Mercedes-Benz Forum banner

1 - 19 of 19 Posts

·
Registered
1991 190e 2.3
Joined
·
344 Posts
Probably not the BP sensor. Mine was doing the same thing and it turned out to be vacuum leaks plus a failing fuel distributor.

Start with a thorough inspection of all vacuum lines.
 

·
Registered
1982 240D manual 4spd
Joined
·
42 Posts
Perhaps a moot question but are you sure you're reading it correctly? Maybe it's 20%?
Yes, it is 80%.But yesterday when I took reading it was showing 1.4%.
It is very confusing.
I have replaced the FD with a quality rebuilt one.
Any ideas?
 

·
Registered
1991 190e 2.3
Joined
·
344 Posts
Yes, it is 80%.But yesterday when I took reading it was showing 1.4%.
It is very confusing.
I have replaced the FD with a quality rebuilt one.
Any ideas?
Sorry for the slow response. I've been off the grid for a few weeks doing family things.

When you measure the DC do you get continuously fluctuating readings or does it stay at a constant voltage all the time?


You're profile doesn't indicate what model car you have. What year and model w201?
 

·
W201, W212, W221, & W222 Moderator
'84 Euro 500SE, '85 Euro 2.3-16, '51 Euro 170S, '97 Jeep Wrangler Sport, '15 G63 AMG
Joined
·
8,446 Posts
Where in SA are you?
 

·
Registered
1982 240D manual 4spd
Joined
·
42 Posts
Sorry for the slow response. I've been off the grid for a few weeks doing family things.

When you measure the DC do you get continuously fluctuating readings or does it stay at a constant voltage all the time?


You're profile doesn't indicate what model car you have. What year and model w201?
I have '86 560sl R107.
It is a constant voltage when it is over riched but would start fluctuating if I lean it out a bit.But the problem is the car would bog down and not accelerate normally if it is not over riched.
 

·
Registered
1991 190e 2.3
Joined
·
344 Posts
I have '86 560sl R107.
It is a constant voltage when it is over riched but would start fluctuating if I lean it out a bit.But the problem is the car would bog down and not accelerate normally if it is not over riched.


I would suggest posting over in the R107 forum but since we're here I can tell you how the w201 works.

There are different methods to trigger the DC reading depending on the generation of the vehicle, where it came from, and in the USA there is a NAM version (and the CA version is different). Some you could just hook up the meter and without doing anything else you could get the DC reading. On others (like mine) you have to trigger the DC reading using a device. On the CA versions they actually have a LED and push button located near the battery.

In any case, once the car is in the DC mode then the meter should bounce around a bit. If you are getting steady voltage then it's most likely not in DC mode.

There are numerous posts on this and other boards about how to build the box as well as error code legends.


Having said all of that, I personally spent a few months trying to track down my rough idle issues and I was getting 80% DC as well. Turned out to be a combination of vacuum leaks and a failing fuel distributor.
 

·
Moderator
190E, SLK350
Joined
·
66,915 Posts
Back to being stuck at 80% duty cycle: when that happened to me, it turned out to be the atmospheric pressure sensor.
 

·
Outstanding Contributor
'85 300D Turbo, '91 420SEL, '92 190E 2.6, '09 C350, '12 E63 Wagon
Joined
·
3,027 Posts
For the M103 (with fault code diagnosis) it’s supposed to be IAT Sensor failure or short to ground. I’ll attach the test procedure. Mine’s accompanied by an unusually high idle. Will try to post info in an attachment. Can’t get the darn connector off the CFI Control Module to test continuity of pin #11! (Technically mine's reading 81.x% for full disclosure)...
 

Attachments

·
Registered
Mercedes-Benz 190E 2.6, 300E, Volkswagen Jetta 1.4T
Joined
·
909 Posts
For the M103 (with fault code diagnosis) it’s supposed to be IAT Sensor failure or short to ground. I’ll attach the test procedure. Mine’s accompanied by an unusually high idle. Will try to post info in an attachment. Can’t get the darn connector off the CFI Control Module to test continuity of pin #11! (Technically mine's reading 81.x% for full disclosure)...
An 88 has just the altitude sensor which is behind the battery tray. There's an easier way to test it then pulling the plug off of the CFI unit, just check each pinout for continuity against another pin on the sensor. There are 3 pins and 3 possible arrangements where you need to have continuity. If only 2 or less combinations of pins yield continuity then the sensor is bad.

The nice part about the FSM procedure is it allows you to rule the wiring out as well, but the altitude sensors are known to fail while the harness is less likely to.

You can also test the temp sensor for resistivity at varying temperatures and see if it matches the thermal curve.
 

·
Outstanding Contributor
'85 300D Turbo, '91 420SEL, '92 190E 2.6, '09 C350, '12 E63 Wagon
Joined
·
3,027 Posts
An 88 has just the altitude sensor which is behind the battery tray. There's an easier way to test it then pulling the plug off of the CFI unit, just check each pinout for continuity against another pin on the sensor. There are 3 pins and 3 possible arrangements where you need to have continuity. If only 2 or less combinations of pins yield continuity then the sensor is bad.

The nice part about the FSM procedure is it allows you to rule the wiring out as well, but the altitude sensors are known to fail while the harness is less likely to.

You can also test the temp sensor for resistivity at varying temperatures and see if it matches the thermal curve.

The sensor reads fine, or at least close enough I would not expect any error. (I’m going off approximate ambient temps via a weather app).

Thanks for the tip about measuring resistance across the two pins of the IAT Sensor connector/harness to look for a short. I actually forgot about that. It’s been upwards of 19 years since I was last studying EE....

I can’t find the proper diagrams/wiring, even though I now have an ALLDATAdiy.com subscription for the W201. I may have to use the “Contact/Request Info” feature. (“Not exact wording”). Makes for a hell of a time diagnosing/repairing something that is most likely damaged somehow. I simply connected one end to the socket/wire in question, and probed every single (all pinned connectors on the 55 pin harness plug): toned on 2 pins for #2 IIRC, and none on #1! There is no sign that anything was ever connected at socket #14. It’s clean and empty, like the others that aren’t connected. I did note a possible contradiction, as the altitude sensor is noted to connect to the MAS, but changed at different years, to the IAT I believe......

Thank you for the tips!!!

Unfortunately for me, now if the car sits for a month it gets extremely hard to start. Something else developed/broke as I tried to solve the constant Check Engine light due to the Air Flow Pot...
 

·
W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
Joined
·
2,323 Posts
IAT sensor = Intake air temperature Sensor - '90 - '94 (post-EGR)
APS sensor = Altitude Pressure Sensor - '84 - '89 (pre-EGR)

Just trying to get the acronyms and their valid production years lined up.
 

·
Outstanding Contributor
'85 300D Turbo, '91 420SEL, '92 190E 2.6, '09 C350, '12 E63 Wagon
Joined
·
3,027 Posts
I was originally working with a diagram for a ‘87, then Paul gave me one for a ‘92. PN gave another one, but they both point to pin #14, and no matter how many times I counted there’s nothing there. (I’m not a good counter)...

So I probed the rest and nothing. I also blindly went through PNs Russian site and looked back and forward some years. With ALLDATAdiy you subscribe for a specific year make and model but the wiring shows some of the older stuff. Still sifting through it all....
 
1 - 19 of 19 Posts
Top