The CAN data bus is a two wire network that basically loops around the car linking through each control module, one wire is CAN high the other is CAN low. It was primarily designed to reduce the amount of wires used in electronic/computer controlled motor cars.
basically, for an example of how one signal is distributed throughout the car (pre CAN data bus), I will use the crankshaft signal, the crankshaft sensor is wired to the HFM so it can determine fuel and ignition timing and then the HFM is wired to the instrument cluster so it can produce a tacho value and the HFM is wired to the trans control unit so it knows what speed the engine is doing and so on and so on, this would require miles of wiring.
Now how it is done using a CAN data bus;
if you can imagine all the control units are interlinked via two data transmitting wires (CAN data bus) all that has to be done now is the crank angle signal is put onto the CAN data bus via the HFM which continues on through all the modules and then any module that requires an engine RPM signal is programmed to accept that signal if the control unit doesn't need that signal say, like seat adjustment module then that CAN signal will pass straight thru. so its a little bit like sending a letter, you address the letter (crank angle signal) you put the letter in the mail box and it gets sorted (HFM) it then gets delivered (CAN data bus) and goes to the address on the letter (control units programmed to accept that signal) if you receive a letter thats not addressed to you, you put it back into the postal system (control units not requiring this signal)
The crank angle signal is a signal that would be constantly active on the CAN data bus, so it would be constantly going around the loop, and is given HIGH priority, a signal such as when you push the button to close a window is only on the CAN data bus whilst you have your finger on the button, this signal would be given LOW priority.
The CAN bus is a fairly bullet proof system but you can still have problems with it, such as the z connectors that connect the CAN wires, you sometimes have to cut these connectors out and solder them directly to rectify CAN signal issues
I hope this helps you to understand what and how the CAN data bus works, If the new IFZ and key doesn't fix your problem, I would suggest you start looking at CAN data bus and possibly faulty HFM. :)

Thank you very much for the very detailed elaboration Ozzy, I have fully
understand your explanation but I have one last (annoying and apology) query for you :)

1. Referring to the bolded sentence, may I ask : since it will be put back into the postal system (control units not requiring this signal), I understand that the proposed letter (control units programmed to accept that signal) will not function but how come the right signal will come back again 5-10mins after several attempts? Does it mean that the mail box and sorter (HFM) has reset itself (and the fault is stored)?

2. Where are you from anyway Ozz? I am from Brunei Darussalam, located in the island of Borneo.

Many advance apologies and and thank yous again for your patient comments.

Yes thats basically what happens, the CAN signals are blocks of data put onto the CAN data bus by each control module, so for instance the crank angle sensor signal would be a constant block of data on the CAN line, were as the signal from the IFZ to release the HFM would be one block of data sent once, I should also explain that CAN high is data sent from a control module and CAN low is data that is received from other modules. So when the IFZ puts the signal on the data bus it basically goes around once, If the HFM misses the signal :sleeping: for whatever reason then it wont release and the engine will not start, you then have to turn off ignition, remove key and then try again.
Where do i live? I come from a land downunder :sun_smile ;)

Thank you for the clarifications !

I think I understand now how the logistics work. So meaning to say that CAN High is from the IFZ and CAN Low from the HFM. Is it a norm for Mercedes vehicles on their HFM to miss signals sent from other control modules? Could insufficient power supply lead to this?

So cutting connectors and soldering them directly; is this a customised modification or a normal "plan B" approach for rectification? Because replacing the CAN Bus will not make any difference huh? I still doubt that the ECU that is at fault here :) Everything performance and function are operating on a normal basis.

By the way Ozz, can you re-attach the picture again in a larger size? The diagrams are visible but I can't make out the words. Thanks!

Which state of Australia are you in? I was in Melbourne, Victoria from 1997 - 2001.

Not really a common problem, but as with any electronic component you can have errors/glitches with them, the HFM may very well be receiving the CAN signal from the IFZ but it may have a problem with processing the data, the HFM is basically a mini computer and we have all experienced that dreaded screen lock up on your home computer, you have to reboot, and away you go again. so i guess you could say it could be a similar problem, you have to turn the key off and try again (rebooting).
the weakest link in any wiring loom is the connectors, but unfortunately it's near impossible for a car to be produced without them. most connectors are basicaly a female and male plug, the trouble with these connectors is they can become loose from heat stress causing bad earth/power problems, the quickest and most reliable solution is to remove the plug altogether and solder the wires directly.
remember earlier in my posts I wrote about how the HFM is always locked until it receives the CAN signal to unlock it, once it is unlocked it will function as normal with no problems at all, just like your computer wont work until you turn the power on.
over the border :)

Thank you for your clarifications again!

Now I understand how the IFZ, CAN BUS and HFM Module works and you know what is happening with my car. (most likely the cause)

I have been doing a lot of homework and researched many forums that could lead to why my car is behaving like this and the similat symptoms happening to other Mercedes models . Do you think other factors such as:

1. Over Voltage Protection (OVP) (Happen to a lot of E W210 classes)
2. Crankshaft Sensor
3. Starter Motor Pinion
4. Fuel Pump
5. Alternator

has/ have any relation to these intermittent starting failures? The MB dealer claimed that these 5 items are ok!? But I would like to hear your comments

Because you are getting the red and green alternating flash we know that the HFM has not been released and for that reason we can eliminate a lot of other possible non start causes. if the engine was cranking but not starting and we had no flashing lights coming from the infrared receiver (mirror) then we would be looking at other possibilities. just to reassure you i will go through and note what normally would result from these 5 items if they were faulty.
1) Overvoltage protection relay; if it was dead car would be stalling or would crank but not start at all.
2) Crankshaft sensor; very much the same , would crank but not start at all or engine would stall once started.
3) Starter motor pinion; would try to crank but make a loud screaching noise or not crank at all.
4) Fuel pump; car would normally still start but would lack performance or run very poorly and stall if fuel pump was very bad.
5) Alternator; if it wasn't charging you would be having battery problems, car would still run if jump started. :)
when are you going back to have the new IFZ and key fitted?

You're good man! This saves me my confusion! I think by now we can narrow down the possibilities! Power supply is out of the question. The most possiblities would be between:
1. IFZ Control Module
2. CAN Bus Wiring Harness
3. HFM (that is the last resolution and i Hope not) :)

The new IFZ and key will be arriving middle of next week but the MB Dealer may gonna keep the car in the workshop for observations (I don't know how long). I was just wondering why would they need a new key coming along together with the new IFZ? Does it mean that my old keys cannot be used anymore after the installation of the new sets?

Not sure why they have ordered a new key, because all keys will have to be initialized (learn in transponder) and then synchronized (learn in IR remote control key) you will still be able to use your old keys, just make sure you have them all on hand when you take the car to the workshop. ;)
just wondering, what were you doing in melb? :)

Will do that, these guys didn't even have the initiative to ask me to bring my old keys along in the first place, luckily i was prepared.


I was studying in college and in Uni. "Over the border" = which state ? How long have you been a Merc Tech/ Mechanic?

Sydney N.S.W, I've been a benz tech for 18 years. :)

18 Years, Cool!
That explains why I could understand all the explanations!

Anyway, I shall be flying to Kuala Lumpur in exactly 24 Hours from now for some meeting (s).

I shall be back on 23rd December 2004 and I hope by then, the parts have arrived and my car would be sent in during this period.

I will keep you posted. Thanks!

By the way, could I have your e-mail should I could not get in touch with you via this forum. My e-mail is thegirlilove@hotmail.com

Just got back from holidays, whats happening with your benz?
Unless I'm away I check this forum daily, I would prefer to assist you via the forum, that way it may help other members with similar problems :)

Happy New Year 2005 and thank you for your reply Ozz!

Hope you did not have your holidays in SEA though.

1. Well, my car is still in the MB workshop since last week, 27th December 2004. They said they need at least 2 days to check everything after the installation of the new IFZ together with the new and existing keys. So I guess I have to wait but I will call them up tomorrow for an update.

2. The other C280 that I mentioned that was re-coded in it ECU by the Service Manager (Ref. Post #13, 12-10-2004, 08:10 PM) was not rectified at all ( you were right). So therefore, the dealer is trying to replace the existing wiring harness of that car but Service Manager still insist that wiring harness is not the culprit.

3. I think from the above, we can conclude that the ECU is out of the “process of elimination” as it could not be a coincidence that both ECUs on 2 vehicles are defective?

4. What are the methods to verify that the DAS mode is functioning properly?

Thanks.

Hey Ozzy,

1 Just had a meeting with the MB mechanic this morning, apparently he has switched the rear view interior mirror/ receiver with their company’s C280 and took the car for a test. But the starting failure happened again.

2. I asked about the status of the other car as mentioned in my last post, he said that though it has the same symptom (alternate red & green lights), they just replaced the OVP relay in that car and it has not returned for a complaint since.

3. The mechanic advised that the reading from the IFZ transfer to the ECU on my car was sufficient but a failing OVP Relay may cause intermittent ampere rate fluctuations from the alternator?

4. All this time he thought that I have changed the OVP Relay and he was very surprised when I informed that I only changed the Voltage regulator in the alternator. Does this make sense to you?

Please comment.