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Discussion Starter #1
I try to establish if the above error (error 8 on pin 3 of the 16-pin OBDII) causes my engine to perform poorly on low revs. By searching and reading dozens of docs and threads, I think my car 1991 SL300-24v (104.981) has no altitude correction module. It seems to be the CIS-E control unit (N3) that sends altitude correction data to the EZL/AKR (N1/3). Question: how do I switch this correction data off? Is there a wire or module to disconnect?

Regards, Frank

Reference: Data Exchange
 

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The 129 DVD says about pin 3 / code 8 "see the DI module". Have you checked to see if there are codes on pin 8?
 

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I try to establish if the above error (error 8 on pin 3 of the 16-pin OBDII) causes my engine to perform poorly on low revs. By searching and reading dozens of docs and threads, I think my car 1991 SL300-24v (104.981) has no altitude correction module. It seems to be the CIS-E control unit (N3) that sends altitude correction data to the EZL/AKR (N1/3). Question: how do I switch this correction data off? Is there a wire or module to disconnect?

Regards, Frank

Reference: Data Exchange
Hi Frank,

Several things could be wrong. Since we have the same car, here is what I did when I had then same code.
Check for vacuum leak and ensure the vacuum line connected to the EZL is O.K. incl. the rubber connectors.
Remove the computer and ensure the air holes/pressure port is not clogged
If all vacuum connectors are good, you might need a new(used) computer or EZL. Try to swap with a friend (my buddy drives a 91 300SL-24V) and see if the code goes away before you buy one. The EZL is very expensive and difficult to find for our 300SL-24V version. If you are thinking about a used computer...write down the Bosch number and the part-number and get the exact same one. Federal vs California versions could make your day.

I got a different EZL....that fixed the problem...and I renewed all rubber connectors on all vacuum lines too.

Good luck
 

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Discussion Starter #6
Thanks for the reply but after some more searching i found this article (three years rough idle sl320 r129 v6 m112 - MBClub UK - Bringing together Mercedes Enthusiasts). The symptoms are exactly the same (engine shaking at idle, loss of power at low refs and fuel smell in exhaust) and you can see that this guy has tried almost everything to fix it (without success). There are some other articles to be found with exactly the same problem accompanied with a huge amount of good suggestions but no result. I think I'll have to spend some money and take it to an MB dealer. I'll open a new thread because the title of this thread doesn't reflect the problem very well...
 

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O.K...?!

I know the SL320 even with the M104 engine has a different fuel injection system. We have a CFI/CIS-E system vs the HFM-SFI or LH-SFI system on the later models. As I mentioned your problems could have many root causes. I just gave you my 2 cents. Did I mention I had the same symptoms and have the same exact car than you?

Please accept my apologies, disregard my post and visit the dealer.
 

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Discussion Starter #9
UncleWolfy,

so you car also had the vibrations (shaking) at idle (750rpm)?

PS: I looked at the vacuum line into the EZL and it looked fine. It's going to be a problem finding a spare EZL to test (but I'll start the search)

Thanks
 

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300SL-24 1991 Vibrating at idle + fuel smell in exhaust

...
I have a 1990 500SL.
The vacuum line to the EZL, simple comes from a port on top of the intake manifold.
This I presume is for ignition advance.

My code #8 is listed as "Altitude pressure signal FROM ignition control module illogical".
So, it must be a wire coming from the EZL, to the computer...
The thinking is that the computer has that altitude pressure signal generator "Inside" the computer?

Where is the computer?? Ha!
 

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The CIS, is located in the computer coffin. Remove plastic cover, passenger back side.
But lets put the horse in front of the wagon.

Do you have vacuum on the EZL?
If you don't have.
Look on the radiator support for a vacuum diagram sticker.
If missing,
You have three lines two small color coded no longer available so they may be clear, and one black larger.
They start at the engine intake manifold, end on and next to the EZL.

Starting , engine OFF..
Left engine side,in order to see and work, you have to remove the third injector line.
Don't damage/bend it.
Have a looong nose pliers. and about 25feet of the thin tubing


0001581435

Cohline Vacuum Hose/Line; White; 1mm ID x 4mm OD; BULK

, and about 10 of the rubber connectors, size like the one on the EZL..
1170780281

Rein O.E.M. Vacuum Hose/Line; 80mm Length Low Pressure Vacuum Line with Elbow

As you will discover more vacuum problems...
(autohauseaz.com has good prices.)
Deep down, the hole, you will see, a special size rubber elbow/ need, two.


1170780481

URO Parts Vacuum Hose Connector


In Stock

Ships within 1 Business Day



5 mm I.D. Small End, 7 mm I.D. Large End


$3.56



(Most likely, leaking.)
That is one of the starter lines.
Now you will see an electric switch over valve, this and one of the thin lines and black, goes to the charcoal canister/ right side to EZL.
Do that, reset, see if the code, disappears. Aha!

Regards,
aam.

PS,
There are two more switch over valves located in the coffin/ computer compartment.
One step at the time....
 

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The CIS, is located in the computer coffin. Remove plastic cover, passenger back side.
But lets put the horse in front of the wagon.

Do you have vacuum on the EZL?
If you don't have.
Look on the radiator support for a vacuum diagram sticker.
If missing,
You have three lines two small color coded no longer available so they may be clear, and one black larger.
They start at the engine intake manifold, end on and next to the EZL.

Starting , engine OFF..
Left engine side,in order to see and work, you have to remove the third injector line.
Don't damage/bend it.
Have a looong nose pliers. and about 25feet of the thin tubing


0001581435

Cohline Vacuum Hose/Line; White; 1mm ID x 4mm OD; BULK

, and about 10 of the rubber connectors, size like the one on the EZL..
1170780281

Rein O.E.M. Vacuum Hose/Line; 80mm Length Low Pressure Vacuum Line with Elbow

As you will discover more vacuum problems...
(autohauseaz.com has good prices.)
Deep down, the hole, you will see, a special size rubber elbow/ need, two.


1170780481

URO Parts Vacuum Hose Connector


In Stock

Ships within 1 Business Day



5 mm I.D. Small End, 7 mm I.D. Large End


$3.56



(Most likely, leaking.)
That is one of the starter lines.
Now you will see an electric switch over valve, this and one of the thin lines and black, goes to the charcoal canister/ right side to EZL.
Do that, reset, see if the code, disappears. Aha!

Regards,
aam.

PS,
There are two more switch over valves located in the coffin/ computer compartment.
One step at the time....
 

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I DO have vacuum at the EZL! My EZL is on the left side (USA model) of the engine, not the right. The elbow on top of the intake manifold would be a very likely suspect to leak, so I will replace it. Maybe it leaks once in a while?

I shall look for a "switch-over valve" too.

The two switch over valves in the compartment at the firewall, check out correctly.

This car, though being old (1990), was a garaged one owner in Los Gatos, CA. So, there is NO rust or even regular corrosion anywhere... a real joy to work on.

I just pulled the dizzies and rotors, and pulled the bracket and dust shield.
The dust shield had a pretty good layer of oil on them, so cleaned with acetone.
WOW! It made a huge difference, it's damn hard to believe!

So now, the car is fairly driveable, except this bogging after the idle switch is off.
Yes, I even put in a new throttle switch (idle/full) and that wasn't it.

So, the symptoms are an occasional severe bog down, when leaving a traffic light, let's say.
If I pull the throttle off and reapply, it will start going again.
Cruises well on the freeway.

Thanks for you reply, I shall look for a perished rubber elbow within the intake manifold.
 

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Greek God of the R129
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8,364 Posts
Symptoms are occasional severe bog down.

1. I would start from checking the fuel pumps, replacing the fuel filter. Unplug/plug the MAS module.
MAS module is in the computer coffin. One of its jobs, is to control the fuel pumps.
(Not a bad idea to spray clean, the pins, and connector.)
2. Fuel pressure, volume, test.
3. Testing the other two switch over valves, one is going to the EGR valve. The other to the air pump.
Ck especially the EGR valve, function, leaks.
Remember that all three switch over valves, are not working all the time!
4.Long shot,
Take a 13mm flare wrench, ENGINE idling, crack open the fittings on the fuel distributor.
Check for rusty fuel.

Regards,
aam.
 
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