Bosch air flow meter potentiometer N.L.A..... - PeachParts Mercedes-Benz Forum

Tinker · #1 03-27-2007, 08:57 PM

I've talk to a few people and it appears one of our favorite Bosch parts (3-437-224-035 ) is NLA. Any other sources for this part out there?

The word I got was Bosch discontinued it. The alternative now is to purchase the whole air flow meter assembly for $600+.

EFI is sounding better every day.

Tinker

just-n-time · #2 03-28-2007, 02:15 AM

Quote:

Originally Posted by Tinker

I've talk to a few people and it appears one of our favorite Bosch parts (3-437-224-035 ) is NLA. Any other sources for this part out there?

The word I got was Bosch discontinued it. The alternative now is to purchase the whole air flow meter assembly for $600+.

EFI is sounding better every day.

Tinker

I just put one in my wifes 420 sel and got it through Phil,We did the work two months ago,Have you tried him?

brewtoo · #3 03-28-2007, 10:41 AM

Yes, that part can still be had. I put one on my '91 a year or so ago.

deanyel · #4 03-28-2007, 11:07 AM

It has been discontinued but there is still some inventory around - just have to find it.

Tinker · #5 03-28-2007, 12:46 PM

Yes, I spoke with Phil and 5 other places. Same story from them, part was discontinued - N.L.A..

Any ideas who might have it in stock?

Tinker

bhatt · #6 03-28-2007, 09:00 PM

Paid $90+shipping for one as I needed it quickly. I think they went for about $75 at the time but I was in a rush.

Put it on, didn't fix the problem (turned out to be something totally different) so I took it off and sat it on the shelf where it is now.

Make me an offer and it's yours. It's brand new except for 15 seconds of testing

Neal

just-n-time · #7 04-05-2007, 03:43 PM

Tinker, try this www.boschusa.com,or www.olyonline.com

Rick76 · #8 04-05-2007, 06:18 PM

Olympus had them until 2 months ago, but no longer.

just-n-time · #9 04-06-2007, 12:30 PM

Well I can see that this is not an item they can do away with,so if you must replace the entire unit. Dose that mean the new units have a different system in them?Some one is pulling something,on us,Perhaps they are trying to get the older cars off the road! No then they would not make any money on parts sales,that has to be a large sum of money each year!JNT

wbain5280 · #10 04-07-2007, 12:52 AM

Time to change over to EFI. I'm looking at Megasquirt, or Bosch LH 2.2 from a Volvo 740T.

wbain5280 · #11 05-08-2007, 06:31 AM

I'm thinking that a MAP sensor would do the trick. I'm going to check this out on one of my cars. The air flap measures the air flow which translates to engine load which is vacuum.

A Freescale MAP sensor MPX4100A measures pressure to 14.7, no boost allowed.

www.freescale.com

The output signal may have to be scaled since idle on our cars is .7 volts output from the potentiometer wiper. An op-amp or a transistor could be used.

Does anyone want to chime in on this?

dpetryk · #12 05-08-2007, 09:59 AM

I have tossed this one around in my head for a while now. Here are my thoughts.

The MB pot arrangement from what I can understand measures the position of the air mass flow device. The signals are processed by the computer to determine the rate of change in airflow. As you stomp on the accelerator, air flow increases, computer translates this into a shot of fuel to get things moving. It is analogous to the way carbs shoot fuel directly into the manifold when you stomp on the accelerator. Under normal driving conditions where the accelerator moves slowly, the POT does not really have much effect.

A Manifold Absolute Pressure (MAP) sensor measures manifold pressure. Modern computerized engines monitor RPM and MAP then using these values as an index into a "fuel map" the computer determines how much fuel should be injected.

If we replace the MB pot with a MAP and do the appropriate electrical interface so that the MB computer "sees" the same input I don't think it would work like it does now. Take the scenario where the engine is at idle and you floor it. Now the engine is at wide open throttle (WOT) but at very low RPM there is very little air flow and there is NO manifold pressure. Initially the signal would jump up as the manifold pressure is lost but then it stays there as the engine begins to wind up. There would be a initial short shot of fuel then nothing. I think the engine would stall or hesitate or not have much power.

I think a better solution would be to make a module that monitored RPM and MAP then fed a signal to the MB computer. It would be more involved but could provide a customized response to the MB computer and could be tuned as needed. Using a microprocessor you could "map" the response and make it totally programmable.

If there is a market for such a device I could design and build them. Any ideas on how many could be sold? Is seems to me that in the grand scheme of things the market segment would be to guys like yourself who like to tinker with their cars.

Any thoughts?

wbain5280 · #13 05-08-2007, 01:32 PM

Yes, my thoughts are that in a more modern FI car, with EFI, what you wrote would be true, but our cars (W126's) have CIS-E which is mechanical in nature, air flow through the horn deflects the plate which injects more fuel. The computer is only monitoring inputs like the O2 sensor, temps, air pressure and some others. Its output is to vary the fuel pressue via the EHA. The pot on the side tell the computer the flap position which is analagous to the load. A MAP sensor would perform a similar function. High vacuum, idle for instance, plate at its resting position produces 0.7 volts on the wiper. Higher air flow, lower vacuum, produces higher voltage, up to approx 5.0 volts or therabouts. This is what the map sensor does too.

Your thoughts...

Fixed the part number:

A Freescale MAP sensor MPX4100A measures pressure to 14.7, no boost allowed.

dpetryk · #14 05-08-2007, 03:32 PM

The pot as it is now does not measure load. It measures air mass flow. You can have a huge load on the engine and very little airflow. The electrical signal from the pot is differentiated and produces a signal that is the "rate of change in position". As the engine revs up the airflow increases and the pot steadily move up as the RPM does. A MAP sensor will not do that. The point I was trying to make is that I think it will take two parameters to determine "load". They are RPM and MAP. Maybe I got it all wrong. The best thing might be to wire one in and try it. I do know that if you remove the EHA connection the car will stumble when trying to accelerate. I know that the pot it critical to getting smooth response off the line. The EHA needs a signal to add fuel during acceleration.

Give it a shot and wire one up. Let us know what you find.

Samuel M. Ross · #15 08-04-2007, 12:31 PM

I was giving my MBz INDY friend a hand in trying to find out how to adjust this infamous "air flow meter potentiometer" and after reviewing what the MBz W126b CD manuals and finding the usual inadequately written and German-to-English translated confusion I then went to Google and searched on the word group [ "Cis-E" Mercedes 300SE potentiometer ]... and Vowalla!?... there were only 4 hits including the link to this THREAD.

The first link was: http://www.sg-motorsports.com/AFS%20Pot%20write-up.htm... and since this is an excellent step-by-step procedure for R&R and adjustment of this "pot"... I thought I would make this POST and share this info with all who venture here in the future.

I'll sign up for automatic e-mail notices for this THREAD and hopefully someone will come up with a source for this critical part of these CIS-E systems. Our subject MBz is a 1990 300SE.
Regards,