w126 blinking CEL

[Das Benz]

Hey folks, just thought I would run a quick problem by you.
Just my luck, I am soon going for our provincial etest for my 1990 420sel, and im having an odd issue with my check engine light.

Only from a cold start, when I start the car, I get a 7 blink code from the engine light, which indicates "TNA-signal (RPM-signal) read by CIS-E implausible"
Any other time I start the car within the day I get no engine light at all and no codes. FYI this is not a California car.

I built the diagnostic led/switch harness, however on my car, pin 3 on X92 under the hood has no connector (no metal sleeve inside), and thus no way to read the code "as per the mb instructions" via this diag port, and no way to reset the code. Like I mentioned above, the engine light on the dash is blinking this code to me, only when the car is first started for the day, it does not re-appear or come on again any other time during the day.

The car runs fine, no issues at all, and the tach works as always. SO this leads me to believe something odd is up. Seeing that this car has only one CPS to read the rpm info from, if the tach is working as expected, and theres no sputtering/misfiring.. What else can the CIS be cranky about?
Car starts right up, and within 2-3 seconds, settles down to 500-600rpm.


Today im going to take the car out for a spirited spin and double check my duty cycle and such, just to make sure everything is working as expected, then off to testing D-day!
Honesty, etesting is like the most dreadful thing for car guys.. I absolutely hate it, ob2 versions no problem, but the sniff test on these older cars, you have no ideal if you will pass.

[Elektri]

Could be the RPM signal wire going to the engine computer, which on my car is further away, than the EZL and RPM dash gauge is (computer to right of passenger's feet).

So maybe a poor connection on the RPM wire going to the engine computer or a weak signal - strong enough to get to the closer devices, but not strong enough to get to the computer?

I can read my RPM signal on the round X-11 connector on the driver's side of the engine. As I recall per the factory manual, you can check the "strongness" of that signal with an oscilloscope.

I would clean all connections, especially those going to the EZL.

[Das Benz]

Well looks like this issue came back, and bit me in the butt..

Started the car this morning, and had to head into town. No issues what so ever, car started like it always has.
Go to my first stop, started up fine, go to my second stop.. 10 min later go to start up, and the car was jumping and stuttering like ive never heard before. It would barely run without the starter helping it turn over, and stuttering and jumping all over. Instantly to me it seemed like a spark or ignition timing issue.

Had to get the car towed home today.. First time for it

Then I started my troubleshooting. First culprit was the CPS on the back of the bellhousing (fyi theres 2 CPS on this car, but the 2nd one on the front balancer is just used for TDC diagnostics thru the X11 plug, it is not connected to anything.)
I followed all the diag steps via the MB manual
http://www.startekinfo.com/StarTek/outside/12253/disc%202/program/Engine/107/M117_56/15-540.pdf

I tested the CPS using a mulitimeter first to check resistance.. ~700Ω
-Check voltage going to EZL- OK
-Check crank pulse from CPS using multimeter - All OK (looked just like the sample on pg 13 of the PDF above.
-Checked CPS insulation resistance - OK.

SO then I stopped and took a break. I noticed that with the CPS unplugged from the EZL, there was still spark coming from the coil (the engine would fire randomly, and jump about)... How would it know to fire the coil if there was no crank position feedback to the EZL..
Immediately I removed the EZL to try and test it in my 90-560sel.. And sure enough.. not even a hint of fire.. What caught me slightly off guard though was in my 420 with the CPS disconnected it would randomly fire, in my 560 with the CPS connected and disconnected, there was no hit of a spark or fire at all.. Odd I thought...
Oh and the thermal paste on the back of the EZL was toast.. so that could be the reason for the death of the EZL.

I haven't put my 560 EZL in my 420, I don't want to risk blowing it up. the 560 runs perfectly the way it is... But I have 2 420EZL's on the way.. Should be here next week.

Guys set me straight if im not mistaken, doesn't the EZL disconnect the primary side of the coil's ground to collapse the field and initiate a spark?
http://www.startekinfo.com/StarTek/outside/12253/disc%202/program/ETM/126from86.pdf
Pg 16 in the pdf or PG 108 on the sheet..

Ive dealt mainly with modern coil on plug ignition systems which is all logic controlled for firing and dwell time, so please bear with me for the questions on these older systems, just need a bit of time to get back up to speed with these older systems.


In the mean time, Im going to put my old EZL in my mill, and open her up and see inside

[Das Benz]

Just confirmed it was the EZL.

Decided to check the EZL on my 560 to see what condition the thermal paste was, sure enough, needed replacement.. While it was removed, I thought midas well give it a shot.. Popped it in my 420, car fired right up!.

I put it back in my 560, ill wait till the proper P/N EZL's come in before driving my 420.

[Das Benz]

Well I thought I would give a quick update.

I was able to get 2 good EZL's for my 420. I finally had some time last week and dropped one in. Took it for a quick spin, and everything is back to normal.. Actually better then normal. To me since ive owned the car, it always felt a bit sluggish when accelerating, and there was a slight noticeable power boost when you let off the throttle. Which had led me to believe something was up with the fuel delivery or ignition timing. But the timing always checked good at idle. (its impossible to check timing under load unless you have access to a dyno)

Well with the new/used ezl, the car now has even power delivery throughout the power band, and has some surprising punch when you stomp on it, and it kicks down.... especially above 3800rpm. Im impressed with an engine that has over 255,000+miles. I would say the car has easily picked up another 20-30hp now that the ezl feels like it is properly controlling the timing in relation to manifold vacuum.

I also finally found the timing resistor (been looking for a few years), and the timing does advance when removed.. But ill leave it in for now, until I have some time to fiddle with it.

[Elektri]

2 good EZL's!

Were those delivered in an armored truck? One alone is worth a small fortune!