I have a problem with the EHA and the current going through it. the car is a 1985 190E 2.3 so the current in only positive. I always thought that the current should go higher when taking off from a stop to richen the mixture and then go down a bit as the car gains speed with even acceleration. My car does the opposite. When I step on the gas with a MM hooked up to the EHA, the current jumps up to 2ma and then drops to 0, this is when warm already. It will start going up slowly once the engine goes over 2k rpm, but before that 2k it hesitates and pops due to running lean from what I have found out. This is not always the case but 95% of the time, and it pops a few times a week under hard acceleration. Another thing I noticed, when cruising above 60kmph, when I left go off the gas, the EHA current jumps up to 56ma, which I thought was only supposed to happen on start up. I'm trying to figure this out to get rid of my hesitation and this is where I found the problem, but I'm not sure what is causing this current to be so sparatic. I have new copper spark plugs, distributor cap and rotor arm coming in in the next few days, new spark plug wires are already on, but for some reason I don't think this will help me at all. I think I'll go pull out my OVP and see if it works at all. Maybe that's the culprit. Any help would be greatly appreciated. xp

I'm going to beat you guys till you understand this.

If you start at zero or wind up at zero with the early 190 system then the system is pegged. For the only positive current systems the nominal adjustment would have you at 8ma. Again pull the O2 sensor and you will see the middle of the road position for either system. The 190 will go to 8ma and if you ground the sensor input to the controller the system will correct to its capability 16ma. if you do the voltage through the body bit you will have it removing enrichment and it will bottom at zero ma.

What you have to remember is that in a perfect system the needed correction at idle would be the same as at 2000 and at 4000 rpm. The correction due to feedback (O2 sensor is in the exhaust and its is after the fact - feedback) should only affect discrepancies to ideal. Other aspects of the system will change mixture. When you step on the gas, instantly the airflap drops. The speed and ammount is proportional to how fast and how much gas one gave it. The airflow potentiometer gives this reading to the controller and increases the momentary mixture correction (adds fuel correction) by adding ma. Sooo... at any point if you stomp the gas there will be an increase in current, once the moment is over the correction will go back to steady state correction which is buried in lean correction if at zero ma on a positive current only 190 system. I think this 190 system stopped in 1986, so don't the rest of you get confused. They quickly decided that better control and a much superior limp home was achieved by having plus/minus current correction.

The plus/minus correction allows zero ma to be the center and a well adjusted car that looses electronic control does great in limp home since all steady state mixture would be at exactly where it was supposed to be anyway - zero. The pos only systems are in the middle at 8ma, when electronic control leaves it goes to zero current and this is the maximum lean correction which doesn't make a good limp home condition.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

stevebfl just one more thing that does not make sense to me.

if for the pre 86 cars the middle is 8ma, why does my car drop down all the way to 0ma when I'm coming to a stop, and then it refuses to go back to 8ma until it goes over 2k rpms. The car will hesitate and miss because of this, but as soon as the current jumps up, it takes off like a rocket. Would adjusting the Air flow sensor pot correct this? Because it won't matter how much gas I give it from a stop, the current will remian at 0 until 2k rpm is reached.

Tomorrow I'm getting my tune up kit, so I will see how the car does, but the parts that will be replaced have nothing to do with the electronic controls of the engine.

If it does not help, after I do my driveshaft, I will get a new pot for the Air Flow sensor, I believe it to be faulty, but I can still try to adjust it a bit.

Can you suggest anything?

xp

Your car drops all the way to zero because it is not in closed loop. If the system is functioning your car is doing this because you are toooo rich. Zero current is the maximun lean correction possible within the lambda system.

I can explain this but that is not to say that your car will run at the required mixture. Most of my life I dealt with cars that in no way could have been made to run as lean as is required to go closed loop at lambda. If you really wish to understand this, give me a 500 word essay explaining that last sentence; most importantly: going closed loop at lambda! I feel that most do not understand that concept.

I can't say I blame people for not understanding it. The concept came out in 1979 (with Volvos - 1980 for MB). I worked around the system for five years before in 1984 we started bringing in grey market cars and had to design feedback control systems to feed three way cats. We were tested on the results. Nothing like failure and money to breed understanding. I worked in the dark for nearly 5 years on these systems.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanx for the explanation stevebfl, it seems that you know a lot about this, I've been trying to figure this thing out for quite some time now but I never got any concrete answers, so maybe you can provide some insight to this if it's not too much trouble.

So far you are exactly right, the car does run rich, very rich actually, I pulled the spark plugs and every single one was covered in a LOT of black powdery residue. I also know that the mixture adjustment screw has been tampered with since the cap is gone, and who knows how it was adjusted. I'd like to get the car running with the right mixture.

I think the car can be much more economical then it is right now. I'm no mechanic yet, so I try to take things slow and make sure I got all scenarios covered before I proceed because I simply don't want to mess anything up.

Would you say it would be a good or bad idea to try and lean out the mixture by adjusting it with the screw on top of the fuel distributor since the electronic controls can't lean it out anymore even when the EHA current drops to 0. I know you can verify this with the duty cycle meter hooked up to pin #3 on the diagnostic port, you also mentioned you always richen the mixture a bit for performance reasons, I guess I should follow your lead.

If this adjustment does fix the problem of the engine running rich, will it put the system back into closed loop? I understand that failure of any of the subsystems will throw it out of closed loop.

One last thing, why would the EHA current jump to 56ma when the decel switch is engaged while cruising above 60kmph, once the speed drops below 60kmph, the current drops to 8 and heads down to 0, any idea what could be causing such a big step there?

I would appreciate any help you can give, I hate to be a bother but if you could just point me in the right direction, or a source where I could find out more about this system, I would be forever grateful.

xp

The simple answer is.. yes, lean the mixture out. If everything works as it should you would turn the 3mm adjuster counter clockwise till the EHA current started to rise. You would be doing this slowly and the first sign would be a few ma 2-3 say. In small steps one would then continue til the average EHA current was 8ma (except I like it richer so stop at 6ma).

The problem is it only works this way on a clean system and yours is most likely fouled. To resurrect the system I advise disconnecting the O2 sensor and hook your volt meter to the sensor end. Before adjustments you must be seeing about 1v. It takes this kind of reading to cause the EHA current to go to 0.0ma full lean correction. Now adjust the mixture (CCW -lean). Rev and hold the motor at 2000rpm. Did the O2 sensor voltage change (be sure to hold a constant throttle when reading O2 sensor voltage as the voltage always goes off scale with throttle movement - the scale is 0,0v to 1,0v)? As you hold the car at 2000rpms continue to adjust the mixture lean. You eventually will lower the voltage (or the engine will run badly). If you reach running badly before a voltage change, we have a problem.

This problem will either be a sensor problem (be sure to leave the heater coil connected when reading O2 sensor voltage - even though voltage dropping to the sensor from this circuit is the common problem of such faults) or a separate problem that won't allow the engie or individual cylinders to run that lean. Remeber the amount of fuel we are trying to achieve is the absoilute minimum for efficient running. At such settings any restriction or added air causes fouling. Cause the flame to burn out (if it were a candle).

Anyway, aftyer you get the voltage to start reading, get the number as low as the engine will run sort of good. Rev the motor a few times to 3k or more, hold it at 2k for 10-15 seconds and then reconnect the O2 sensor and see if closed loop can be maintained at around 6-8ma.

Remember closed loop is the distinct cycle of lean rich lean rich correcting. Once this is happening the current will be real sensitive to mixture. Placing ones finger lightly on the airflow meter should be seen on the mixture correction. This will show you how much the system has the ability to correct. If you place a finger on it for say ten seconds and the mixture corrects back toward zero ma what will happen when you let up on your finger???? The system will have corrected lean and will be lean and correction will run way above 8ma for a while til recovered. Watch all this happen and it starts making sense.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

God bless you Steve. You must have children......you have the patience of a saint.

I think the contributions you and a few other senior members make keep several of us in Benz cars we would have abandoned/sold long ago.

Thanks again.

Tinker

God bless you Steve indeed.

And a thousand Thank yous.

I think this is the answer I was looking for for the past 9 months.

xp

Now, If I could only find out what's causing my puddle of fuel inside the manifold!

Oh Stephen. You poor soul. You did get thread-jacked.

I hope you get this sorted-out...

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Well for some things you just have to be there. I have never looked into a manifold that wasn't wet but if you have raw gas accumulating there in any sort of quantity, what we have been talking about is irrelevent.

Clean up the mess and check to see if it returns. Unbolt the fuel distributor and lift it while bypassing the fuel pump relay to maintain system pressure and see if it drips from the plunger. If you have a leak you must address it.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I have already separated the upper and lower halves of the fuel distributor, dried out the manifold, and reassembled. Fuel puddle came right back. Fuel is leaking inside the distributor. There are no external leaks. And for what it's worth, I rechecked the voltage for the 02 sensor heater today with the engine cold and had 11.85 volts while cranking, so I have to assume the heater is working. If I do have a leaking fuel distributor, is it even possible to set Lambda?

Very nicely said Tinker!!

Cap,

The only thing I could think of that would cause a pooling of gas in the manifold is the O ring that seals the plunger in the fuel distributor. DON`T take it apart without measuring the plunger height. It is a critical adjustment. BUT the O ring is available from the dealer.

Just a thought.

Tinker

Steve, does this concur with your thinking? Can a defective EHA cause this sympton? As stated there are no external leaks.