Easy way to test MAF / MAS / AMS
 

I thought it would be helpful to put this in a separate thread. Some of you may have seen my parallel thread in which my M104 MAF is suspected to be faulty: http://www.peachparts.com/shopforum/tech-help/191793-1997-e320-m104-flat-low-rpms-backfiring-post1543618.html#post1543618

In my research I stumbled upon this VERY HELPFUL document which outlines exactly how to test a MAF. No more guessing - it is a very exact procedure: http://www.x-desk.net/ximages/21302_pgsi0079en.pdf

The output voltage of a healthy MAF is supposed to be around 1.00 plus or minus 02 with key on engine off. Upon starting the engine, it should rise to 1.6 and then travel all the way to between 3.8 and 4.4 at full throttle.

When I tested mine, the output voltage was 1.462 with key on engine off, which is way high. When I blew a little bit into the MAF, the voltage would go both up and down (it should only go up) and was very erratic. Upon starting the engine, it was at 1.64 (a tad bit high). I didn't bother with the full-throttle test, as it was very evident to me that the MAF was malfunctioning.

At least I think I know what's going on now. My E320 M104 is still driveable, you just can't accelerate slowly. Have to give it lots of throttle from a standstill or it will go flat. I will be ordering a new MAF as soon as I can.

Oh, and the document also settles the debate over whether or not there is actually a difference between the $130 MBZ element and the $59 generic (VW, Ford, Audi, etc.) element. Definitely different part numbers, so I wouldn't even mess with the generic since I like my catalytic converter the way it is.

Good find. Great article. Here's another, more basic one: http://www.answers.com/topic/mass-flow-sensor

Did you try to clean it and then retest??

I had similiar problems with my MAF which was contaminated . . . cleaned up perfectly. Check MENU#4a for details.

My test was actually run AFTER I cleaned it. I have no idea how bad it was before cleaning, unfortunately I didn't stumble upon the document until afterwards. Based on the out-of-spec measurements I feel pretty confident that a new one will eliminate the problem.

JimF, do I read correctly that your MAF issue wasn't triggering a CEL? I ask because my CEL is not illuminated either, yet it would appear the MAF is faulty. I guess I'm just surprised the MAF can be bad with no CEL illuminated.

Did you just pierce the wires to take readings?

No CEL . . but the car was not running as it should. As you read the 'contamination' was due to silicone . . .AND . . K&N oil and both were coating the IAT sensor.

I will be interested in your readings from the new MAF. Look forward to your posting of the measurements esp the ignition on . . of "1.00V +/- 0.02".

Yes, using sharp probes.

No worries - I will probably be ordering it next week and will probably install and take the readings the week after. Normally I would run out and do it ASAP, but I am extremely swamped with household tasks this weekend and have several other cars to drive in the meantime. I will report back.

is there a MAF in M103/W124?

Thanks!

I was under impression E320 had Bosch AMS. The procedure in the article is for Pierburg Hot Film sensor. Are you sure it is applicable?
Mike

Here is info for Bosch sensors. I saved it some time ago from Bosch OZ web site, but can't find it now to post links.

page 2

page 3

page 4

Mike, tnx for posting . . . .good info :) If you want, I'll combine those into one (1) PDF. . ???

The output appears to be something greater than 1V at zero air flow; page 4 shows 1.3 - 1.4 V or so.

I personally wouldn't be concerned with the output at zero air flow but more with the flow at various air-masses. And the best way is to see what the fuel system "thinks" is Lambda. If it maintains 50% (and oscillating) at idle and 2500rpm, then that MAF is good as far as I'm concerned.