration and trans jerking issues - PeachParts Mercedes-Benz Forum

tinypanzer · #1 03-22-2017, 07:04 PM

Hello all,

There's a lot of variables here, so I'll try to break this down into a sane set of parameters.

Background: Just bought a '97 S420 with 265k on the clock from my local indy. Yes, that's a lot of miles however the body is in absolutely stunning condition, and my indy just did a complete rebuild on the transmission. It's a car he wound up with after the previous owner decided they didn't want to pay for a new trans.

A host of performance issues plagued the car when I got it, for starters it had been sitting for a while and was just starting to feel the effects of lot rot. My main complaints with the car so far are these things:

- Poor acceleration, weird and inconsistent throttle response
- Transmission sometimes causes the car to jerk back and forth a little bit when coasting down to a slower speed
- Lifter tick, likely the oiler tubes although 8oz of MMO made it go away for a while

I've done several things to the car, and some improvements have been made. The throttle body was badly fouled, so I cleaned it. I put a new MAF sensor in. I reset the adaptive transmission data using the standard key/throttle method

The poor acceleration issue is a lot better, but not 100% cured. Sometimes it feels as though the transmission has simply shifted too soon and doesn't want to kick down unless I give a LOT more throttle. Overall however, the MAF sensor replacement made a very big improvement.

I very much doubt that the actual transmission is the issue here. For starters, my indy used to be a dealer tech and he has done lots of these. His work is excellent, and I do not yet think that the problem lies in the transmission because to me, it feels much more like the ECU is confused about how much throttle I am giving, or is getting bad sensor data somewhere.

The jerking motion I feel during coasting down to a lower speed is intermittent, and it almost feels like the ECU can't decide if I'm stepping on the gas or not, as if it's failing to allow what I will ignorantly refer to as 'overrun'.

My main suspects at this point are the throttle position sensor and the electronic throttle assembly.

Can anyone offer any suggestions and/or test procedures to help me isolate the fault? Is there some other part I didn't mention that needs to be checked?

97 SL320 · #2 03-23-2017, 06:54 PM

I'd have a look at fuel pressure while driving and engine vacuum. I had a GM car with a plugged cat that ran fine most of the time and didn't have any of the typical exhaust hiss.

With a good scan tool you should be able to see throttle percentage.

What do the spark plugs and fuel filter look like?

tinypanzer · #3 03-23-2017, 07:31 PM

Thanks for the reply.

Fuel filter is fresh (~100 miles on it or so) and my indy who I bought it from did an injector purge too. Mostly the acceleration issues are gone, except when the trans decides to shift too early. Plugs will get done this weekend as they just arrived today. Hoping for a slight improvement in a slight miss I occasionally get, but not thinking that will do anything about the trans jerking as I coast down in speed.

I need to get a better scan tool. Apart from my '96 Impala SS that I sold like a decade ago, I haven't owned any OBD-II cars until now and I just have the crappy little Bosch code reader with no realtime anything.

I do however have good electronics skills, and high accuracy multimeters (Keithly 2016THD among others) so I can certainly measure resistance values accurately. Anyone have a chart for values and pinout of the TPS?

Also, are there any tests or resets that can be performed on the throttle body?

Q - How does this car know when you let off the gas? Is there an idle switch somewhere like on older Benzes or one built into either the TPS or ETA?

97 SL320 · #4 03-24-2017, 05:21 PM

97 should be total drive by wire, ( the inline 6 was by then ) The pedal position sensor is on the left fender. This is a dual pot system where the computer compares both to confirm one has not failed so I'd expect a code to be tripped.

I'd pull the trans computer and look for oil. When the trans connector bushing fails, oil wicks up the wires. Oil on the circuit board isn't the problem, it is the graphite from clutches mixed with the oil that cause issues. Cleaning the board should work although I've seen an oiled board with no ill affects. If you swap trans computers, get one from another 420 because the smaller engined cars had different trans ratios and you will go into limp mode. You will also need to match rear end ratio also.

Does the ASR triangle blink on hard acceleration? Do you have an ASR off switch? My 97 SL320 engages ASR too early and will pull the engine down. I'd try driving with the ASR off. Well, the switch actually mutes ASR rather than a full torn off, you would need to jumper a pin on the 38 pin under hood connector.

tinypanzer · #5 03-24-2017, 07:30 PM

Thanks for the reply and suggestions. Since my indy just rebuilt this trans, I would suspect that any fluid migration issues would have been addressed, but it's certainly worth a check. Where is that module on my car?

Yes, this '97 is totally drive by wire even though it was made in late '96. I have found the throttle pot near the firewall on the driver's side. Interesting that it's a dual track pot, I'll check out the resistance values even though I agree, there should be a code if the values don't agree with each other.

My car has ESP, but not ASR (thank god). Not sure I would have bought an ASR car from this era after all the stuff I have read about the problems with it. ESP triangle does not come on during these episodes, and WOT acceleration is pretty good. It's more the problem of being in 4th or 5th at 35mph and wanting to accelerate... Often I have to give more than 1/2 throttle before it finally decides it needs to kick down. There's no feathering the pedal to go a little faster, you're either accelerating hard or not at all. Now, I'm not sure whether it really does need to kick down, or if it is lacking power in that RPM range for some reason and just doesn't have the oomph to speed up without a kickdown. The car being new to me doesn't help, I have no baseline to compare it against.

97 SL320 · #6 03-25-2017, 07:08 AM

Actually, ASR ( Acceleration Slip Control ) is base and ESP ( Electronic Stability Program ? ) just adds software. 96 and older cars had the less than better half mechanical / half drive by wire. 97 had the much better full drive by wire.

tinypanzer · #7 03-25-2017, 11:50 PM

Quote:

Originally Posted by 97 SL320

Actually, ASR ( Acceleration Slip Control ) is base and ESP ( Electronic Stability Program ? ) just adds software. 96 and older cars had the less than better half mechanical / half drive by wire. 97 had the much better full drive by wire.

Well, we're drifting off topic a bit here, but I don't think that is correct. ESP uses intertia sensors, yaw sensors, and steering angle sensors to decide when the vehicle is skidding. ASR as far as I know just uses the ABS sensors to detect wheel spin. At any rate, my car has neither an ASR light or ASR switch.

Put the new plugs in today... Passenger side plug sockets were drowned with oil. I sucked about 2oz out of those four plug wells using a mityvac bleed kit. No doubt those coils are suspect, but despite a slight miss there's no misfire code to speak of.

One thing I noticed today is that it really does seem to get worse as the engine heats up. What a pain. Wasn't expecting to have to deal with this kind of crap so soon, the car looked to be much better maintained. Still pretty clean overall, but sigh.. I hate a weak throttle.

Still looking for suggestions and or tests to perform.

tinypanzer · #8 03-27-2017, 02:23 PM

So, dunno if this helps in the diagnosis at all, but I noticed over the weekend that the jerking issue only happens if I have the gear selector in 3rd or higher. If I keep it in 2nd, the problem does not happen. What is weird is that I'm pretty sure the jerking is happening in 2nd in both cases. I need to verify that tonight, but I am starting to narrow down exactly when this happens.

Again, the jerking is like a back and forth rocking that happens when I start coasting down to a lower speed, usually when traveling about ~30 mph or so to start with. Shows up a lot approaching stop signs.

97 SL320 · #9 03-27-2017, 08:31 PM

The shifter on this car is mechanical for park through drive then electric switches from 4 to 1. I wonder if the switch got something spilled on it. ( Later cars with the move left and right to shift 4 to 1 had a different set up. )

Try to get a hold of a scan tool that used the 38 pin under hood connection and access the trans computer. ( A 38 pin to OBD 2 cable won't do the job as it is the same as plugging in under the dash. ) Look at selected gear position and see if it is changing without moving the shifter.

tinypanzer · #10 03-27-2017, 08:55 PM

Quote:

Originally Posted by 97 SL320

The shifter on this car is mechanical for park through drive then electric switches from 4 to 1. I wonder if the switch got something spilled on it. ( Later cars with the move left and right to shift 4 to 1 had a different set up. )

Try to get a hold of a scan tool that used the 38 pin under hood connection and access the trans computer. ( A 38 pin to OBD 2 cable won't do the job as it is the same as plugging in under the dash. ) Look at selected gear position and see if it is changing without moving the shifter.

Now that is an interesting bit of info, thanks! I will look for that. The PO was a slob and there was much evidence of spillage in places, although it cleaned up pretty well. So it is entirely possible there is coke in those switches.

Also been doing some reading about the whole ATF wicking problem with the trans module. I am going to pull the module tonight and see if there is any evidence of that. Since it is completely separate from the trans, I suppose it's possible my indy did the rebuild and never checked the module for fluid migration up the wires.

Thanks for the leads, I'll try and narrow down some possibilities. While I do not have a 28 pin connector, much less the correct tool to use with it, I will see if I can use a DMM to assess if those switches are intermittent or not. I'm pretty good at finding intermittents like that, provided the switches are accessible without too much pain.

97 SL320 · #11 03-27-2017, 09:08 PM

All of that era cars had the 38 pin under hood. Look for a under hood box near the firewall with what looks like an oil cap with a hex in the middle, it's under there.

tinypanzer · #12 03-28-2017, 12:51 PM

Quote:

Originally Posted by 97 SL320

All of that era cars had the 38 pin under hood. Look for a under hood box near the firewall with what looks like an oil cap with a hex in the middle, it's under there.

Meh, I didn't word that correctly. What I meant to say was 'I don't have a connector to plug into that 38 pin socket under the hood'

At any rate, looked at the trans module yesterday, and it was bone dry and spotlessly clean, as was the wiring harness. So there goes that possibility...

tinypanzer · #13 03-29-2017, 03:42 PM

Well, I am starting to narrow things down a bit. Did some careful observation and experimenting, and it turns out that the jerking issue only happens when the trans is in 3rd. Not talking about the shifter position, I'm referring to what gear it is in.

If you start slowing down while its in 3rd, that's when it starts that sort of bucking back and forth behavior. I'm starting to think that perhaps there is something mechanically amiss in the trans.

97 SL320 · #14 03-29-2017, 07:03 PM

I'd still find a scan tool that will read the trans and look at shifter position , requested gear , actual gear when this is occurring.

tinypanzer · #15 03-29-2017, 08:13 PM

Quote:

Originally Posted by 97 SL320

I'd still find a scan tool that will read the trans and look at shifter position , requested gear , actual gear when this is occurring.

I will try to do that, might be time to invest in a better scanner. When you say 'requested gear' does that refer to the position of the gear lever or what the ECU is 'requesting' of the transmission? Am I troubleshooting just the gear select switches here, or am I also seeing if the trans is refusing to shift even though the computer is telling it to, or both?