1991 SL500 - Issues with idle, cold starting, rough running - Mercedes-Benz Forum
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post #1 of 20 (permalink) Old 07-08-2019, 09:58 PM Thread Starter
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1991 SL500 - Issues with idle, cold starting, rough running

Hello,

My friend has a 1991 SL500 with under 200k, well maintained that has recently been having some issues. He noticed that there is no voltage going to the idle control valve at all. Also we've performed the diagnostic test at the X11 port (pin's 3 and 2) and get .1v.. which is VERY odd... as BobTerry's post say it should be fluctuating and near half the battery voltage. Can a bad OVP relay cause this? Let me know the next steps I should perform and thanks in advance.

Last edited by marco455; 07-08-2019 at 10:01 PM.
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post #2 of 20 (permalink) Old 07-14-2019, 05:04 AM
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Also we've performed the diagnostic test at the X11 port (pin's 3 and 2) and get .1v.. which is VERY odd..
The OVP relay could be bad, so I would momentarily bypass it for a test. To do that remove the relay and jumper together its sockets #2, #4, and #6 with wires or paper clips, and then check the voltage on X11.
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post #3 of 20 (permalink) Old 07-15-2019, 02:08 AM
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The ICV receives power from the N16 MAS module via input from the N3 KE injection control module.

Power to the above modules (and others) is via the OVP, which protects them from excessive current.

Consider that while bridging the pins for the OVP, these modules will not be ‘protected’.

If the OVP is bad (typically because of a leaking Zener diode) to the extent that the MAS cannot supply power to the ICV, I would surmise that the fuel pumps would also not receive power from the MAS, and the engine would not run.

From your description, and whilst a proper diagnosis would be required, I suspect you have a defective N3 KE module.
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post #4 of 20 (permalink) Old 07-15-2019, 06:09 AM
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In regard to the engine management, the MAS powers the fuel pumps and oxygen sensor heater but nothing else. The MAS receives power directly from the battery via a power distribution block located a few inches away from it. As an aside, a faulty MAS too can be diagnosed with a bypass by removing the module and jumpering together its sockets #1 and #2 (to run the fuel pumps).

The OVP relay is not a critical component. In fact, the engine can run perfectly fine with it removed from the vehicle. But since the OVP powers the fuel injection controller, cold start enrichment would not exist.

A bypassed OVP leaves the fuel injection and ABS controllers unprotected from voltage transients exceeding c. 22 volts. I suppose the risk of a transient occurring during a brief test is akin to a person being struck by lightening in the midst of a thunderstorm.
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post #5 of 20 (permalink) Old 07-15-2019, 01:46 PM Thread Starter
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Thank you very much. I will bypass the OVP (as directed above) and test ports 3 and 2 on the X11 (3 positive, 2 negative). If it still comes back as 0.01V, what is my next step? How do I test the N3 KE module? Thank you again.
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post #6 of 20 (permalink) Old 07-16-2019, 12:31 AM
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Quote:
Originally Posted by bobterry99 View Post
In regard to the engine management, the MAS powers the fuel pumps and oxygen sensor heater but nothing else. The MAS receives power directly from the battery via a power distribution block located a few inches away from it. As an aside, a faulty MAS too can be diagnosed with a bypass by removing the module and jumpering together its sockets #1 and #2 (to run the fuel pumps).

The OVP relay is not a critical component. In fact, the engine can run perfectly fine with it removed from the vehicle. But since the OVP powers the fuel injection controller, cold start enrichment would not exist.

A bypassed OVP leaves the fuel injection and ABS controllers unprotected from voltage transients exceeding c. 22 volts. I suppose the risk of a transient occurring during a brief test is akin to a person being struck by lightening in the midst of a thunderstorm.
Cheers Bob. Specifically the ICV Y6 receives power via connector block X26/2 pins 10 and 14 from pins 4 and 23 of N3 KE module. Not MAS. Sorry to OP, and thank you Bob.

.. and yes, the engine can be started and run with the OVP unplugged.
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post #7 of 20 (permalink) Old 07-16-2019, 12:35 AM
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If the fuse on the OVP is blown, I would not recommend that you jumper the pins until the cause of the overvoltage has been found. If the fuse is intact, then the risks are akin to being struck by lightening as in Bob’s example.
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post #8 of 20 (permalink) Old 07-16-2019, 06:56 AM
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How do I test the N3 KE module?
I think the definitive test is to measure its output current to the EHA located on the side of the fuel distributor. With key on and engine off it should me 75mA DC.
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post #9 of 20 (permalink) Old 07-16-2019, 10:00 AM
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Thank you very much. I will bypass the OVP (as directed above) and test ports 3 and 2 on the X11 (3 positive, 2 negative). If it still comes back as 0.01V, what is my next step? How do I test the N3 KE module? Thank you again.
The following link describes some tests on the N3 KE module that you could probably do. In this case the member confirmed a dead KE ECU module. I suggest you read from the point where Bob and H.D. step in: -

https://www.benzworld.org/forums/r12...l#post16677393

Also this link : -

https://www.benzworld.org/forums/r12...y-cycle-4.html

When stuff sucks... make it right!
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post #10 of 20 (permalink) Old 07-16-2019, 04:23 PM Thread Starter
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I've re-tested the voltage with bypassing the noted ports and it reads 0.01V. I'd like to move on to testing these other modules, however, my skill level is beginner in diagnosing.

Can you please describe with pics how to now move onto testing my N3 KE Module? I am reading through the posts but sort of confused. Is this an entire ECU/computer? Am I testing this with keys on and not engine running?
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