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  #1  
Old 02-13-2006, 04:23 PM
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Please help! Snap-on MT2500 and DTC P0170 questions.

Hello,

I'd like to ask some questions:

1) Can (Snap-on MT2500 + Mercedes module) be used to RETRIEVE and RESET fuel trim adaptation values on a 1994 c220?

2) If that combo cannot be used to RESET fuel trim adaptation values, is there any "hack" method that can be used to reset them without using SDS?

3) Do new MAF replacements (such as those from Fastlane) also use thin-film instead of hot wire as the sensor element? I bought a non-Bosch MAF for my 1994 c220 from Ebay, and it uses a hot wire. It seems to be the constant cause of the DTC P1070 that persists after the MAF replacement.

4) Can long-term fuel trims values that are obtained through OBDII be interpreted as approximate fuel trim adaptation values?

5) After the MAF replacement, the code P0170 keeps coming up every time after I clear it using an ODBII scanner and drive for about 10 minutes. The car runs very well on highway as well as at idle. However, its only symptom is that it often sputters or hesitates for a fraction of a second when I give it gas to accelerate from a stop (usually after exiting a highway and stopping at the red light). This problem only happens when the car has warmed up after driving for about 5 minutes. It does not seem to happen when the engine is still cold. I have checked the fuel pressure regulator, EGR (it has a red dot indicating it was replaced in the past) and made sure there's no vacuum leaks in the engine compartment. BTW, the car's PSE pump is inoperative and I blocked both of its vacuum and pressure lines. I wonder whether this non-working PSE pump may play a role in the aforementioned symptom and DTC P0170?

FYI, I also replaced both pre-cat and post-cat oxygen sensors.

Thanks in advance for all your precious information.

Best regards,

Eric
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  #2  
Old 02-13-2006, 04:46 PM
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Exclamation

THE mass air flow sensor IS designed as a hot film on the C220! Never seen any thing other than that!....

That sensor is most likely your problem..

OBD2 on that car is only talking to the diagnostic module... the computer that needs reset is the HFM-sfi unit...
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  #3  
Old 02-13-2006, 05:54 PM
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why get failure codes with incorrect MAF?

You MUST use exactly the same MAF as the engine managemetn system is DESIGNED to use--no substitutes. A hot wire WILL NOT substitute for HOT FILM MAF. You are running on 'open loop' fuel ratio control.
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  #4  
Old 02-13-2006, 06:43 PM
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I think the Snap-On MT2500 with MB module should set adaptations, I'm relatively sure it should allow one to view them in actual values.

It is very unlikely that a MAF for your car could be anything but the hot hilm that it came with, but there is no scienticfic reason for that fact. The output of hot wire sensors and hot film are both voltage values between about 1v and about 4v. It's the "about" that causes problems. The situation is precise and the calibration can only be a ways off before the adaptation of the system runs out of range. You also may be faced with an early control unit that has the software that could only make an 18% correction. The new units run to 32% correction, either rich or lean.
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  #5  
Old 02-13-2006, 08:49 PM
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Wow I learn something new every day :-)

Dear Mr. Brotherton: your reply really solves one of the puzzles that I faced, because I always got the long-term fuel trim value of (-18%) using my OBDII scanner. I thought it might be a bug in my OBDII scanner. In some of your posts in other threads, you mentioned something about the ability of Mercedes control units to make a correction of as much as -32% or +32%, while I could only get a LTFT of -18% from my car. Now everything is clear: my car must have an early control unit that can only make a correction down to -18% and up to 18%. Thank you very much for that precious piece of information!

Many thanks to M.B.DOC, Kebowers, and Steveblf for your expert replies. I will send my complaints to the Ebay seller regarding the HOT WIRE MAF. I should expect a HOT FILM MAF only (anyway, that's what HFM stands for :-)

From your extensive experience working with Mercedes cars, what are the average corrections on a new Mercedes and an used working Mercedes? Is a 0% correction very commonly seen? If a car is running with a correction of as much as -30% or +30% (supposedly with new control units), is it still considered as being in good shape?

Are you aware of any "hack" that allows the resetting of the HFM-sfi unit without using a Snap-on MT2500 or a SDS?

Many thanks again.

Best regards,

Eric

Last edited by ericnguyen; 02-13-2006 at 08:54 PM.
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  #6  
Old 02-13-2006, 09:38 PM
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>

You can reset/re-activate the memory of the HFM/SFI module with a home made led tool.

http://members.aol.com/ajdalton7/ledtool.jpg

This tool will let you get flash codes at HFM at pin #4 of the 38 pin connector. After you retrieve the codes [ write them down] , you can clear them, one at a time .
After all codes have been cleared , you will get a Single flash, verifying all stored code have been erased.
Now the trick..
After you get the single flash, turn the key OFF for 30 secs .... Key back On and get the single flash again.. wait 2 secs and hold button on tool for 6-8 secs.. turn key Off for 2 secs , On for at least 10 secs , and then start car.
The memory on the HFM module will now be reset to zero/mean...
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  #7  
Old 02-13-2006, 10:08 PM
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Arthur,

Your led method sounds much like the beginning steps of adaptation clearing using SDS. It does leave one step out. After the ten second clearing and the turning off of the key the key is then turned on for 30 seconds to relearn the throttle. The key is then again turned off and then can be driven.

Does your method not clear throttle adaptation?

As to the concept of aging. I would consider a car with greater than 10% part load adaptation to be heading to failure. Cars usually run less than 6-7%. The thing about adapting is that it is really a static/steady state function. A system doing perfect with high adaptation numbers, is likely to not do as well in various load rpm ranges. There is after all only one correction. Domestic vehicles adapt in load rpm blocks. There are numerous of them and it is often the case that a system will have widely different numbers in each block. I have always assumed the Germans have a more stable system needing less variety of correction, but fast acting changes are the ones most notably not corrected. This accounts for misfires from cars near the end of adaptation but within.
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Last edited by stevebfl; 02-13-2006 at 10:18 PM.
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  #8  
Old 02-13-2006, 10:19 PM
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SB

You are correct .. I forgot that step.
Yes . it does clear throttle ..different pin

It is basic procedure for reset after single flash only .. will not work on DM as that has to go through self test cycles after code clearing .
[ you already know all that, but I mention it for others interested]

Hey, it's COLD down here............
Have some friends coming down for Daytona , so hope it gets back to temp...........
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  #9  
Old 02-13-2006, 10:50 PM
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Supposed to get to 80 by the weekend
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  #10  
Old 02-14-2006, 04:46 AM
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This is really good!

Many thanks to Mr. Dalton for your wonderful trick, and to Mr. Brotherton for your expert knowledge of Mercedes cars.

Since there is some discontinuity between your posts, I would like to ask you to confirm the following sequence of steps to reset the HFM module:
--------------------------------------------------------
1. Get the single flash code.
2. Turn off the key for 30 seconds.
3. Turn on the key, and get the single flash code again.
4. Wait 2 seconds and hold the button for 6-8 seconds.
5. Turn off the key for 2 seconds.
6. Turn on the key for at least 10 seconds.
7. Turn off the key.
8. Turn on the key for 30 seconds.
9. Turn off the key.
10. Turn on the key and start the car!

Result: The memory on the HFM module will now be reset to zero/mean...
--------------------------------------------------------

Merci beaucoup!

Eric
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  #11  
Old 02-14-2006, 05:32 AM
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That should do it..
You will have to use smaller test lead ends for the tool, as the tool pictoral I posted is for the 16 pin connector [ vs your 38 pin], which takes the larger bananna plugs. I think you will find a single strand # 20 wire will fit the 38 pin..

Also , on the 38 pin , power [ 12v +/-] for the tool will be pin 1/- and pin 3/+ .. the odd color will go to pin 4 for HFM module initiation...

Here is some good info... go to page 12 for 38 pin connector locator diagram

http://www.autolib.diakom.ru:8000/CARS2/pribor/cs1000/manual/cs1000_mb.pdf
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  #12  
Old 02-14-2006, 08:10 AM
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I have never tried such a manual reset and I might have confused the issue. I did three readaptations yesterday on a 202 car. BUT! it was an ME car.

As Authur pointed out , briefly, the throttle is controlled by a separate controller on HFM cars and OBDII (check engine light) diagnostics are done on a third module. While monitoring of adaptation state is faulted in DM, the actual adaptation is done in HFM. Clearling mixture adaptation in HFM probably won't require a throttle relearn as it does in ME, although it never hurts and is done every time one turns the key on and lets it sit for 30+ seconds with the key then being turned off.
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