Intermittent ABS light + a misfire at idle
 

My 300SEL has been idling a bit rough lately...sometimes it will run perfectly smooth, and other times it will run a bit rough, and then other times, out of nowhere it will misfire horribly for a second or two, then start idling smoothly again...really random but it seems to get a bit worse when I shift into reverse. The ABS light has also been turning on and off randomly...

I'm assuming this is OVP related? Are there any other symptoms I should look for?

Oh, and I dunno if it's related, but my tach is a bit bouncy...

You should start by making sure that the OVPR is supplying power correctly to BOTH computers.

Sure sounds like an OVP relay problem. The contacts sometimes mess up and and the voltage supply to the ABS and CIS-E becomes intermittent which causes the ABS light to come one and your idle smoothness to go south. I would rock the OVP a bit in its socket to see if that does anything. Pull it off the socket and inspect the contact pins. Also, make sure the fuse is ok and properly seated.

I was thinking the same thing, especially on an m103, I've had several go bad with the exact same symptoms, try finding one at the junkyard first. Also there is a slim chance the fuse blew in which case there's your problem :)

If johnathan1 is handy with a soldering iron, he could try and pry open the OVP, and resolder the connecting pins. It seems the internal connections to those pins sometimes develop small metal fatigue cracks that cause the intermittent failures. The same also can happen with the Fuel Pump Relay (I fixed mine some years ago), and to a lesser extent to the Klima relay.

Wow, that was a whole lot easier than I thought it would be...I pulled the OVPR out, and none of the solder joints really looked suspect, but I went ahead and re soldered them all anyway. I reinstalled it, and used some electrical contact cleaner/enhancer on the connector. It looks like the repair was a success!! Car started up perfectly, no ABS light, and it has never idled so smooth. :eek: It's still not perfect (needs a full tuneup) but it is one HELL of an improvement, that's for sure. I can't thank you guys enough!!

Another satisfied customer! :D :beerchug:

Well, I spoke too soon... :( It is now misfiring again, but there is no ABS light this time. I am going to check the connector tomorrow and make sure all of the pins are making good contact.

If there is no ABS light on when the car misfires, the OVP should be fine now.

You mentioned that the car was in need of a tune-up. Why don't you get that out of the way? The distributor cap and rotor, spark plugs (properly gapped), and spark plug cables are important items that need to be in good shape if the engine is to perform correctly. Your miss-fire condition could be any of those parts failing intermittently, specially a badly worn out spark plug, or defective cable. Next step would be to check the CIS-E system, mixture adjustment and possible unmetered air leaks (vacuum hoses, injector seals, brake booster, etc), proper operation of the o2 and temperature sensors, EHA, Idle control valve and associated hoses, and finally if all else checks out ok, maybe and then only maybe your problem might be related to the fuel distributor itself.

<<If there is no ABS light on when the car misfires, the OVP should be fine now>>

Lack of ABS lamp is not an OVP OK verification. It is just that many times a bad OVP will be coupled with an ABS lamp and off idle...but neither need show to still have a bad OVP. ECU protection from OVP is thru zenier resistor spike circuits, not ABS feed fused circuit.

The OVP is simply a relay controlled by the FPR (Fuel Pump Relay) that mainly feeds the ABS and CIS-E modules, plus the ICV (Idle Control Valve). It's protective properties come from a build-in zener diode that will trip the 10A OVP fuse if the main bus voltage reaches above ~15V. The most common fault condition of the OVP is loss of output voltage, either due to the fuse being blown, or bad contacts on the OVP itself. Since all the devices that are powered through the OVP have a common feed point, loss of voltage from the OVP produces the well known ABS light on warning, and also simultaneous loss of the CIS-E mixture and idle valve control which manifests itself as rough idle and to some extent also engine misfiring.

You're right that in general one should not use the ABS lamp as a strict guideline to determine OVP operation, but in this particular case we know that in fact the ABS lamp was a clear indication of the problem, or at least one of them. An even better test would be to measure and confirm that the OVP output voltage is present and within range. This can be done by measuring the voltage at either pin 2 of the OVP (K1) itself, pin 1 of the CIS-E (N3) module, or even easier, between ground and the Red-yellow cable connected to the ICV (Y6).

BTW, I've been using pin and module references for the W124 model CIS-E system, which is the manual I have, but the arrangement in your particular 300SEL car should be very similar, if not almost the same.

But I would definitively still recommend a complete engine tune-up before looking for any other suspects responsible of the engine still misfiring at this point.

<You're right that in general one should not use the ABS lamp as a strict guideline to determine OVP operation, but in this particular case we know that in fact the ABS lamp was a clear indication of the problem, or at least one of them.>

.."or at least ONE of them" is the point I am making.
One can have ABS feed from an OVP and still have poor feed to CIS control.

We have all seen that exact state hundreds of times ...and I agree that OVP output feed monitoring is the correct way for perfect diagnosis, which is why Techs and Shops that are hip to that fact all have a spare/known good OVP in their toolbox...one of the few times that replacement with a known good one is a true diagnostic time saver..
OVP gremlins come in many shapes and sizes and are falsely diagnosed as many times as they are blamed for things they have nothing to do with...but understandable, due too there high fail rate, specially the original, single fused versions.