I read Steve's article about monitoring the EHA current signal.

I had to do the same thing when I bought my 190E 2.6. It got 19MPG and the plugs were black and black smoke on idle.

I monitored the oxygen sensor and EHA and after awhile saw the relation and pin poined the oxygen sensor...the voltage was going down to -20 mV and the EHA current to 12 mA or so. Rich rich rich.

With a new o2 sensor I'm now up to 28MPG /31 hwy, plugs are white/tan. Its a little harder to start cold,5-10-15 sec crank time. 1 -2 sec when hot.

My question concerns the EHA values he reported as GOOD values. Mine are a little off: Here they are:

Cold start 40F goes to 24 mA and as I drive works down to
-2 to -4 mA and stays around there as I drive around town or hwy.

Hot idle goes to -9 to -9.8 mA and then stays at -9.8 mA.

Hot full throttle goes to +4.5 mA and stays.

Coasting down goes to -64 mA.

I think maybe I'm a little on the rich side.

When I put the new o2 sensor in I adjusted the throttle plate to "zero position" (I think is what the Mercedes manual called it) and as I tested and watched the signals I leaned it out 1 or 2 turns until the the hot normal driving was at -2 or so mA. I didn't know where to set it and made the guess for a little rich throttle plate adjustment, and of course now having seen Steve's data...

What do you think ? Should I lean it out another small amount or so ?

I have a scope but cannot drive around with it as it takes 120VAC.

I remember the manual says that you can reach a point(too lean) where it may not start. And as it seems to start a little hard now ????? I'm not sure...

Oh yea when the sensor was bad it started in just a 1-2 sec. crank.

When I say hot I mean normal operating temperature.

Car specs: 1989 190E 2.6, 5 spd manual, 147,000miles, oil might go down 1/2 quart for 3000 miles.

I know this is long and windy but please respond.

Bill Bass
williambassjr@naxs.net

I do not want to oversimplify all of the various things that must happen.

Every model has a different "map" for the various conditions. Your total cold enrichment plus starting enrichment should be more than 24ma (I think). Some models go to a total of 60ma or more. These values must be looked up.

In general there are a couple of comments. The basic adjustment of -2 to -4ma is usually a good enrichment to help during the non feedback time before O2 sensor reacts. BUT, you mention warm idle settings go to -9ma. You state that full load it goes to +4ma. Well full load is a different condition (not closed loop). I see a problem here. First, you should stay around the same correction at most conditions (other than full load). Once closed loop is established, the relative amount of correction should not vary significantly. To me this means more than 3-4ma.

In other words, if the closed loop correction is -2 to -4ma swing at idle,warm, then the correction at 2000rpm unloaded or constantly loaded should be the same. To go to zero or -6 or more would be an indication of a problem. It can get tricky with pressure measurements part of the testing. The difference idle to part load can also be an indicator of vacuum leaks or fuel injector flow problems.

More power to you for trying a testing approach to diagnosis.

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

Thanks for the reply.

On the cold start enrichment.. both temperature sensors read the same within 4-5 ohms, hot or cold and seem to be within the scale shown in the manual.
(I have good test equipment, my work is in industrial process control 22yrs). I would think the computer would set the cold start at a point depending on the temperature feedback. I'll check the manual for values and see....

I understand about full throttle...the switch kicks in...and the loop is opened.

For the normal driving and idle difference > 5 mA... you think MAYBE fuel pressure or injector flow could be somewhat off the normal ? And I should have those two checked ?

Bill

Cold starting enrichment is the sum of starting enrichment and temp enrichment. The starting enrichment goes away as soon as the starter stops. At that moment the temp enrichment is whats left. It goes away fast so look quick (it should drop steadily to zero in ten to twenty seconds - a guess). It usually makes it before the O2 sensor is warm enough for the computer to start correcting.

I use a fifteen foot long harness to break out the EHA current and allow my to view it while driving the car. At any constant speed the EHA correction range should be close to any other. If the rich/lean cycle is -2 to -4ma at idle it should be close to that at any other constant load/rpm.

There are many causes tell me what you got.

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

My only difference now is the run current ver idle current:

run = 0 to -2mA

idle = -6 to -7mA

(I've adjusted the mix screw, more lean, since last email.)

Still that mA difference is present.

I have the EHA readout in the car. For the last month.

Just got back from a short trip, going to be 350 miles on this tank of gas.

I think I'll monitor the "idle device" current next.

Bill