I am going to try and describe my situation as simply as I can. I just replaced the ECU (see earlier thread on cold start valve). My duty cycle will not fluctuate - this is NOT a new problem. I am using a Sears multimeter and I have read so much about whether you put the black probe in #2 or #3 of the X11 connector...2 is the ground but something about the way the Sears meters work and the way Mercedes defines duty cycle...so using meter and just reading the volts using the Landiss formula is what I've been trying.
Key on Engine off - I'm getting 12.4 volts at X11 2 and 6, and 8.62 volts at X11 2 and 3. That converts to a 30% duty cycle - which doesn't make sense. The static tests don't really make sense since I can't really figure out this initial reading...Engine running the volts drop to 3.62 and stays there - no matter what I do to rpms....
Key on Engine off - EHA current is right at 20ma, but when engine runs it stays right at 0.
If I use the duty cycle setting on my multimeter - I get either a static 20% reading (which suggests TPS), or 80% if I switch the leads in X11 2 and 3 (which suggests IAT sensor).
In that case you can read my thread on the W201 forum with the title "EHA valve".
But even more importantly you need to read the following article from "H.D.".
I believe it is the preeminent article on the fuel injection diagnostics. Also explains how the darn thing works very nicely.
Yes, I have bookmarked and read thru H.D's article several times...
20% error suggests TPS - which tested following procedure in manual and it appears to test OK...or an 80% code points to the air temp sensor - which has been replaced and tested. I am having trouble just figuring out how to take the initial static reading - red probe in #2 or #3???
And I can not get the engine in to closed loop - while idling stays at 20% or 80% - AGAIN, depending on the black/red probes being in #2 or #3 of the X11...
O2 sensor has been replaced, but it's possible that it is clogged because I'm fairly certain the car has been running rich...
The definition of "Duty Cycle" of a square wave signal from an electronics point of view is always defined as:
Signal "Hi" time divided by signal "period" which is "Hi time + Lo time"
From a literal point of view duty cycle definition is:
Duty Cycle is the fraction of one period in which a signal or system is active
Most, if not all multimeters are made for the electronic world and follow the first definition.
Mercedes Benz literature and specs use the second definition. Which probably means that when the voltages is "zero" the system is active. So when an ordinary meter is used, the duty cycle read by the meter needs to be subtracted from 100%.
But this is all semantics, for your situation what you need to do is to connect the meter so that you are getting a positive "average" voltage. Use the DC option instead of AC on the meter.
Record the DC voltage, make sure it is a positive value. If not reverse the leads.
Then turn the knob so that %Duty is read out.
If the average voltage was >6.5V and your %duty is >50% then the %Duty needs to be subtracted from 100%
If the average voltage was >6.5V and your %duty is <50% then the %Duty you read is as Mercedes defined it
If the average voltage was <6.5V and your %duty is >50% then the %Duty you read is as Mercedes defined it
If the average voltage was <6.5V and your %duty is <50% then the %Duty needs to be subtracted from 100%
I hope my explanation is clear.
In other words always use the Duty cycle in combination with the average DC voltage.
Just as a quick suggestion: check resistance between ECU connector terminal 46 and ground (with ignition “off” !). If it’s not ‘infinite’, there is a problem with the TPS’s WOT signal … either the switch is made (although the T is not WO), or there is a short of the T46 wire to ground.
Btw, don’t overlook what I pointed out under “Measuring device” in post 2 of the thread dolucasi mentions (appreciatively :thumbsup: “Vp6 = (battery) voltage between X11 port 6 & port 2 (or ground) during the respective rev!”
That means, always set Vp3 in relation to Vp6 in the respective situation ... (Vp6 at ‘ignition on’, Vp6 at idle, Vp6 at 2500 rpm) !
That sentence also reveals that port 2 of X11 is connected to ground … consequently the meter’s ‘V’ cable belongs in port 3 (respectively port 6) of X11. :wink_2:
Thanks to both of you - very helpful posts and I will check on this later today and report back. One clarifying question - when you say "check resistance between ECU connector terminal 46 and ground" - you're talking about the wiring harness plug and NOT the #46 pin on the ECU - right? Probably a dumb question, but just making sure...and I can just use the negative battery post for my ground - right?
... when you say "check resistance between ECU connector terminal 46 and ground" - you're talking about the wiring harness plug and NOT the #46 pin on the ECU - right? ... ... and I can just use the negative battery post for my ground - right? ...
I made some checks just now - with ignition on and engine off, I get positive voltage with black probe in X11 #2 and red in #3 - 8.64 volts and when I switch meter to Duty Cycle reading, I get 69.9%. This doesn't make sense as following the post above this would give me a static reading of 30% - 100% - 70% (Federal car is supposed to 70% with key on engine off).
At idle, duty goes immediately to 3.65v and the duty cycle goes to 19.9% which means a static 80% (again, following the 100% - 20%) reading while the car is idling.
rgnptof, You had me doubt my own readings so I just checked. Mine is a CA car so the expected duty cycle is 85% (instead of 70%) with ignition on, engine not running.
I get
#3 to #2 (gnd) = 1.81V
#6 to #2 (gnd) = 12V
With the formula:
Duty cycle = (1-(1.81/12) * 100% = 85%
Meter says 15%.
My table sent to you suggests because my #3 is lower than 6.5V and meter says <50%, I need to subtract
Duty cyle = 100% - 15% = 85%
So everything is in order for me.
Remember if you push the diagnostic button after the ignition is on and engine is off, the ECU will start sending some sort of codes instead of the default code which is telling us if the ECU is operating and what type it is.
So do not push that button before you make your readings.
If you have not done so, and you have just turned the key on (with ignition off) without pressing that button your readings say your duty cycle is 30% something is definitely wrong.
My best guess is something is wrong with your ECU but at this point I can not help you, hopefully H.D. can.
Dolucasi - I have no 'diagnostic button' to press on my car - I believe that's only on California cars...
I will report back on the ECU resistance tests (I have long suspected the TPS, partially because it's impossible to get to without pulling the motor - slight exaggeration ), but following the procedure from the FSM , it has always tested 'mostly' within specs...
Ths is a new to me ECU (bought used on ebay), but it did come with a lifetime warranty.
Sorry my mistake, you maybe correct only the CA models have the diagnostic button.
You never said if your old ECU has the Ignition on Engine off duty cycle reading problem. Does it? If it does not, then it seems you may have replaced your old (and maybe faulty ECU) with a new/used and faulty ECU.
I was having similar problems with the old ECU - couldn't get the car in to closed loop either, but I think the specifics were a bit different (I replaced it after running in to problems with the cold start valve and didn't check the duty cycle first...but I've had problems with it for a while).
I checked the TPS at pin 46 of the ECU and it reads infinite, so I don't think that's the problem...
I have pulled the replacement ECU and I'm going to put my old one back in and see what happens...
If I were you, I would put the original one in and see if it can read the correct Ignition on engine off Duty cycle reading.
If you do I would return the one you bought from EBAY.
Then start chasing what else is wrong with your system.
If that one has the exact some issue with the Ignition On Engine Off problem then you can start chasing other issues like H.D. suggests.
I replaced the ECU with my old one and I am getting EXACTLY the same readings! I checked my meter on my 560SL and it works properly - 70% duty cycle at KOEO and 50% when I start it and then it starts to cycle...
That, of course, sounds less like a problem with the ECU itself, but more like a problem that affects the ECU ... which might take some patient searching ... which, due to my currently limited time, I can only try to assist in.
First I suggest to check on the disconnected ECU connector:
voltage at T1 and T41 with “ignition on”
resistance to ground at all other terminals with “ignition off”
Again, keep in mind always to disconnect / reconnect the ECU with ignition switched off !
Can anyone provide some guidance here? I have spent some of my weekend looking through wiring diagrams and tracing wires...I've accounted for maybe 30 or so of the 55 pins of my ECU, but have no idea of specs...are specs published somewhere?
Don't have the complete FSM - the online manuals do not have the information needed for the M104 engine...or at least I have not been able to locate it.
Thanks - I understand being busy and just appreciate any assistance you can offer. I think I could troubleshoot some of this, but I just don't know specs...readings are below.
OK - I have replaced the OVP relay within the past year or 2...KAE brand. But, I did blow one of the 10a fuses a little bit ago when I was troubleshooting the cold start valve. Don't know if that has anything to do with anything...
I will replace it again and see what happens.
Again, Thanks so much for the assistance...I'll be sure to report back.
I really never thought a thing about blowing that 10a fuse..I just replaced it with another one thinking that I had overloaded the wire while testing the cold start wiring...I thought the fuse did what it was supposed to do and that the OVP itself was fine.
Two years back, I replaced the OVP hoping it would solve some problems similar to what I'm having now, but I don't think doing so made much difference. Maybe the KAE brand OVP was bad from the start, or maybe this is a separate problem. I don't know...
Is the KAE brand ok - or should I go Mercedes (which I think is Stribel...)
I really never thought a thing about blowing that 10a fuse..I just replaced it with another one thinking that I had overloaded the wire while testing the cold start wiring...I thought the fuse did what it was supposed to do and that the OVP itself was fine ...
... Two years back, I replaced the OVP hoping it would solve some problems similar to what I'm having now, but I don't think doing so made much difference. Maybe the KAE brand OVP was bad from the start, or maybe this is a separate problem. I don't know ...
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