Hi,

I have a new airflow meter in my hands, the potentiometer on the old one was intermittant and erratic.

Can anyone please tell me what the resistance values should be for the potentiometer on pins 1 & 2? Bear in mind this is not an 8V but a 2.5 16V.

I see 1K at zero position and a maximum 9.6K at 3/4 deflection and a slight decrease in resistance at maximum deflection. I have a W201 CD and it has resistance values for the 8 V air flow meter, I am not sure what they are supposed to be for the 16 V 2.5. The Bosch part number is 0438121 072.

Also I need some information on if it is necessary for me to set the zero position of the airflow meter plate. Are they pre-set at the factory? I notice on my old one, it is not in the position shown on the CD. At the top of the vertical machined surface. In fact it is a couple of mm over. It is the same on the new air flow meter. This doesn't match with what the CD says.

I notice that if I depress the plate to where the CD says to set the zero position, the resistance value does not change, indicating that there is some period of movement prior to the potentiometer reporting any change. Is it supposed to have this "idle period" - or is it supposed to respond due to any change of airflow? I would suspect it is supposed to be in the zero position but I wonder why both of my air flow meters are not correctly set!

Hoping someone can help me here!

Cheers,
Neil

Haven't done it yet, but here's what I would do...

If you haven't removed the old pot yet, it is reasonable to assume contact is intermittent ONLY at a few isolated spots, where wear has taken its toll. The total resistance is unlikely to have changed significantly, and contact at other points should be good enough to take accurate measurements of 'percentage of full-scale Ohms' corresponding to distance of deflection of the metering vane. These will constitute your calibration table.

Steve

Steve,

Thanks again, but unfortunately the pot is totally fried in the old one. At zero position, most of the time on 1 & 2 I see infinity! If I move the plate up and down a bit I can sometimes get a reading, but it more often than not is around 5-6K and when I do get a reading it varies constantly even though the plate is sitting at zero! Not good. It also ranges in value when deflected to over 16K!

I just tested it again now, and I saw 13K at idle - LOL - then after a few pushes infinity. Unfortunately I can't use this one as a base.

I've read you need to have the fuel distributor hooked up, fuel pump bridged to get pressure in the fuel distributor when testing this. Not sure how on earth that works. I guess the fuel distributor control tube erm or whatever it's called must drop a little under pressure so therefore you can't zero the thing without pressure..

hmmm..

Neil

Steve,

My $0.02. Why did your new air flow meter come without the potentiometer?

I have read that you can not accurately set the potentiometer, they are set when assembled by Bosch. I believe Stevebfl has mentioned this several times. HOWEVER, I also read that others have set theirs by installing it on the car and playing with it until they got the car to run right. Probably not what you want to hear since removing it is a pain in the arse! Been there done that.....a few times.

I also have a 16V and do all the work on it by myself. I also believe that my potentiometer has a problem. I would expect it after 145k. I have managed to avoid its symptoms by other methods.

There was an other member who followed my trials and journey of hell trying to get it to run right. He emailed me and offered to sell me a new potientiometer that he bought for his 16V. His mechanic installed it and played with it and couldn't get the car to run right. He ended up buying the whole air flow meter from the dealer with the potentio installed and...... his car ran fine after that. I know it is a bit antidotal(?), but I think there was a lesson there somewhere.

I have also watched the resistence values on mine jump all over the place when checking between those two pins. Drop me an email and I'll check mine out installed to see what values I get along the curve. Be aware that if our resting heights are not the same, even those values could be different.

You can clean the old one, if it is not totally destroyed, by using a erasure. Have you tried that??

Tinker

Hi Tinker,

It was myself with the problem not Steve. I didn't buy a potentiometer by itself, but a complete new airflow meter.

I'd really appreciate it if you could tell me the resistance values you see at zero position, the maximum resistance you see with the plate depressed, and the resistance at maximum deflection of the plate.

I want to see if my airflow meter is within range. I don't have the spec. for these. The parts comes with no instructions on either setting the zero position or on what the resistance values should be as new. I olnly have data for the 8V and I know the units are different as the part numbers are also different.

Cheers,
Neil

Steve,

So it's won't be necessary for me to set the zero position either?

I will take some readings and mark them down, post it up here. Unfortunately I don't have a caliper with a depth gauge to do anything accurately. Will try and get one. But as I said I see 1.03K at zero, up to 9.6K to about 3/4 travel then it starts to drop off again.

I'm guessing the inline resistor you're talking about is 1K as initially there is a dead spot of about 2-3mm before the resistance readings start to change. I guess once fuel pressure is applied, the control column in the fuel distriubutor lifts and the air flow sensor plate drops, putting the plate much nearer to the real zero position. I'll test the resistance again at zero when it's back in the car. I remember the air flow meter plate dropping with the ignition switched on. Wondered why that was, now I know!

Thanks again,
Neil

Not sure whats going on here but my post appeared ABOVE yours Steve and I replied after your post!.... weird..

"I'm guessing the inline resistor you're talking about is 1K as initially there is a dead spot of about 2-3mm before the resistance readings start to change. I guess once fuel pressure is applied, the control column in the fuel distriubutor lifts and the air flow sensor plate drops, putting the plate much nearer to the real zero position."

The plate is designed to allow 'backflow' that would occur with an intake backfire without damage to either the mechanism or the pot.

Steve

Neil, I misunderstood - I was under the impression you were just replacing the pot. If it is the entire airflow meter ass'y, and it is the correct part# for your car, it should be a direct plug-in replacement, no cal required.

Since you have the new one in hand, though, taking some readings and making up a table of your own would be very handy for future reference. According to Bosch, the pot taper is non-linear, providing a simplified acceleration enrichment program in the ECU (more enrichment needed near idle than at higher rpms). For this reason, the more measurements you take - and the more accurate - the more useful the results. Note though that there is a resistor in series with the wiper arm for protection. That means you can expect resistance readings to always be offset by the value of this resistor.

Steve

O.k. I got my Neil and Steve squared away.

Neil,

I was also under the impression that it was potentiometer you were replacing. Steve is correct in his descriptions. Ideally you should be able to plug in the new air flow meter and go. You might need to reset the resting height, depending on how it starts when its warm, but you will not know until after its installed and running. Look at the used one and examine the wear marks the sensor plate left in the funnel. That will give you an idea where the old one`s resting height was set. Once its installed and pressure is in the fuel distributor, you want to see about 1mm of free play before resistence is felt when you deflect the sensor plate by hand. If its close and runs/starts fine, both hot and cold, don`t touch it!

Please post the resistence values on your new meter. It would be helpful to all members as a reference point. I will test mine and post my readings this evening.

Tinker

Hi,

Got the reading from my new air flow sensor. This is for Bosch Part number 0438121 072. My car is a German 1989 190 Sport Evolution 1. However this part as far as I know was fitted to all 16V's including the stock 2.5 and the Evolution II. I have a Bosch specification for the unit kindly supplied by Cor de jong in Holland. For a start I'll list those values, but bear in mind this was in German and I've done my best to translate it.

Supply Voltage: Pin 1 & 3 of Airflow meter Potentiometer - 4.35-5.35V
Tension Signal: Pins 1 & 2 of Airflow meter Potentiometer:

This is a graph which I unfortunately can't scan in. I'll give the ranges. With a supply of 4.35v volts on 1 & 2 should be _approximately 0.435V to 0.78V. With a supply of 5.35V volts on 1 & 2 should be _approximately 0.54v to 0.92v. These numbers have been derived from simply looking at values on a graph. I'd say they should be accurate to around 0.20v.

Now to the airflow meter plate. The Zero position of the air flow meter plate should be 4.2-4.4mm from the rest position.

There should be 0.1-2.0mm free travel. More on this later.

Right, thats it for the Bosch details, now for my testing on my new airflow meter.

I have used a vernier caliper to take these readings. They are taken from the centre of the rubber rest on the very top of the meter, down to the washer under the nut securing the airflow meter plate.

The distance to the washer from the top of the rubber stop in the centre is 30mm. Be aware this isn't science as it's very difficult to measure this accurately, but I'd say the numbers are good for maybe 10-15% variation - ie 15 ohmns.

Here are the values.

30mm 1.022K (2K scale)

This area the readings fluctuate slightly but pretty much fall to the 30mm position.

34.2mm 1.000k (2K scale - according to Bosch the Zero position)

I'm a little concerned as to what Bosch consider the "Free Travel" as plainly there is 4.2mm of free travel in mine, but at 4.2mm that is exactly where Zero position is supposed to be according to the literature. I may have to adjust the pin to lower the height of mine by at least 2mm to take up some of the free travel. But if I do, it will no longer contact the rubber stop. Bit of a mystery... Anyone? I think I will leave it for now.

35mm 1.070k (First place I see a change)
35.80mm 2.00k (Very rapid ramp up to 2.0K from 1K in the space of less than a single mm)

36mm 2.35k
37mm 3.35k
38mm 4.25k
39mm 4.95k
40mm 5.55k
41mm 5.95k
42mm 6.35k
43mm 6.80k
44mm 7.10k
45mm 7.30k
46mm 7.60k
47mm 7.80k
48mm 7.90k
49mm 8.15k
50mm 8.20k
51mm 8.20k
52mm 8.28k
53mm 8.28k
54mm 8.32k
55mm 8.50k
60mm 9.40k

65mm 9.50k (peaks here, then begins to fall)

68mm 9.40k
70mm 9.20k
71mm 9.10k
72mm 8.90k
73mm 8.80k
74mm 8.60k
75mm 8.45k
76mm 8.25k
77mm 8.10k
78mm 7.85k
79mm 7.60k

80mm 7.50k (Plate bottoms out - no more travel).

Well, I hope this helps somebody!

Cheers,
Neil

Thanks for the data, Neil. I've saved it and printed it for possible future reference.

Steve

D'oh! I think I've worked out what the "free travel" is after reading Steve's post about "backflow". I imagine thats the area between the stop and the zero position. The free travel is the area between the zero position and the first reading from the pot - which in my case id 0.8mm - within spec.

Well I got my Evolution 1 back together this weekend. New injectors, airflow meter all new rubber etc etc.

At first with the EHA connector and the harness from the airflow meter pot connected she wouldn't barely hold an idle and the idle would jump from 500rpm to 1500 rpm - exactly the same as before. Raarrgghhhh......

Except of course the airflow meter pot _was doing it's stuff correctly. I was a little stumped, as when I removed the idle speed regulator she would hold a warm idle no problem. Began to think it might be the regulator after all but it just didn't ring true. Decided to RTFM and pulled out my Bosch FI manual by Charles Probst.

Seems that the factory does not set the air flow meter's zero position and basic position. Maybe because they can't as both positions have to be checked and set with fuel pressure. Zero position of the sensor plate is at the top of the machined vertical cylinder. The plate side furthermost from the fuel distributor. The airflow meter plate must be at the very top of this tiny cylinder. The cylinder is only 2-3mm in total height. My plate was about 3mm above the zero position. I dropped the sensor plate height by driving in the guide pin adjuster a mm or so, this put the plate in the correct place.

For basic position I depressed the plate until I felt the resistance increase, ie when I was moving the control plunger in the fuel distributor. It was way off, no gap between zero and basic. I adjusted this by leaning out the lambda adjustment down to basic position which is the bottom of the "cylinder" I mentioned above. All of these tests were done after I had run the car for 30 seconds, switched off, ignition on to keep pressure in the system. At the bottom of the cylinder, the max allowable distance between basic position and zero position is 2mm. At 2mm she was too lean to start. I richened it up approx. 0.5mm, ie 1.5mm zero-basic and she started smoothly. A bit of further tweaking by watching the EHA current and duty cycle and I now have her running at 45-55%... VERY HAPPY. It seems I have finally exorcised her demons. All digits crossed!

Just thought I'd report back here my findings and result. Whats really weird is the original air flow meter I removed was not correctly set for zero position, and it exhibited the same weird up/down idle! The potentiometer was still toast of course, but it seems that the zero position was causing all the idle strangeness.. Be interested if anyone else can corroborate this!

Cheers,
Neil
87 190 E
89 190 Sport Evolution (Factory)

Neil,

Wow, thanks for posting the resistance values. That was MOST appreciated. I have not had the time to compare mine, but plan to and will post my results.

Your zero position setting sound right. Especially if its starting well when hot. Some incorrectly set the resting height a bit lower to cure stuborn cold start problems. But this usually results in problems with hot starts as the slightest pressure on the control plunger causes the fuel pressure to bleed off. The manual states that there should be slight free play ( the 0 - 2mm you mentioned) of the sensor plate before resistence is felt

One other question for you. When you set your duty cycle, did leaning the mixture (a counter clockwise adjustment at the mixture hex) result in a lower or higher duty cycle reading?

Thanks,

Tinker