OM603.971 EDS Codes and Diagnosis
 

I pulled codes on my 93 300SD, from the 38 pin diagnostic plug.

I got the following:
Pin 4: EDS
8 - Engine coolant temperature sensor (B11/4)
9 - Intake air temperature sensor (B2/1a)
2 - Fuel rack position sensor (L7)
3 - Air flow sensor signal (B2/1)
7 - Starter ring gear speed sensor (L3)
5 - Exhaust gas recirculation valve vacuum transducer (Y31/1) or fault in exhaust gas recirculation (EGR) control circuit

Kind of surprised that I got so many codes. All I know is that I idle around 550rpm, and the cluster temperature, tachometer, and speedometer all work.

Pulling other codes, I found that the HVAC codes indicated, amongst others:
43 - Engine Coolant Temperature Sensor (B10/8)

I would assume that B11/4 and B10/8 imply two different sensors, as we know there are two.

My primary interest is, what of these are required to be fixed in order to get idling properly? I haven’t tested boost levels, but the car drives very well, cruises at speed effortlessly, and now starts well (replaced GPs and cleaned the intake soot out).

I primarily bought this car because it has a factory refurbished engine and AT; and I want those. The car runs great and all systems are functional. But idle isn’t high enough, and the governor overshoots to a low RPM before bouncing back to 550. When stone cold it does crank and idle at a bit higher, but as soon as it’s slightly warm, and everything warmer it’s always 550ish.

So, recommendations on where to start?

Thanks!

Code 7 is why you have a low idle most likely. Does the tach in the cluster work?

Now that you've pulled codes, you should clear them and see which ones come back. Some of those might be old.

The governor in the IP really should never control the idle speed, it should be the EDS.

-J

The tach, speedometer, and MPG gauge all work fine. Thus why code 7 was suspect to me...

I suspect that the signal may make it to tach but not to someplace else (???). Im having an issue like that with the alternator light.

I do need to clear all codes and try again...

Thanks for the hint on #7. Need to find where the wiring harness goes...

On the W126 the Tach gets its signal from the EDS computer. So if the tach works, the pickup is good and the computer should be controlling idle speed.

Mine overshoots the governor then bounces back to 550 rpm as well. Following…

I’m doing this on my 93, w140. Think it’s different?

Other thing is no guarantee the wife from the tach (or engine harness) to the computer is good. Need to figure out that diagram.

Dieseal300 had a thread where he replaced the caps in the ECU to get his idle operating correctly.

Yes Im aware. But I dont think blindly recapping the board is the right first step. Diagnosis of the causes and issues is prudent. Thats what Id like to be able to do....

I tried to look up W140 wiring diagrams, but could't find one in an easily readable format. I'll be able to look more later.

Why not test it before blindly changing the caps?

That's what Im trying to do!

Thus why I pulled codes from the w140 38 pin connector and posted above.

I would assume that those codes help with the test. If there's another basis of testing that you recommend, I'm all ears!

Now that you know what the codes are, clear them and see if they come back. Then you'll know what the computer is currently upset about.

That’s the plan once the snow clears a bit…

I started to diagnose.

My codes were:
Pin 4: EDS
8 - Engine coolant temperature sensor (B11/4)
9 - Intake air temperature sensor (B2/1a)
2 - Fuel rack position sensor (L7)
3 - Air flow sensor signal (B2/1)
7 - Starter ring gear speed sensor (L3)
5 - Exhaust gas recirculation valve vacuum transducer (Y31/1) or fault in exhaust gas recirculation (EGR) control circuit

I figure code 7 and 8 are critical. 7 supports the RPM control loop, and 8 may make some decisions.

I went through the FSM, somewhat out of order. The pin out on the connector seems to be straight across with #1-10 being the group all in a row, and then it just keeps going and continues below.

https://i.imgur.com/1PO0xY3l.jpg

I first checked the main fuses which are all fine.

https://i.imgur.com/538lFI9l.jpg

Then I checked the EDS to the IP wiring which was fine. 3.5 ohms.

https://i.imgur.com/NHyeRcll.jpg

Then I check code 8, and it checks out fine.

https://i.imgur.com/6wcPBJRl.jpg

https://i.imgur.com/19SMGBvl.jpg

https://i.imgur.com/ZX5IZ75l.jpg

So code 8 seems like a fluke.

Then I try to diagnose code 7, and I cannot get conductivity between the pins for the speed sensor L3. The pins the FSM say to check don’t give any resistance in the range for the Bosch or Beru sensor.

But here’s the thing… the tachometer works perfectly. And it seems like the tachometer gets its signal from the N39 connector which is the EDS.

How can my tachometer work, and the EDS not have a speed signal?

Harnesses all look perfect.

https://i.imgur.com/8B7aRGhl.jpg

After cleaning and checking everything, I still get:

Pin 4: EDS
9 - Intake air temperature sensor (B2/1a)
3 - Air flow sensor signal (B2/1)
7 - Starter ring gear speed sensor (L3)

How does my tach work, yet I can’t measure the right resistance on the pins to diagnose code 7?

Any ideas?

How does my tach work, yet I can’t measure the right resistance on the pins to diagnose code 7?

Operator error measuring signal?
You should get 150 to 300 ohms but it's a shielded signal measure ohms between the pin and ground.