HELP: air flow meter pot - PeachParts Mercedes-Benz Forum

ThomH · #1 12-20-2003, 01:37 PM

Hi!

I currently feel lost with my problem. I have exchanged the pot of my afm and tried to adjust it according to the describtions of different sources. It's a M103 engine of 1986.

What happend? I adjusted the trim pot of my new afm pot according to my old afm pot (131 Ohm). Then I inserted the new afm pot and rotated it in such a way that the resistance would rise directly after the idle position of the flap (flap 1mm below cylinder). A MB direction says to check for U12 = 0.2-0.5V (voltage between pin 1 and 2) with the top surface flap at the buttom edge of the cylinder. I had 0.15V. I started the engine and had to modify the position a bit to improve idle. However U12 was 6V at idle. When I rotated the pot slightly more, the engine stalled until off at about 3,5V. Hence I had no chance to set to 0.7V at idle, what Steve suggests here in this forum. However, with engine off there are 0.7V at the idle position of the flap. So at the same flap position I have a completely different resistance readings depending on engine on/off. But U13 almost doesn't change.

With EHA unplugged the engine runs just wonderful.

Do you have any idea what to do (besides keeping it unplugged)?

Thanks a lot
Tom

stevebfl · #2 12-20-2003, 01:54 PM

The problem is that the on and the off position of the airflow plate aren't the same. It needs .7v running to enter the idle map strategy. If you are at .7v key on engine off then you will be much higher voltage when idling.

What is happening to EHA at the time all this is taking place? If it likes 0ma (disconnected) what IS happening when it don't like it?

ThomH · #3 12-20-2003, 02:06 PM

Oh, Maestro himself!

>It needs .7v running to enter the idle map strategy.

Thats's exactly what I am trying to reach - but how???

>the on and the off position of the airflow plate aren't the same

I deflected the plate by hand with engine off to the idle position if engine on. Hence it was approx the same position. I had .7V eng off and 6V eng on at this position.

And as I said, with less then 3.5V at idle the engine stalls. So how can I reach 0.7V?

When EHA is connected, idle is not steady. I am still searching my rev meter so I can only guess it's about 50 to 100 rpm. However, it sounds as if idle would stall due to a load (like from a/c, what I don't have). Idle stalls very quickly, remains there for 1 to 2s and then goes quickly up again. The sound of the engine is as if it would pull something.

stevebfl · #4 12-20-2003, 02:20 PM

Tuff stuff.

The airflow pot does a couple things. Its actual voltage is used to trip the strategy for idle control. The change of voltage adds to fuel correction at the EHA. You will need to separate the functions to evaluate them.

Start with the EHA off. What happens to the airflow pot voltage as you rev the motor. it should start low (below .7v to be exact) and it should rise to above 3-4v under snap acceleration. This can also be verified with the ngine off and key on by deflecting the flap with ones hand.

I don't know the pin numbers but there are only three. One is probably 5v and another ground and the third is the output voltage which should vary with flap position. IS this the way you see it working?

The end result is that the idle is controlled by openning and closing the idle air valve. The current used to do this should be about 6-900ma. It should be rather steady at above 700 until the airflow pot gets below .7v. At that time it should open (more ma) or close (less ma) as a load is added. It should add 50ma with A/C compressor "on" signal. What happens to the idle control current as you do the voltage adjustments?

ThomH · #5 12-20-2003, 02:42 PM

>Start with the EHA off. What happens to the airflow pot voltage as you rev the motor. it should start low (below .7v to be exact) and it should rise to above 3-4v under snap acceleration. This can also be verified with the ngine off and key on by deflecting the flap with ones hand.

Unfortunately not as the voltage changes when eng on. However, I will try if EHA off when Eng on changes this.

Positive, 3 pins. 1 (-), 2 slider, 3 (+). U13 is 4.8V (in range). The voltages we were talking about are U12.

I will check the idle valve and refine my measurements tomorrow.

Thanks a lot already for your support!

Tom

ThomH · #6 12-21-2003, 12:16 PM

Steve, I don't know what happened yesterday, I repeated the measurements a couple of times and always had the same results. Today however, I got U12=1.3V at idle and was able to reduce it to .65-.75V. Either I repeated the mistake or the ECU or the multimeter acted different this time.

This pushed idle stability a bit forward to this silkiness that pursue, but still left enough space for improvement.

The idle air valve received 665mA, MB ref is 600+-50. Do you see a problem here?

Unfortunately I didn't read enough about the EHA current before I measured it today

I did not look for the polarization...
First, at idle, I read a stable .7mA. After a short gaz kick and return to idle the current fluctuated between .7 and 7mA (maybe -.7 to -7mA). The idle didn't feel less stable after the kick than before. At 2000rpm it was between 3 and 8.5mA.

Since I have the same rough idle with a cold and a warm engine but a smooth idle with EHA off, I would not focus on the idle valve unless you say, 665 mA is too much. Unfortunately I don't have a duty cycle meter nor can I find my rpm meter (lent?). I intend to check the following next:
-EHA current eng off ign on 20mA to check the OVP relay
-injection cone and throughput of the valves
-0.8mm spacing of sparks (resistance of wiring has been checked already)
-air leaks
Are you aware of anything else I could check without the duty cycle meter that makes sense?

Cheers
Tom

stevebfl · #7 12-21-2003, 01:00 PM

You don't need to see duty cycle unless looking for a fault code. The EHA gives the best view of the adjustment and response to mixture anyway. And there will be no codes if you are in closed loop.

If you had +20ma KOEO then you have nthe right polarity. Your readings indicate slightly lean mixtures with are being controlled in closed loop. This means that is all there is! This does not mean your engine likes it that lean (lambda is exactly as lean as the motor likes it.

Its strange though. If your readings are right, and you are correcting for slightly lean the car shouldn't run better with the EHA removed. If it were slightly richer disconnected that would make more sense.

ThomH · #8 12-25-2003, 03:38 PM

Hi Steve, marry Xmas first of all!

I checked the EHA again and watched for the polarization this time. KonEoff, I first had a stable 10mA which then switched suddenly to a stable 19mA after 5-10s.

Eon, the current was between 0 and -8mA, sometimes even +0.2mA. When the current is around 0, the engine sounds like in unloaded idle. However, as I described above, the revs decline periodically and the engine sounds as if it would be loaded. The EHA current then quickly rises up to 8mA. I would guess, it sounds abou 1s unloaded and 2s loaded.

I also checked the spacing of the sparks and adjusted the spacing to 0.8mm (the manual states 0.8, but MB mechanics sometimes adjust it to 1mm for a better idle). When I started the engine, the idle felt so gentle. Amazing, that's what I was looking for. However, finally I discovered that I had forgot to reconnect the EHA. And after reconnecting I had the same rough idle back again. =-(

What do you think, Steve, isn't -8mA during idle a bit too much?

Best
Tom

stevebfl · #9 12-25-2003, 04:07 PM

The actual correcting current (EHA) is not as much a problem as the variation. During similar steady state activities (idling..holding at 2000; for two examples) the EHA current range shouldn't change more than ten percent. In other words if your are idling one minute at -8ma +or- a couple ma for lambda closed loop correction it should stay in that same range anytime you check over both the long and short term. It should be the same in three weeks or months or years. Obviously over time it can wander some but not more than 10% without explanation.

This also applies to idle versus 2000 steady rpm versus 3000 etc. All steady state conditions should be within 10% or something is wrong.

Consider this example. The engine runs smothly at 700rpms at idle with a plus 4ma correction. Running at 30mph down the road it runs at a steady rpm but is now running at -4ma. A simple explanation is a fixed size airleak (vacuum leak). This makes the car lean at idle which is corrected by the 4ma additive corrective. When moved to road speed the relativeley small same size leak does affect mixture much so the mix is actually slightly rich and a subtractive correction is maintained.

In another post about the effect of the OVP in stalling a engine I mentioned it would be hard to imagine a car dieing on the road due to OVP failure. I said I could dream of a condition but it would be extremely out of adjustment. Let me explain such a senario.

As it turns out the total lambda correction is only a little over 20ma. THis changes the differentail pressure about .2bar from lean to rich. On decel cut-off the fuel is stopped by a -60ma subtraction. Given these relative values, a totally disconnected EHA can only change the lambda correction half of the 20 or 10ma. So the only way that can make a huge difference is if the car is massively set lean and lambda is using all its capability to keep it running (a better than 10ma + correction). This would probably not be enough to kill the motor when removed unless the correction at 10ma wasn't achieving closed loop and just was keeping it running. A real certainty for such a happening would be if the car was stuck in a 60ma starting enrichment. If such were the case then the car wouldn't run (that is how much it takes). But if the mixture was then leaned way out mechanically then the car could be running fine when the OVP dropped out removing the stuck on 60ma enrichment and leaving the car running at zero ma and way too lean.

This system works better when defeated unless purposely set wrong, except for cold starting/running and emissions control.