Setting up the EHA - PeachParts Mercedes-Benz Forum

Joreto · #1 08-01-2003, 07:35 AM

A friend of mine recently got a 2.3-16v, the car is in good shape exept that previous owner has tampered with the EHA (with the screw which rises/lowers the fuel pressure). Can anyone tell me what is the correct procedure for setting up of the EHA. The car has the following problem, when driving in 2, 3 or any other gear and you floor the gas pedal (WOT) the car hesitates for 1-2 sec and then takes off like crazy. I hooked up an A/F ratio gauge which shows that during the period of the hesitation the car is very lean (off the scale), then it enriches the mixture and the car takes off. I disconnected the wires of the EHA (so it dosn't get a signal from the ECU)and the A/F ratio gauge showed that the mixture is very rich a idle (all of the lights light up, shows around 0.96 / 1.00 volt on the amp meter). Shoundn't the car with the EHA disconnected be around the 14.7 : 1 A/F at idle ?

mbdoc · #2 08-01-2003, 08:06 AM

The CIS injection system operates on basic fuel pressures. The lower chamber pressure must be .4 bar lower than system pressure. The EHA is the adjustment for that lower chamber pressure. Whatever the system pressure measures set the EHA to govern the lower chamber 0.4 bar lower with the electrical connection disconnected.

psfred · #3 08-01-2003, 09:58 PM

Go to Sears and get the multimeter that has a duty cycle funtion (there is a thread in the last month or so with details).

Remove air cleaner and unscrew the cover of the diagnostic socket on the fender (on the fuel/ignition box there). It's the only one with a round screw-on cover with a retaining chain.

Set the meter for duty cycle and put the red probe in the #3 hole and the black probe in the #2 hole in the diagnostic socket.

Switch ignition on but do not start engine. Should read 70% (there are other readings -- see the DIY article by Steve Brotherton for details).

Start engine. Duty cycle will be 50% until the oxygen sensor warms up, at which point it will go way down to 30% or less if you are running rich and the electronics are working.

When the computer is controlling the mixture, it will read something other than 50% and will change all the time. At this point, you can use a 3mm Allen wrench to adjust the mechanical fuel mixture at the fuel distributor. Insert allen, then press down GENTLY to engage the key in the screw. Don't press very hard, you will shove the mass flow sensor flap down and add extra fuel.

I believe clockwise is lean, CCW is rich, but I'm pretty dyslexic and it could easily be the other way.

Turn the allen slowly and release the pressure, then wait a bit to see where the reading goes. At idle it will take 10 sec or so for the change to show up, and if you get carried away, you will have trouble getting the correct reading.

Adjust in small increments, 1/16 turn or so at a time, until you get a reading close to 50% duty cycle. This should set the EHA current at close to 0, giving you proper fuel mixture control.

Now, the fun starts. If you cannot obtain anything but 50% duty cycle with no changes, the O2 sensor is bad or unhooked (connector is under the floor mat in front of the passneger side front seat). If it changes, but you cannot set it to 50%, something else is wrong.

You should get momentary enrichment (lower duty cycle %) when you open the throttle, going back to 50% quickly. If you get the opposite, you probably have a bad mass air flow potentiometer, so the computer doesn't get signaled you opened the throttle.

You may also have a bad throttle switch.

Peter

Cap'n Carageous · #4 08-03-2003, 08:35 AM

Quote:

Originally posted by Joreto
A friend of mine recently got a 2.3-16v, the car is in good shape exept that previous owner has tampered with the EHA (with the screw which rises/lowers the fuel pressure). Can anyone tell me what is the correct procedure for setting up of the EHA.

I asked this same question once. All I was looking for was some info on which way to turn the screw and how to measure it, thus returning to 'factory' setting.

psfred · #5 08-04-2003, 08:12 PM

Duty cycle, EHA current (best) or CO in the exhaust with the EHA unplugged. No other way to do it.

Peter

Cap'n Carageous · #6 08-04-2003, 10:43 PM

Quote:

Originally posted by psfred
Duty cycle, EHA current (best) or CO in the exhaust with the EHA unplugged. No other way to do it.

Peter

Well.... why did they put the dadgum adjustment screw in there for????

psfred · #7 08-05-2003, 09:09 PM

The screw is how you change the mixture.

The EHA current (the electronic control system), the duty cycle (electronic control system) or CO in the exhaust is how you measure the mixture to get the correct setting. It cannot be set "by ear" on these cars with any real success.

Just like ignition timing -- you turn the distributor to change the setting, but you use a timing light and the scale on the crank pulley to get the corret setting.

Peter

pentoman · #8 07-30-2007, 09:41 AM

Quote:

Originally Posted by Joreto

A friend of mine recently got a 2.3-16v, the car is in good shape exept that previous owner has tampered with the EHA (with the screw which rises/lowers the fuel pressure). Can anyone tell me what is the correct procedure for setting up of the EHA. The car has the following problem, when driving in 2, 3 or any other gear and you floor the gas pedal (WOT) the car hesitates for 1-2 sec and then takes off like crazy. I hooked up an A/F ratio gauge which shows that during the period of the hesitation the car is very lean (off the scale), then it enriches the mixture and the car takes off. I disconnected the wires of the EHA (so it dosn't get a signal from the ECU)and the A/F ratio gauge showed that the mixture is very rich a idle (all of the lights light up, shows around 0.96 / 1.00 volt on the amp meter). Shoundn't the car with the EHA disconnected be around the 14.7 : 1 A/F at idle ?

So.... was this solved?

just-n-time · #9 07-30-2007, 03:02 PM

Ok I see that this is a dang if you do type question, I reset the EHA on my wifes 420 sel,this is what I did,I made a harness to place in line with the EHA harness this gave me the ability to place the meter between the EHA and harness.I took a reading and adjusted the internal screw till I got .0+ or -0.then reset the air mass to get 50%

crhenkel · #10 08-03-2007, 11:11 AM

bump

crhenkel · #11 08-03-2007, 07:09 PM

I started to set my air flow / tower setting by reading the duty cycle and found that it would not move off 50%, indicating the O2 sensor is bad...RIGHT? SO until I can get the sensor in, can you set the co correctly by reading EHA voltage instead or will the bad O2 sensor also effect the EHA reading?

just-n-time · #12 08-03-2007, 08:49 PM

I will be totally honest I do not know,But it seems to me that if you are trying to get to a place of balance,all items in the equation would have to work.

just-n-time · #13 08-03-2007, 08:51 PM

If you wish you can go to my post's and see how I was coached threw the EHA settings.

pentoman · #14 08-15-2007, 08:03 AM

No you cant. If the O2 sensor is bad the EHA will not be adjusting the mixture at idle. Its readings will not change if you adjust the mixture. You need the new sensor.

crhenkel · #15 08-21-2007, 09:03 PM

Got my EHA test harness from the MB Dealer today.
Part number 102 589 04 63 00
Total cost to have it UPS to my house, $32
Part name: test cable
WIll be testing the EHA voltage. Anyone used this harness before?
There is not labeling or directions and two of the ends will fit into the EHA harness. One is a straight connector and one is a 90 degree connector, but they will both fit the male connector on the EHA itself.