Retrieving Pre-OBDII Diagnostic Data From ’94 SL500

[MB-Dude]

Infamous ‘Check Engine’ light is on for my ’94 SL500. Have tried a couple simple things including a new wiring harness (it needed it anyway), new gas cap, tune-up, etc. – all to no avail. If I reset the CEL, it will remain off through 3 or 4 engine starts. Then during the 4th or 5th engine start, the CEL will remain illuminated – the CEL never comes on while driving. If I reset the CEL again, the same sequence repeats. Also, the car runs great and gets reasonable gas mileage (~15 mpg). It even passes smog checks as a very clean vehicle - as long as I reset the CEL immediately before the inspection.

With no OBDII port, there is no straight-forward way to retrieve diagnostic data. I know there are methods for a DIY guy to this and would appreciate hearing from folks on how I can retrieve diagnostic data from a ’94 SL500. Thanks in advance.

Cheers,
Jeff

[deanyel]

You're resetting the CEL how? You can pull the codes on your 38 pin diagnostic connector, in a black box near where the battery would be. Try a search - lots of info, or go to menu item #2, bottom, at:

http://pages.prodigy.net/jforgione/MB_S500.html

[MB-Dude]

Originally Posted by deanyel

You're resetting the CEL how? You can pull the codes on your 38 pin diagnostic connector, in a black box near where the battery would be. Try a search - lots of info, or go to menu item #2, bottom, at:

http://pages.prodigy.net/jforgione/MB_S500.html

As corny as it sounds…
1. Remove the negative battery cable from the battery post
2. Using a long screwdriver, touch the disconnected negative cable to the positive cable for 10 seconds
3. Remove long screwdriver
4. Reconnect negative battery cable

Somehow, this technique resets the CEL; at least for 3-4 engine starts. Obviously I have to reset the radio code each time, but it has worked for the dozen or so times I’ve done this.

Regarding the codes, thanks for the link. I am familiar with the 38-pin connector and box location. The problem with a search is you end up with WAY more things to plod through than necessary. This is why I simply asked the question.

Cheers,
Jeff

[JimF]

Originally Posted by MB-Dude

Infamous ‘Check Engine’ light is on for my ’94 SL500.

I know there are methods for a DIY guy to this and would appreciate hearing from folks on how I can retrieve diagnostic data from a ’94 SL500. Thanks in advance.

Cheers,
Jeff

You may have a built-in LED/SW combo so you can read the DM codes (Menu#6). That would get you a summary of the overall problems.

Since it has the basic engine as my '94 S500 so you find out what the flashes mean by using the 'home-brew' DTC reader (use deanyel's link) and using the various menus on my pages.

You did ask on how to retreive diagnostic data. . .

[bobterry99]

Stevebfl has a DIY article that explains how to read codes here: http://www.peachparts.com/Wikka/EngineControls.

On a '94 the socket for diagnosing the CE light is #19 of the 38-socket connector. Steve mentions using an LED to see the diagnostic pulses, and for this I would recommend a particular LED from Radio Shack part number 276-209. It has a built-in resistor that makes it operate at 12VDC. This LED would have its (-) terminal connected to socket #19 and its (+) terminal connected to socket #3 (#3 is 12VDC).

If you send me an email, I can reply with a PDF document that has the meaning of all of the diagnostic codes.

[MB-Dude]

Thanks deanyel, Jim and Bob – Yes, I did ask how to retrieve the codes! Boy did I ever. Obviously not as straightforward as with OBDII, but hey… that’s the fun of it. The information is great and am going to begin work on it later today. Thanks again.
Cheers,
Jeff

[MB-Dude]

Thanks everyone for the information. Jim's web page is very informative and I look forward to exploring more of it this weekend.

However, my '94 SL500 (R129) doesn't match up well with the information on Jim's web site regarding module layout - my Control Modules are not in the same order as the diagram shown in Jim's web page. I suspect this is simply a difference between the W140 and R129 models. Nevertheless, the attached pic (94SL500 ECU Layout.jpg) shows my computer box. For clarification, the next two pics (94SL500 Top 3 Modules.jpg & 94SL500 Bottom 2 Modules.jpg) show the module numbers for the five installed modules. The only two I can definitely identify is the 'Base Module' and the 'ASR module', which is at the top and second position, respectively. Other than the base module, all the others seem out of order.

QUESTION: Can someone help me identify which modules are which?

Good news is my vehicle has a Diagnostic Switch installed. Refer to attached pic (94SL500 Diag Switch.jpg), although it does not have a cover and was not 'mounted' to anything; simply laying in the bottom of module bay.

Bad news is, following the directions outlined in Jim's web page (menu #2, specifically), I get zero blinking... none... nada. I've tried holding the button 2 secs, 4 secs, 5 secs, 10 secs... nothing. Not one single blink.

QUESTION: Does this mean there are no codes stored?

Lastly, after all the above poking and prodding, I disconnected the negative cable from the battery to measure battery voltage without any load. Voltage was 12.42vDC. However, when I reconnected the battery cable and started the engine... the CEL operated normally. Meaning lit up when ignition turns on, and goes out when the engine starts.

QUESTION: Is it possible a weak or marginal battery could cause the Diagnostic Module (whichever it is) to 'latch' the output circuit for the CEL, even though there is not a fault?

Any and all information will be greatly appreciated. Thanks in advance.
Cheers,
Jeff

[JimF]

Yes, the module arrangment is a lot different.

The picture shows what's what . . . that answers your 1st question.

When you press the 'button' does the LED light up?? If NOT, it looks like it's not connected. If it lights when the button is pushed for 4 - 6 secs, then it should start the readout process. Sometimes if the DM has been cleared, it takes some time for the 'system' to get into sync.

When the battery gets weak, it does do very strange things to the electronic modules. One thing that I watch for: when you start the car watch the IC lamps. If they really loose "ALOT" of brightness, then it's time to replace the battery. Not very scientific but you get the idea.

If it drops to 8V - 9V range, then mysterious DTCs get recorded. Such things as the windows loose their sync; ie don't know where the top is which happens w/ the coupe, so the doors won't close.

[MB-Dude]

Thanks Jim for the reply - especially for providing the order of the control modules.

When I press the Diagnostic Button, the LED does NOT light at all. In fact, at first I thought the LED may be burnt out. I will try it again tomorrow, using some longer wait times and hold-the-button times; different combinations based upon your idea of longer sync time needed.

Also, I swapped the battery between this car and my '79 240D. Ironically, they use the same aftermarket battery. After 3 engine starts today (after the battery swap), the CEL has not stayed lit. One of the things that always stuck in my mind (and this will sound crazy) is that whenever the car would start, I could 'sense' that the CEL would stay on. This is because I could perceive an ever-so-slight change in the sound of the starter as the engine cranked. My electronics background tells me that... maybe... I have a marginal cell in the battery that fails periodically/occasionally; especially under load – such as under starter load. In my case, the CEL only latches on during engine start – never during operation/driving. Thus, the reason I asked question #3. And remember, my CEL problem resets simply by removing the negative battery cable – this is not considered normal for the CEL. Please correct me if I am wrong on that point.

I'll report more tomorrow as I continue testing. Again, thanks to all.

Cheers,
Jeff

[JimF]

Originally Posted by MB-Dude

Thanks Jim for the reply - especially for providing the order of the control modules.

When I press the Diagnostic Button, the LED does NOT light at all. In fact, at first I thought the LED may be burnt out. I will try it again tomorrow, using some longer wait times and hold-the-button times; different combinations based upon your idea of longer sync time needed.

The fact that it doesn't light was why I asked; that means that it's not connected and is open-circuit. So to use it you'll have to connect it to the proper point. That may be more difficult than it appears.

It might be easier to construct the "home-brew" reader in MENU#2. Then you can use it on any pin. The "hard" part is the small connector pins to properly interface into the 38 pin Diag Connector.

Originally Posted by MB-Dude

Also, I swapped the battery between this car and my '79 240D. Ironically, they use the same aftermarket battery. After 3 engine starts today (after the battery swap), the CEL has not stayed lit. One of the things that always stuck in my mind (and this will sound crazy) is that whenever the car would start, I could 'sense' that the CEL would stay on. This is because I could perceive an ever-so-slight change in the sound of the starter as the engine cranked. My electronics background tells me that... maybe... I have a marginal cell in the battery that fails periodically/occasionally; especially under load – such as under starter load. In my case, the CEL only latches on during engine start – never during operation/driving. Thus, the reason I asked question #3. And remember, my CEL problem resets simply by removing the negative battery cable – this is not considered normal for the CEL. Please correct me if I am wrong on that point.
Cheers,
Jeff

You can test it with a hygrometer (available at most auto stores) to see if the cell is bad or use a battery tester that can load it properly. If it's low, then best to invest in a new one.

[MB-Dude]

Originally Posted by JimF

... It might be easier to construct the "home-brew" reader in MENU#2. Then you can use it on any pin. The "hard" part is the small connector pins to properly interface into the 38 pin Diag Connector.

No problem. Will build the 'Home Brew Box' today (assuming the local 'Shack has all the parts). The schematic is straightforward, however when complete, which pins of the 38-pin Diagnostic Connector are J1, J2 & J3 (as shown on the schematic) connected to? I'm assuming that once the 'Home Brew Box' is connected to the 38-pin Diagnostic Connector, operation is identical to the installed MB switch X11/21s1.
Cheers,
Jeff

[JimF]

If you refer to the picture in MENU#2 of the 38pin Diag connector,
1 = ground <-- connect to J3
3 = battery 12V <-- connect to J1

J2 connects to the ECU under test. You'll want to test the following 'systems'

Pin ---------- Name
4 ----------- LH-SFi
7 ----------- EA/CC/ISC
19 ---------- Diag Module

and anything else that you feel that the car has.

[MB-Dude]

Originally Posted by JimF

If you refer to the picture in MENU#2 of the 38pin Diag connector,
1 = ground <-- connect to J3
3 = battery 12V <-- connect to J1

J2 connects to the ECU under test. You'll want to test the following 'systems'

Pin ---------- Name
4 ----------- LH-SFi
7 ----------- EA/CC/ISC
19 ---------- Diag Module

and anything else that you feel that that in the car.

AH-HA! Excellent. Thanks - it all makes sense now. For some reason, I had a mental block regarding the 'test lead' location.
Cheers,
Jeff

[MB-Dude]

First things first… If you own a pre-OBDII Mercedes and have *not* made one of these ‘Home Brew DTC Readers', you are missing one of the best values for your tool box. All the parts are standard at Radio Shack and with an hour of your time, you can build a great little test box. See attached pic. Thanks JimF for basic information and thanks also to Bob Terry for additional info.

Ok, so I made my ‘Home Brew DTC Reader’ and plugged it in. See attached pic. Here are some things I learned the first time I used it:

1. When you plug in the ‘Home Brew DTC Reader’, the LED will be lit until you turn the ignition on. It should then go out immediately with the ignition on.

2. The LED should light and stay on for as long as you hold the button down.

3. If you press the button for less than 3 seconds, the LED will generate a quick, faint flash of light. This indicates you did not hold the button down long enough and no codes have been sent.

Other than that, the operation is exactly as Jim describes on his web page.

So what codes did my ’94 SL500 generate?
Test Pin #19 (Diagnostic Module) – 19 LED blinks
Test Pin #4 (LH-SFI) – 1 LED blink
Test Pin #7 (EA/CC/ISC w/ASR) – 3 blinks, 5 blinks, 14 blinks

The diagnostic code data I have (if I am reading these correctly) indicates (in the order listed above):

A) Injector open or short circuit or emission control system adaptation at limit.

B) Right EA/CC/ISC Actuator (left cylinder bank) (M16/1)

C) Stop Lamp Switch (S9/1)

D) Closed Throttle Position Contact Switch

Now for the fun part… what does it all mean? What I can tell you is what the problem isn’t. I have replaced the main engine wiring harness, gas cap, battery, and did a general tune-up. The car runs great, gets reasonable mileage and passes smog when I 'reset' the CEL. One of the things I have suspected is the wiring that is part of the Main Throttle Actuator. This may help satisfy D and perhaps A.

I’m open to any and all thoughts, feedback and input.

Cheers,
Jeff

[JimF]

Originally Posted by MB-Dude

First things first… If you own a pre-OBDII Mercedes and have *not* made one of these ‘Home Brew DTC Readers', you are missing one of the best values for your tool box. All the parts are standard at Radio Shack and with an hour of your time, you can build a great little test box.

Wow, you're good! Not just good but a

Originally Posted by MB-Dude

So what codes did my ’94 SL500 generate?a
Test Pin #19 (Diagnostic Module) – 19 LED blinks
Test Pin #4 (LH-SFI) – 1 LED blink
Test Pin #7 (EA/CC/ISC w/ASR) – 3 blinks, 5 blinks, 14 blinks

The diagnostic code data I have (if I am reading these correctly) indicates (in the order listed above):

A) Injector open or short circuit or emission control system adaptation at limit.

B) Right EA/CC/ISC Actuator (left cylinder bank) (M16/1)

C) Stop Lamp Switch (S9/1)

D) Closed Throttle Position Contact Switch

Now for the fun part… what does it all mean? What I can tell you is what the problem isn’t.

One of the things I have suspected is the wiring that is part of the Main Throttle Actuator. This may help satisfy D and perhaps A.

I’m open to any and all thoughts, feedback and input.

Cheers,
Jeff

Ahh . . . the infamous DTC#19 . . . check my page about halfway down and that can be fixed so you won't have to reset the CEL to pass smog!

I'd next replacet the brake stop lamp switch . . cheap to do and it has fixed a lot of suspect ETAs . . . Just as in my case, bad Neutral Safety Switch, that one also in in the 'path'. As you saw in MENU#24 (update: 10/19), replacing the NSW fixed my "ETA" problem.

Of course, reset all the codes and see what returns. . . .

I added your (re-sized) pics to my MENU#2 . . .