R172 Problem Diagnosis 101 - Mercedes Benz SLK Forum

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Old 07-05-2017
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Cool R172 Problem Diagnosis 101

Sooner or later your R172 is going to encounter a fault, maybe more. It is imperative that you have a good scanner tool (not all are created the same). The main purpose of this thread is just to demonstrate the necessity and use of a reliable one. In my case I'll be using the well known and highly recommended Autel MD-802 All-System. Not to go into the best scanner tool as there are threads on the "best scanner tools", some vehicle specific, but in this thread you will also get to see the versatility as well as some of the capabilities of the All-System scanner tool. My dash is lit up so bright right now with MIL (malfunction indicator lights) I got to wear sunglasses Time to bust out the 802 and get to it
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Old 07-05-2017
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Posting pics in order of occurrence from start to finish you can see this scanner is versatile and covers many different makes and models. I'll be using the Auto Scan feature which scans all modules and systems available on the car. On the R172 it scans over 30 different modules and lists all DTC's (default trouble codes)associated with each.
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Old 07-05-2017
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Here I have a fault shown so I'll hit the display DTC button to display all faults associated with this module.
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Old 07-05-2017
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Continuing on with the Auto Scan and displaying faults. Didn't realize I had a problem with the passenger side rear view mirror heater, but then again I don't ever use it Ok and the EPB communication error with ESP is going to be associated with the sensor issue as well. In fact the longer you let an issue go unchecked, the more likely you are going to have more DTC's set as each module has trouble communicating with the other.
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Old 07-05-2017
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Although not a huge issue, the cooling fan is still concerning to me because it keeps the amplifier cool so I WILL be having a look into this later. On the keyless go issue, I replaced the battery and that DTC cleared. The keyless go has been working fine although these two DTC's with teachin and door sensor remain uncleared because the scanner will not clear unchecked (not repaired) codes which is a feature I like. Not sure why these two issues remain but again I am having no problems with the keyless go whatsoever so I'll let it ride for now. If any body has any info on this it would be most nice to go ahead and post it here for all to see
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Old 07-05-2017
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Continuing the scan.
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Old 07-05-2017
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Not sure if this is from the last time I cleared DTC's as this shows up as an event fault, or if it is related to the audio 20 control unit fan issue
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Old 07-05-2017
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This one has 3 faults. The first and last one are obviously related to the sensor issue. The second one is from me messing around with the SOS button as it is not in service
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Old 07-05-2017
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And last but not least. So all in all, mostly minor stuff but the speed sensor issue is the prioritized problem for me now as it affects a lot of the systems and is the cause of all the MIL lights that I have including the check engine light. I am going to focus on it for now and as you can see from all the information the scanner provided there is a definite problem with the signal coming from the left front speed sensor so its on to find out what the problem is. To be continued on the thread R172 VSS repair
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Old 07-05-2017
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Looks like a great tool Rob! How much did that set you back?
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Old 07-05-2017
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Looks like a great tool Rob! How much did that set you back?
A buck seventy-five ($175) on Amazon. I'm very pleased, next best thing to me would be a DAS/STAR setup.
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Old 07-05-2017
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Oh ya, for any interested DIY'ers here is a link to an awesome site that offers all kinds of trouble shooting and problem solving techniques, and detailed descriptions of how things work on just about any system. The site is underhoodservice.com and here is a link to the VSS (vehicle speed sensor) or WSS (wheel speed sensor) page: Wheel Speed Sensor Diagnostics for Meters and Scopes
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Old 07-07-2017
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UPDATE: LMAO, your not going to believe this peeps. So I haven't done the DIY VSS sensor repair thread yet because I am still dealing with this bs (but I will). So I replaced the left front VSS, reset the codes, all cleared, all functioning fine. Next day I took about a 30 mile drive to the Dr.'s office and all is well until on the way back home. Not even 2 miles from the house, BAM! ABS/ESP all in my face again so I'm like WTH must be something else wrong. Hooked the scanner back up and guess what, the DTC for the right front came up. Put the scanner in Live Data mode to view the sensors working and all is fine starting out with very little variation but there was a slight difference in the signals from the front two(mostly from turning as one wheel will turn slightly faster than the other-1mph difference). When I turned around and got up to speed, all of a sudden there was no signal coming from the right front at all. This sensor was just replaced at 8800 miles last year and the car now has only 12000 miles on it. So to sum it up, here is how bad the ABS/VSS sensor situation is for me after owning this car for two years, buying it used with only 3300 miles on it, and putting roughly 9000 miles on it: Right front goes out at 8800 miles, left front at 12000 and right front again shortly after. At this point I am highly suspicious of the right front just being a bad connection by the way it was working one minute and not at all the next. This one was replaced by MB and I just happened to be watching the tech put the part on and noticed his fumbling around trying to reconnect the connector with a long handle screwdriver because he didn't want to take the time to remove the fender liner out of the way some first. I will have a look at it first b4 I order another sensor and report back my findings. I'm betting bad connection, hope so, really don't want to drop another $68 on a crap aTe sensor. aTe makes the sensors for MB and it would be nice to see the failure rate of their sensors compared to other manufacturers. I have found some as low as $30-40 and am almost tempted to try one and see how long it lasts.
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Old 07-08-2017
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UPDATE: I did the VSS repair thread. Ima hold off for a bit on this right front sensor issue for now but I will follow up, its hot as hell here I am looking at this Audio 20 or Command control unit fan issue (DTC A386 above). I am wondering if part #8 in the pic in this link is what is being referred to: https://mercedespartscenter.com/part...agramCallOut=8 Also I see that it is not fused at any of the SAMs (wonder if the Command unit powers it?) and I have to remove the glove box to get to the fan as well as the display screen to get to the connector to check the connection.

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Old 07-09-2017
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Here is some nice info on OBDII
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Old 07-13-2017
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UPDATE on the #1 cooling fan issue: I went into the Comand engineering mode and was able to view the fan settings. I could change the fan speed settings from auto to slow to fast to fastest (Autel scanner tool would do this too). There are two cooling fans both of which are controlled and powered by the Comand HU (head unit), #1 fan being the Comand HU internal fan and #2 being the external fan. I could easily hear the #2 (external) fan running as I changed the speeds but could not hear any fan operation at all from the internal fan even on fastest speed and all quiet inside. So it looks as though the internal fan is bad which will require removal of the Comand HU to repair. The code was A386 (fan 1 of the control unit has a malfunction) and it didn't lie. Not a big enough deal for me to remove the HU at this time, still got the external cooling fan so I think I'll let it ride
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Old 07-14-2017
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Hey! Good to hear it wasn't the #2 fan. That would have taken a lot more stuff to take apart. I did take the NTG2.5 (R171) apart (to replace the scratched screen) and it wasn't a big deal. Assuming they still have the same supplier, it's straigh forward taking it apart. You should be able to buy the internal fan from any electronics shop. Its connections may be soldered to the board though.
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Old 07-14-2017
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Hey! Good to hear it wasn't the #2 fan. That would have taken a lot more stuff to take apart. I did take the NTG2.5 (R171) apart (to replace the scratched screen) and it wasn't a big deal. Assuming they still have the same supplier, it's straigh forward taking it apart. You should be able to buy the internal fan from any electronics shop. Its connections may be soldered to the board though.
Ya one day when or if I have to pull the HU for any reason I will have a look at the fan ( Associates degree in electronics makes soldering easy). I'm on to the next two issues for now ( right front VSS and mirror heater output ). Stay tuned for results
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Old 07-16-2017
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UPDATE: Right Front VSS- After checking all electrical connections (ABS module, VSS connector) for good connection, I hooked up the scan tool again to observe VSS signals. This car is only 5 years old and only has 12K miles on it so everything is in pristine condition and highly unlikely to have bad connection or wiring issue, but its always a good idea to check first any way. I also opted to do this b4 condemning another sensor as this one was just replaced around 4000 miles or so by dealer. Upon observing the function of the right front VSS again with the scanner tool, sitting still in the driveway it was giving all kinds of erratic readings. At one time it was registering 167 MPH, thats pretty fast for sitting still Yep another bad ABS/VSS folks, third one in less than 10000 miles.
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