Tachometer Not Working After Jump Start and ACC is Weird Too... - Page 2 - Mercedes-Benz Forum
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post #11 of 27 (permalink) Old 08-29-2015, 08:31 AM
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Originally Posted by kelley312c1 View Post
Thanks Floobydust! I have an OVP on the way!
I just see, with another thread you start asking again about this weird issue. Why you don,t wait until you will change the OVP, the greatest expert in the forum already said what is the problem.

Also in the future you should also use the adopted names and descriptions. A/C= air condition, AAC automatic air condition. Using ccu (??) is definitely wrong because CC more and less mean clutch control or cruise control ...
Your innovation ccu could be a Climate Control Unit ? or what? Using adopted abbreviations would make the troubleshooting easier and faster.
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post #12 of 27 (permalink) Old 08-30-2015, 12:46 PM Thread Starter
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Well, I go the new OVP, installed it...and I still have the same problems. Tachometer is not working and the Climate Control Unit still doesn't turn on initially, the turns on/off, then turns on. Coincidentally, I have the old Climate Control Unit [which works, but has other problems, such as A/C compressor does not shut off and defrost flap does not open (the vac pod works fine)] and it would work where the other one didn't. So, I assume the issue is in the climate control unit. I will try resoldering the connections and hope for the best; if it still doesn't work I will have to fin a new climate control unit.

But the tachometer still doesn't work....hmmmmmm...any ideas? Thanks!
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post #13 of 27 (permalink) Old 08-30-2015, 01:08 PM
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According the MB official drawings for Automatic Climate (A/C) the "...Climate Control Unit.." in the center of the dashboard NAME is a Pushbuttom Control Assembly , inside N/22 is a Pushbuttom Control Modul
Like I said, if A/C not start, could be wrong the pushbuttom control modul (N/22) . Assume overvoltage or some shortage of blower motor could destroy it, need to replace. If the blower motor not working, this could destroy the replaced module also.
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File Type: pdf A:C clutch control 87 diesel .pdf (405.2 KB, 85 views)
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post #14 of 27 (permalink) Old 09-01-2015, 02:05 PM
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Originally Posted by kelley312c1 View Post
Well, I go the new OVP, installed it...and I still have the same problems. Tachometer is not working and the Climate Control Unit still doesn't turn on initially, the turns on/off, then turns on. Coincidentally, I have the old Climate Control Unit [which works, but has other problems, such as A/C compressor does not shut off and defrost flap does not open (the vac pod works fine)] and it would work where the other one didn't. So, I assume the issue is in the climate control unit. I will try resoldering the connections and hope for the best; if it still doesn't work I will have to fin a new climate control unit.

But the tachometer still doesn't work....hmmmmmm...any ideas? Thanks!
So the plot, as they say, coagulates . . .

Because the tach signal is used by the Klima relay, I would start by troubleshooting that first. Since the OVP did not resolve the tach signal problem, you will have to determine whether the tach is bad or the signal to it from the EDS is bad. Although I am not a true "diesel" person, I would suggest building/borrowing the LED scanner and try to read any faults codes present on pin 4 (EDS) on the diagnostic connector. The attached snippet from the ETM for the EDS also implies that the tach signal is also present of pin 2 of the diagnostic connector, even though that does not seem to be documented elsewhere. If you own/can borrow and digital VOM with a frequency counting function on it you should be able to check for the tach signal.
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Floobydust: a contemporary term derived from the archaic Latin miscellaneous, whose disputed history probably springs from Greek origins (influenced, of course by Egyptian linguists) - meaning here "a mixed bag." - National Semiconductor Audio Handbook
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post #15 of 27 (permalink) Old 09-01-2015, 02:56 PM
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So the plot, as they say, coagulates . . .
I was taught never to get excited over wounds, because all bleeding eventually stops.
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post #16 of 27 (permalink) Old 09-01-2015, 07:41 PM Thread Starter
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Yes it does...I ended up fixing the Climate Control Unit Issue...there were two 33uf/16v capacitors that were blown; I replaced them and now the Automatic Climate Control works just fine!

The information you sent is great, although i am lost on how to build/obtain the necessary tools to scheck the function of the tachometer. It seems, however, from the diagram that the "test connector" travels to "N6 A/C Compressor Control Unit" before going to the tachometer...since my compressor works just fine, I can rule that connection out...correct?

Here is my line of reasoning...if I am geting a bad tach signal that travels through "N6" then I would have issues with the compressor. Since I don't have issues with my compressor, then I can assume (again only assuming because I do not know how to build/obtain the necessary scanning tools) that "N6" is receiving a tach signal?

Last edited by kelley312c1; 09-01-2015 at 07:43 PM.
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post #17 of 27 (permalink) Old 09-01-2015, 07:53 PM
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Originally Posted by kelley312c1 View Post
Yes it does...I ended up fixing the Climate Control Unit Issue...there were two 33uf/16v capacitors that were blown; I replaced them and now the Automatic Climate Control works just fine!

The information you sent is great, although i am lost on how to build/obtain the necessary tools to scheck the function of the tachometer. It seems, however, from the diagram that the "test connector" travels to "N6 A/C Compressor Control Unit" before going to the tachometer...since my compressor works just fine, I can rule that connection out...correct?

Here is my line of reasoning...if I am geting a bad tach signal that travels through "N6" then I would have issues with the compressor. Since I don't have issues with my compressor, then I can assume (again only assuming because I do not know how to build/obtain the necessary scanning tools) that "N6" is receiving a tach signal?
your conclusion is correct: without tach signal A/C would not work. Let,s go find out why the tachometer don,t work. You need drawing?
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post #18 of 27 (permalink) Old 09-01-2015, 07:59 PM
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It has been my experience that fresh electrolytic capacitors tend to be fairly tolerant of brief periods of over voltage, but they tend to go bang when even lower than rated working voltage is applied in reverse. Well aged, dried out caps (alas, like most in our W124s) can fail at the slightest provocation. Keep in mind that the OVP protects only those components it powers - the EDS and the ABS from over/reverse voltage, so any of the other electronics in the car like the CCU or tach may have been damaged by a voltage spike or momentary reverse voltage during the jump. Given this and the functioning A/C, it is certainly more likely that the tach itself is the problem, but I would still try to check for the tach signal and/or EDS codes before I replaced it.

Floobydust: a contemporary term derived from the archaic Latin miscellaneous, whose disputed history probably springs from Greek origins (influenced, of course by Egyptian linguists) - meaning here "a mixed bag." - National Semiconductor Audio Handbook
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post #19 of 27 (permalink) Old 09-01-2015, 08:11 PM
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after all, EDS need tach signal also. Without signal engine could confused and - running bed.....
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post #20 of 27 (permalink) Old 09-01-2015, 08:11 PM Thread Starter
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Yes I would love a drawing! Thank you!

@Floobydust ...I am still lost on exactly how I would test the tach signal, I am not familiar with the tools you mentioned; sorry
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