10%= new airflow meter? - Page 3 - PeachParts Mercedes-Benz Forum

brewtoo · #31 12-25-2003, 05:54 PM

today...

tested air flow sensor, both resistance and voltage. Perfect.

When I first turned on key, koeo, I got 70% DC. After the car sat and warmed up, I got DC varying around 65% while running. Then it went to varying around 10%. No change in the way the engine was running.

Turned engine off, koeo, 10% fixed DC. Pressed down air sensor plate...went to 70% fixed. Let plate up...back to 10% fixed. BUT, air flow pot tested perfectly and had continuity to computer. Unplugged temp sensor...went to 70%...plugged it back in...went to 10%...unplugged idle speed control...went to 70%...plugged it back in...went back to 10%.

All the while it's stuck at +2ma on the EHA anytime it's running or koeo.

After the car sits a while it will go back to the proper 70% koeo.

sigh.......

brewtoo · #32 12-28-2003, 12:04 AM

UPDATE

Just so happens that we own a '90 and a '91 300E. The '91 is the one giving trouble.

Today, even though I've read that the ECU rarely goes bad, I decided to swap the ECUs between the cars.

To my amazement, the problems moved to the other car.

So now, the '90 gives the goofy 10% readouts on the DC.

The '91 still requires that I "wake up" the ECU by flooring the gas pedal once for just an instant, but after that it runs fine (it's been this way for three years). Before I do the "floor-it" maneuver, it just sits forever at 50% fixed DC. After I floor it once, it comes to life.

I'm still gonna replace the airflow sensor pot to see if it clears up this original problem.

Wonder where's a good place to get a replacement ECU.........

mhingram · #33 12-28-2003, 06:17 PM

test your throttle switch before you replace the potentiometer. It is very easy and I think 50 is the throttle switch or temp sensor not the potentiometer.

stevebfl · #34 12-28-2003, 07:53 PM

I don't memorize code numbers but 50% is easy it is for a dead O2 sensor. It's easy because all cars exhibit 50% until the O2 sensor warms up. Sometimes at idle they even cool enough to go back to 50%. A bad heater circuit will contribute greatly to slow sensors

brewtoo · #35 12-28-2003, 09:22 PM

Quote:

Originally posted by stevebfl
I don't memorize code numbers but 50% is easy it is for a dead O2 sensor. It's easy because all cars exhibit 50% until the O2 sensor warms up. Sometimes at idle they even cool enough to go back to 50%. A bad heater circuit will contribute greatly to slow sensors

Oops! Sorry guys, I didn't make myself clear. OK, when you start a cold engine the DC stays at 50% for a bit until the engine warms up and then the closed-loop system begins to do its thing, adjusting the mixture with the DC and EHA Ma bouncing around.

On my car, the system does not come to life until I floor the accelerator one time, even if it is fully warmed up. It just stays asleep until I "wake" it.

So I didn't mean 50% as an error code but as an indication that the system had not yet started to work.

Thanks

stevebfl · #36 12-28-2003, 09:31 PM

Fifty percent IS a fault code on a warm motor. If you have to rev a car with a heated O2 sensor, it is real likely the heater circuit isn't working. Once warm the O2 sensor should never cool until shut down.

brewtoo · #37 12-28-2003, 10:56 PM

Quote:

Originally posted by stevebfl
Fifty percent IS a fault code on a warm motor. If you have to rev a car with a heated O2 sensor, it is real likely the heater circuit isn't working. Once warm the O2 sensor should never cool until shut down.

Hmmmmm......You know, Steve, that is an interesting thought. I had not considered the heater circuit. I'll check it with the ohm meter it and see if it's open. It would be ironic if it was the 02 sensor all this time.

Thanks

OK, I checked the heater wires going into the O2 sensor. Resistance is almost none...2 ohms. It's the same as an old sensor I had lying around and checked for comparison. Battery voltage is going to the O2 sensor when the engine is running.

That means the O2 sensor heater circuit is good?

MARIO FARIAS · #38 12-29-2003, 05:36 AM

Thanks guys for this interesting threadid.

Please, I have two questions:

1º- What does means "heater circuit" and how to test?

2º.- If We don´t have the harness for testing the EHA, how to

connect the DMM to the EHA for reading the amps?

Thanks in advance.

Mario Farias
laprefar@cantv.net

MARIO FARIAS · #39 12-29-2003, 06:06 AM

Mr. Mhingram, Please I tryed to see

http://mingram.com/mb/Engine/107/M117_56/073-121.pdf

but I couldn't because it say that we are not authorized to view

this page.

Please how to view your web page.

Thanks

Mario Farias
laprefar@cantv.net

stevebfl · #40 12-29-2003, 08:04 AM

Two ohms is probably right for the sensor, but that doesn't confirm the circuit or whether it actually heats.

The best way to test a heater circuit is with an inductive ammeter. The circuit isn't turned on all the time either and in newer cars is often pulsed. I will show a GMM (graphing multimeter) image I took of the rear O2 sensor heater circuit on a 328i BMW (it is the same as many late MBs):

fahrgewehr2 · #41 12-29-2003, 04:12 PM

Brewtoo- What are your cold start symptoms? My problem has progressed to the point that it takes at least 5 starts to get her to idle. Eventually it warms up and idles fine. 10% duty cycle entire time (except 70% KOEO).

Unplugged the pot. today and noticed no difference in symptoms. Should I go ahead and pull it anyways, just to take a look?

Mike

mhingram · #42 12-29-2003, 04:17 PM

mark it and pull it see if it is damaged. Easy to do and quick check. did you get eha numbers yet?
m

fahrgewehr2 · #43 12-29-2003, 05:10 PM

I pulled eha numbers again this afternoon. I get nothing- 0.0mA. unplugging EHA does nothing to the way the cars starts runs.

Maybe my setup is incorrect?

fahrgewehr2 · #44 12-29-2003, 05:14 PM

here is how I am taking the reading you see in the big picture above.

stevebfl · #45 12-29-2003, 05:36 PM

Sure doesn't look like proper test set-up.

You are measuring CURRENT. This means the circuit must be in place with your meter acting as part of the wiring. There are two wires one in and one out. If they are both disconnected from the EHA one meter lead hooks to the connect (removed from EHA) and the other lead must hook to the EHA position that wire came from. The other wire of the connector must be hooked straight from the connector to the EHA. It does not matter which wire is which but their position on the EHA can not be reversed or the polarity of the current will change and screw with the running.

If you think of it theoretically the meter is inserted into one of the wires and carries the current that the wire carries.