Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Tech Help

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 04-05-2008, 07:21 PM
Registered User
 
Join Date: Sep 2004
Location: Seattle
Posts: 1,942
Short term trim vs. Long term vs. O2 calculated trim

I am curious about what the O2 sensor's "calculated trim" is. My short term fuel trim hovers around 0 as it should and the long term trim is at 19. The front O2 sensor's calculated trim is 64. What does that mean? I don't have any trouble, I am just curious what it means.

By watching the long term trim and the O2 trims can anything be determined about the "health" of the MAF sensor or the O2 sensors?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
Reply With Quote
  #2  
Old 04-07-2008, 08:15 AM
Moderator
 
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
The long term trim is usually a good way to determine the MAF's ability to read volume correctly. Having a "clean" air intake is very important.

Vacuum leaks & fuel pressure can FOOL even good technicians on those readings.
__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator
Reply With Quote
  #3  
Old 04-07-2008, 10:17 AM
Super Moderator
 
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Just for comparison, on my SL500 problem child's most recent reading, the long term trim on the left is .8% and the right -.8%. I did read that an absolute value less than 10% for LTFT is "good."
__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
Reply With Quote
  #4  
Old 04-07-2008, 04:29 PM
Registered User
 
Join Date: Sep 2004
Location: Seattle
Posts: 1,942
Hmmmmm. Well that is interesting. So are you guys saying that since my Long term trim is 19 that means the MAF is getting "old". Is there any point to watching this number?

Now I still don't have an answer to my original question. What is the O2 sensors "calculated trim"? Is that number good for anything?

Thanks
__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
Reply With Quote
  #5  
Old 04-07-2008, 05:22 PM
Moderator
 
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
O2 sensors react only to "fuel mixture" changes. their "adapation" is of little OR no concern. When an O2 sensor has a problem YOU will see a check engine light quickly!
__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator
Reply With Quote
  #6  
Old 04-07-2008, 08:01 PM
Super Moderator
 
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Where are you seeing the "sensor calculated trim"? That one is new to me, but that's not saying much. The sensor is pretty dumb, it can't calculate anything. It simply sends a voltage to the the ECU indicating the amount of oxygen in the exhaust stream.
__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
Reply With Quote
  #7  
Old 04-07-2008, 09:33 PM
Registered User
 
Join Date: Sep 2004
Location: Seattle
Posts: 1,942
It is part of the data stored by the car's computer relating to the O2 sensor. Technically it is byte "B" of PID 14 of the Mode 1 OBDII commands:

"PID 14 Oxy. sensor voltage bank1 sensor1
This PID returns the Oxygen sensor voltage for the sensor located in Bank 1, Sensor 1. There are 7 other PIDs which are similar, but for other locations. In each case, Two bytes are returned. Data A represents the sensor voltage (defined below), and Data B is the short term fuel trim associated with the sensor, or Hex FF in the case where the sensor is not used in the calculation.
Oxygen Sensor Voltage = .005 * A
Short Term Fuel Trim% = .7812 * ( B-128 )"

Other sites referred to this as the "calculated trim". In any case it is not same as the the Short Term fuel trim associated with the bank or the Long Term fuel trim. Those are PID 6 and 7:

"PID 06 Short term fuel % trim Bank 1
The fuel trim is a value from -100% (lean) to +99.22% (rich)
Fuel Trim% = .7812 * ( A-128 )

PID 07 Long term fuel % trim Bank 1"

This is from http://obddiagnostics.com/obdinfo/pids1-2.html
__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
Reply With Quote
  #8  
Old 04-08-2008, 09:46 AM
Super Moderator
 
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Well, that helped, a little. I think what you are referring to is the value "Fuel trim or lambda" plotted in blue on the attached graph, which is from my S500 idling. It looks to me like the computer records two data values, the sensor voltage and the fuel trim, which is a measure of the correction needed at that particular instant.

Most the fuel trim values in the plot are between -2 and +2, which I assume is a %, so I don't know how to relate that to your value of 64. Maybe that's the raw value of "B" in your equation. But that doesn't make a lot of sense, since plugging 64 into the equation

Short Term Fuel Trim% = .7812 * ( 64 -128 )
STFT = -50 (rounded)

Even if the 64 is a hex value (100 decimal), plugging 100 in gives a value of -21.9.

Maybe someone who understands this stuff will enlighten us.
Attached Thumbnails
Short term trim vs. Long term vs. O2 calculated trim-s500-idle-o2-plot.jpg  

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Last edited by ctaylor738; 04-08-2008 at 10:05 AM.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 05:54 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page