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Outstanding Contributor
1988 300CE
1,544 Posts
Discussion Starter #1
This thread refers solely to the OVP (OverVoltage Protection relay) in CIS-E cars !

Despite the fact that the number of those who are interested in proper testing & diagnosing has further dropped here at BenzWorld over the past couple of months and that the preferred approach to fix problems is still to change parts based on guessing, I‘d like to try once more to spark interest in logical approaches based on facts (real facts, not alternative ones ;)). After all, this is a car forum, not the … sorry, I digress. :unsure:

The “E“ in KE-Jetronic is provided with power solely via OVP. A failure of this component can cause a number of symptoms, of which I mentioned a few in my KE-Jetronic on-board test device thread. … It can, for instance, cause cold start problems, or acceleration problems with the engine not warmed up yet. But, contrary to alternative info here at BenzWorld, with an otherwise flawless system and the engine at operating temperature, a faulty OVP can not cause the engine to stop and, besides a slightly higher idle speed, you‘ll even hardly notice OVP failure (meaning failure of the complete “E“ in KE-Jetronic) during driving at all … thanks to the KE-Jetronic‘s unmatched limp home mode. … Neither does the OVP have anything to do with the ignition system. The OVP is frequently suspected in case of symptoms that the OVP has nothing to do with, making it a frequent victim of guessing games and one of the most often unnecessarily replaced parts on CIS-E cars.

In case of engine related symptoms (or duty cycle readings ;)) that do make the OVP a suspect, the first thing that should be done is a supply voltage test at two, engine dependent, test spots on the disconnected connectors as shown in the (symbolic) diagram below (which is applicable to all MB CIS-E engines). With the key turned to “ignition on“, you should see battery voltage at the following test spots:

  • In case of M102 & M103 engine: (1) & (2)
  • In case of M104 & M119 engine: (1) & (3)
  • In case of M116 & M117 engine: (1) & (4)

IACV = Idle Air Control Valve
CSV = Cold Start Valve

Attention: Always disconnect / reconnect ECU connectors with ignition switched off !

CIS-E MBs are, engine dependently, equipped with a 25-pin or a 55-pin CIS-ECU (N3).
(See pin assignments in the diagrams attached below)

In ^this quick & easy voltage test at the above test spots, not only the OVP itself, but also all involved connections & wires to & from the OVP are covered. After all, what's the use of a perfectly intact OVP if there‘s a problem with those things !? ;)

IF you see battery voltage at the above test spots, the OVP is doing what it‘s supposed to do. However, consider that there might be an intermittent OVP related problem that just didn‘t appear during the test ! … BTW, with a test device like the one I introduced in the above mentioned thread (or with an alternative that I mentioned in that thread) you can very comfortably monitor and easily detect, not only OVP related, intermittent problems during driving. ;)

IF you do not see battery voltage at the above test spots:

Check the OVP‘s fuse(s)

2) If the fuse(s) is/are blown, remove the OVP and check for short(s)
  • between ground and each of the two above mentioned engine dependent test spots
  • and between OVP pin “31“ and each of its pins labeled “87..“.
Fix short(s) between the former (if any), respectively replace the OVP in case of short(s) between the latter.
Then replace the fuse(s) and repeat the above supply voltage test.

3) If the fuse(s) is/are good, remove the OVP and make sure that there is
  • continuity between the above test spots & the corresponding OVP socket port(s)
  • continuity between ground & (solely!) OVP socket port 5
  • battery voltage at OVP socket port 1
  • voltage ‘15‘ (battery voltage) at OVP socket port 3 (solely) with key turned to ‘ignition on‘ -- (If port 3 is empty, use port 6)
  • no mechanical problem with the OVP socket & the connector ports
4) If ^these things are in order, then there is a problem with the OVP itself. In that case either replace it, or open it and check for cold solder joints on its circuit board. In by far most cases it‘s just a cold solder joint on the circuit board. With a little soldering skill, that can (preventively along with all other solder joints) easily be fixed within 15 minutes. Properly resoldered original 30+ years old OVPs usually work problem-free for more years to come than today‘s new OVPs. ;)

If you want to test a replacement/resoldered OVP before you install it, here‘s a simple bench test:
  • connect pin 30 to the positive terminal of a 12 V battery
  • connect pin 31 and the voltmeter‘s ground lead to its negative terminal
  • now there should be battery voltage at pins 30a, 30b
  • there should be no voltage at pins 15, 87E, 87L -- (often overlooked potential cause of “unexplainable“ battery drain, BTW ! ;))
  • connect pin 15 to the positive terminal of the battery (you should hear the OVP click)
  • now there should be battery voltage at pins 87E, 87L
For ^this bench test you can also heat the OVP (with a hairdryer) or cool it (in the freezer) to check for temperature dependent / intermittent problems.


25-terminal CIS-ECU connector.jpg 55-terminal CIS-ECU connector.JPG

1986/1990 W126
14,322 Posts
Yes awesome. As is that limp home mode you refer to H.D. It really works, I often wonder if that sort of thing is why these cars were so popular for Heads of State type people, and as armoured cars. It'll almost always get you home.

Premium Member
Daily: 2000 Kleemann Supercharged CLK 430. Seductress: 1988 Jap AMG SEC Widebody
1,201 Posts
Thank you very much H.D. for sharing your knowledge in such a logical way that is so easy to understand and follow.

Outstanding Contributor
1988 300CE
1,544 Posts
Discussion Starter #6
Yes awesome. As is that limp home mode you refer to H.D. It really works, I often wonder if that sort of thing is why these cars were so popular for Heads of State type people, and as armoured cars. It'll almost always get you home.
That may have made these cars popular not only for Heads of State type people, but also for mob type people who treasure reliable getaway cars. … LOL … Come to think of it, some of the former, even democratically elected ones, are of the same type as the latter. … Sorry, I digress again. :unsure:

I talked about the main reason for MB‘s decision to use the KE-Jetronic & the limp home mode in post #2 of my KE-Jetronic Lambda Control Thread. … Nothing represents the spirit ‘golden era‘ MBs were designed with more than that decision.
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