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‘95 S600 Sedan 98K miles / ‘95 S420 Sedan 107K miles (sold) / ‘95 E320 Sedan (sold) -- 157K miles
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Discussion Starter #1
Hey all -- short summary of my issue is that I went to install an A/M head unit in my '95 S600 recently and could not get the head unit to power on. This car was new to me last Summer and this is the first time I've tried using anything other than the factory cassette head unit. I seem to have found the culprit is that I am not getting 12V to the "ignition" pin of the factory stereo wiring harness. While the factory head unit has worked fine to date, odd behavior I had noticed in getting to know the car is that the factory head unit will not turn off on it's own once the car is shut down, even after the key is removed. It must be physically switched off by pressing the volume knob each time. Random as well, but the heated seats only seem to work intermittently, and the owners manual does call out that they will be automatically shut off if draw on the electrical system is overly taxed.

I used a multimeter to confirm I am not getting 12V to pin #7 "Ignition" (black/gray/red) from the factory radio harness attached below, even when the key is in and either in the ACC or ON positions. I am getting 12V to pin #4 "Constant 12V" (red/yellow) at all times. To test my theory, I bridged power from the Constant 12V pin over to the Ignition pin, and the radio powers up and functions just as it should. Million dollar question: why no 12V to the Ignition pin?

The attached PDF below I grabbed from one of Chris' older posts (granted it is for a '98 S500, and maybe wiring changed slightly from that of my '95 S600) appears to show that power for pin #7 "Ignition" (black/gray/red) has dependencies on fuse #22 (tested with a multimeter and no issues) in the forward fuse box, as well as what are labelled as "combination relay module" and a "CTEL transmitter/receiver". The wiring diagram appears to be from Mitchell so the names may be vague for that reason. Any ideas which relay this might be referencing, where it is and how to test it? What about the involvement of the CTEL transmitter, assuming this is related to the car phone it came equipped with?

Thanks in advance for any help you can provide!!


Andrew Kloppel
Olympia, Washington, USA

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93 SL500, 95 SL320, 96 S320, 98 S500, 2002 E320 4Matic Wagon & A little 91 5.0 FORD Mustang
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Yea, no clue where this combination relay is. According to the MB troubleshooting manual their should be a 12v at pin 7 Factory MB schematics are not always accurate in many parts. I cannot confirm or deny which parts are wrong, but I have seen some sections that are dead-wrong in the past.

The other problem related to your old radio is the radio may just be needed to be rebuilt as the capacitors are just old, and are starting to leak or dry-up. I rebuilt one where it was flat-out dead for someone here. One of mine let out a mushroom-cloud one day, and quit working due to electrolytic capacitor ageing.

Next, I would never trust those Mitchell drawings. I would go with the true MB diagrams, and go through the MB troubleshooting guides too.





And here....


Soon I will do a DIY on radio Bluetooth mod with Cell phone capabilities, and media player for our old Becker radios.


Best of luck,

Martin
 

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If I am not mistaken the combination relay is "N10" according to MB legend and is located in the engine fuse box, rectangular unit right behind the fuses. Your problem is clearly not getting 12V at Pin 7 so your radio should be ok. Did you ever measure the voltage at the harness PIN 7? After you identified "N10" measure voltage at Pin 7 of N10. This Pin connects to Pin 7 of your radio. However, as N10 provides ignition 12V to various parts of the car I don't think this is where your problem is (if everything else works fine but the radio).
Good Luck!
 

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Discussion Starter #4
Yea, no clue where this combination relay is. According to the MB troubleshooting manual their should be a 12v at pin 7 Factory MB schematics are not always accurate in many parts. I cannot confirm or deny which parts are wrong, but I have seen some sections that are dead-wrong in the past.

The other problem related to your old radio is the radio may just be needed to be rebuilt as the capacitors are just old, and are starting to leak or dry-up. I rebuilt one where it was flat-out dead for someone here. One of mine let out a mushroom-cloud one day, and quit working due to electrolytic capacitor ageing.

Next, I would never trust those Mitchell drawings. I would go with the true MB diagrams, and go through the MB troubleshooting guides too.





And here....


Soon I will do a DIY on radio Bluetooth mod with Cell phone capabilities, and media player for our old Becker radios.


Best of luck,

Martin
Thanks Martin, sorry for my delayed response, just got around to checking out those resources you shared.

I see reference to the radio wiring harness I'm dealing with which has that same "switched battery power (circuit 15)" at pin #7 of harness A, and test 1.1 on page 23/1 seems to tell me that if I am not getting power then I should check "wiring, Fuse E in A2, A2" but Im not completely following how the fuse in the radio, or the radio itself have anything to do with the voltage that would be supplied by pin #7 in the wiring harness? Am I reading this wrong somehow? Sorry Im not familiar with these testing instructions.

To be clear, the stock radio works fine (it does oddly not turn off even when the car is turned off and key pulled out of the ignition completely) and I am getting power at pin #4 for constant 12V, however the goal I have is to install and A/M head unit which so far wont turn on because of there being no power at pin #7. I've tested providing pin #7 12V and the A/M head unit does come on.
 

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About your problem,

I'm working on audio stuff for these cars. I will go see if that voltage is present in my 98 S500 tomorrow as it could be a pulse trigger with no current caring capacity at all.

About the radio not turning off, it can be the switch on the volume control part, and I replaced several of those for that reason.

The deal with ALL Mercedes vehicles of 90's and above, and basically the radios are always "on", and when one turns it on it is "software" turn on much like your TV/Cell Phone/Desk Top Computer/Laptop Computer.

Their is always a current consumption of one-milliamp (0.001 A) or so of current, and this is operating the Micro Processor ( the brain), and is running in "Sleep Mode" awaiting a few commands to wake-it-up from sleep, or that low low current capacity momentary switch on the dial. The real reason is on these cars is the MB Star Diagnoses system we hook up to the car to read/diagnose/clear codes will wake-up all the modules(radio included), and send out any malfunction codes.

In my opinion, I think it genius for them to do that as any large company with a network of computers. They can set which computers get a software update without ever being there, and physically manually installing the update from desk to desk. The MB Star system wakes-up all the modules which is clever..

Martin
 

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Discussion Starter #6
About your problem,

I'm working on audio stuff for these cars. I will go see if that voltage is present in my 98 S500 tomorrow as it could be a pulse trigger with no current caring capacity at all.

About the radio not turning off, it can be the switch on the volume control part, and I replaced several of those for that reason.

The deal with ALL Mercedes vehicles of 90's and above, and basically the radios are always "on", and when one turns it on it is "software" turn on much like your TV/Cell Phone/Desk Top Computer/Laptop Computer.

Their is always a current consumption of one-milliamp (0.001 A) or so of current, and this is operating the Micro Processor ( the brain), and is running in "Sleep Mode" awaiting a few commands to wake-it-up from sleep, or that low low current capacity momentary switch on the dial. The real reason is on these cars is the MB Star Diagnoses system we hook up to the car to read/diagnose/clear codes will wake-up all the modules(radio included), and send out any malfunction codes.

In my opinion, I think it genius for them to do that as any large company with a network of computers. They can set which computers get a software update without ever being there, and physically manually installing the update from desk to desk. The MB Star system wakes-up all the modules which is clever..

Martin

Thanks for offering to check yours out! I will be interested to hear if you are getting 12V at pin #7 or if it could be the pulse of 12V as you mention. The A/M head unit I am using is looking for a constant 12V coming from that Ignition pin #7 and wont turn on without it. Whats odd to me is that the same head unit, with the same wiring harness adapter was previously installed in another '95 S420 I had and it worked just as I feel it supposed to (with 12V flowing to pin #7 upon ignition to ACC or ON).

If I cant get to the bottom of why I am not getting a consistent 12V at pin #7 I may have to rig something up from an alternate nearby power source to get the extra 12V I need, though that's my last resort.

As an aside, I also have what seem to be intermittent issues with my heated seats so began wondering if my power issue could be related somehow to those -- that thought led me to this thread (Heated seats and Radio are dead) where you can see I recently went pin by pin through the Combination relay wiring plugs and noted some discrepancies. As you pointed out though Martin, perhaps these guides are not always exactly perfect for every car even though they claim to be for all up to a certain production date.

Appreciate any more thoughts you might have, especially around that pesky fuse 22 which seems to have something to do with both the radio and heated seats according to the fuse assignment card in the fuse box...


Andrew
 

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Discussion Starter #7
If I am not mistaken the combination relay is "N10" according to MB legend and is located in the engine fuse box, rectangular unit right behind the fuses. Your problem is clearly not getting 12V at Pin 7 so your radio should be ok. Did you ever measure the voltage at the harness PIN 7? After you identified "N10" measure voltage at Pin 7 of N10. This Pin connects to Pin 7 of your radio. However, as N10 provides ignition 12V to various parts of the car I don't think this is where your problem is (if everything else works fine but the radio).
Good Luck!
Thanks for the note Stephan, how would you recommend measuring voltage on pin #7 and where exactly are you meaning?

I have measured pin #7 in the radio wiring harness, but that is where I am currently not getting voltage even with the key turned to ACC or ON. There are multiple pin #7's in the Combo relay as there are 3 total wiring plugs; 2 of those 3 contain a pin #7:

--In the medium 8 pin plug, #7 refers to "F2 maxi fuse box pin 11" -- this was actually one of the wires that did not line up correctly with the WIS diagram that is linked in the thread I just wrote to Martin above on linked here (Heated seats and Radio are dead)

--In the larger 21 pin plug, #7 refers to "Terminal block (X4/17) terminal 15C" - this wire happens to also be black/gray/red which is the same color as pin #7 in the radio harness, and is correct in my harness, is this the one you mean? Any ideas what the X4/17 and 15C refer to the in the items noted above in the WIS pinout?

If the relay is providing the power to the wires at these pins, I'd almost really need to be measuring voltage coming off the relay pins, rather than the wires themselves.. variables in my mind are power coming into the correlating fuse (which is 22 according to the PDF), the combo relay and the cellular receiver. Im going to go see whether turning the cell receiver on does anything to change voltage at pin #7 in the radio harness....



Andrew
 

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Discussion Starter #8
So quick update, I’m beginning to suspect my N10 relay. The Mitchell diagram I posted in the PDF above is spot on. I followed power through fuse 22, into pin 14 of the 21 pin N10 connector, which should then bring power to pin #7 which is the same pin #7 as the radio harness. To test this I jumped pins 14 and 7 on the 21 pin connector, and voila I have 12V at pin 7 on the radio harness.

I measured for continuity between poles14 and 7 on the relay itself, and I am not getting continuity. Is that a clear indication that the relay between these points has failed? Or is the nature of the relay that there could be another pin, which when combined with the input 12V from pin 14 translates power to pin 7? Searching the forum as we speak for diag instructions to test the N10 relay function short of taking it apart.
 

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Discussion Starter #9
And did I also mention that I had intermittent wiper setting issues, in addition to my lake of radio and heated seat power? All these three things seem in some way to interact with the N10 relay. I think I’m getting closer..
 

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As I stated in my previous mail I do not believe your problem is the radio but the lack of voltage at Pin 7. As among other things wiper and seat heater are supplied from the same pin of N10 I am pretty sure N10 is your problem. However, if the radio does not work at all then wipers and seat heater should not work as well. So I am not sure what "intermittent wiper setting issues" means.

As an outside chance the problem could be the trigger source for N10 to provide voltage to PIN 7, that would require further digging into N10 as a whole.
 

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Discussion Starter #11
As I stated in my previous mail I do not believe your problem is the radio but the lack of voltage at Pin 7. As among other things wiper and seat heater are supplied from the same pin of N10 I am pretty sure N10 is your problem. However, if the radio does not work at all then wipers and seat heater should not work as well. So I am not sure what "intermittent wiper setting issues" means.

As an outside chance the problem could be the trigger source for N10 to provide voltage to PIN 7, that would require further digging into N10 as a whole.
Intermittent setting issues whereas the first click on the wiper stalk for intermittent setting does nothing to move the wiper, yet positions 2 and 3 for normal and high speeds work just fine. Heated seats are intermittent in that sometimes they work, and seemingly randomly other times the switch light doesn’t come on and they don’t warm.

And agreed on the relay likely being the culprit. I just don’t have enough knowledge on how they work to be able to bench test it.

See my other question about about testing between pins 14 and 7, should I have continuity between them? Or as you alluded to, is there some other trigger/input within the relay that closes that bridge between 14 and 7 that could be the culprit?

Im going to reach out to some 140 friends locally to see if they have a spare I might be able to pop in and test. If everything worked normally again with a different relay, seems that would be my solution.
 

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Sure - to test with a spare N10 would be best. What bothers me is that you apparently have intermittently working seat heaters and wipers but the radio never works. Is that correct or is it rather that when the seat heaters (or wipers) work then the radio works too?
To your question about Pins 7 and 14: I do not have the internal circuit of N10 so I do not know how the pins are connected but I would assume that between Pins 7 and 14 is a relay function. So with ignition on there should be continuity between the two pins. What triggers that relay function is the key question. Could be ignition switch...Perhaps other people with more knowledge than me can chip in.
 

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Discussion Starter #13
Sure - to test with a spare N10 would be best. What bothers me is that you apparently have intermittently working seat heaters and wipers but the radio never works. Is that correct or is it rather that when the seat heaters (or wipers) work then the radio works too?
To your question about Pins 7 and 14: I do not have the internal circuit of N10 so I do not know how the pins are connected but I would assume that between Pins 7 and 14 is a relay function. So with ignition on there should be continuity between the two pins. What triggers that relay function is the key question. Could be ignition switch...Perhaps other people with more knowledge than me can chip in.
Yeah sorry for any confusion, the stock radio has always worked somehow. It’s only the A/M head unit that I try to plug in that isn’t getting powered on due to lack of 12V from pin 7 in the radio harness.
I kind of wondered about ignition switch too, though when I did my test last night bridging 14 and 7 in the N10 harness, I got 12V at radio harness #7 once I turned the key to ACC and ON, and 12V even remained in the OFF position, but only until I pulled the key out. This behavior is what I recall from an old 95 S420 with stock radio as well. Music stays on even with the key in OFF position but turns off once you pull the key.
 

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Discussion Starter #14
So I opened the N10 relay (though I can’t figure out how to get the circuit board out and don’t want to break it), and seem to have found that this copper switch (?) is what creates continuity between pins 14 and 7 on the relay, and ultimately provides power to the #7 pin on my radio harness.

So are these internal copper switches what wear out/break in these relays? Everything else I can see appears intact/fine. I have to believe that some input that tells the relay to close the contact (ignition switch perhaps?) so that voltage can flow from pin 14 to pin 7. I even put the relay back in the car with it still open, and was able to get 12V at pin #7 of the radio harness when manually moving the spring loaded contact that’s pictured.

My problem either seems to be the relay itself, or some signal that the relay should be receiving to close the circuit, but isn’t.

Would appreciate any thoughts on whether this confirms the relay is bad or what that input is that is missing so that I can start tracking that down.

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So if you get voltage at Pin 7 when manually connecting the relay contacts then a) the relay is bad or b) the relay is not triggered. I never opened the combination relay so you need to find the two solder points for the relay solenoid. Use a Multimeter to check for voltage at these pins with ignition on. If you have voltage there then the relay is bad. If you do not have voltage then the relay does not receive the required signal. Did you check with the spare combination relay you mentioned earlier?
 

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Discussion Starter #16
So if you get voltage at Pin 7 when manually connecting the relay contacts then a) the relay is bad or b) the relay is not triggered. I never opened the combination relay so you need to find the two solder points for the relay solenoid. Use a Multimeter to check for voltage at these pins with ignition on. If you have voltage there then the relay is bad. If you do not have voltage then the relay does not receive the required signal. Did you check with the spare combination relay you mentioned earlier?
I realized the spare I have through this process is for MY96 and newer, so doesn’t fit my 1995. I didn’t realize the PN’s were different until I tried swapping them. There is a used one on eBay for $40 I’m considering just buying for the sake of having another one to test with a swap out.

I haven’t been able to figure out how to get the circuit board out of the box, but think I might be able to get my multimeter leads squeezed in there to check the solder points since at least that relay is at the front of the opening. Good call on checking for voltage at the relay! Will let you know how that goes.
 

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I saw another post of yours where you struggle with the wiring around the fuse box. If that is the case then there is a chance that the issues are connected i.e. the signal wire to N10 triggering the relay may be not connected.
 

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Discussion Starter #18
I saw another post of yours where you struggle with the wiring around the fuse box. If that is the case then there is a chance that the issues are connected i.e. the signal wire to N10 triggering the relay may be not connected.
Not sure which post you mean? I was able eventually to pull the fuse blocks up enough from their bases to see below them, I saw no loose wires or anything obviously out of sorts.

Fuse 22 is what provides voltage to pin 14 and using a multimeter I confirmed that voltage was present at the fuse block. I suppose though that another wire that is the trigger input could have come off, but I don’t know which other pin/fuse that could be. I measured several other pins for voltage, 1 other pin which appeared to be fed directly by a fused connection from the fuse block (pin 15 I’m pretty sure it was) was the only other one that seems to get voltage.

did you ever get a chance to test voltage in your own S500 to see that you are getting voltage at pin 7 in the radio harness?
 

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I did not measure recently but this is the ignition fed supply so you need that for the radio to operate. And your previous test shorting pins 14 and 7 confirms that. The next thing to do would be to see the N10 connector diagram to identify where the missing trigger signal is coming from (provided that the relay is not the culprit as mentioned in the prior post).
 

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Discussion Starter #20
I did not measure recently but this is the ignition fed supply so you need that for the radio to operate. And your previous test shorting pins 14 and 7 confirms that. The next thing to do would be to see the N10 connector diagram to identify where the missing trigger signal is coming from (provided that the relay is not the culprit as mentioned in the prior post).
The odd thing is that the stock radio does still operate. I haven’t tried testing for voltage at pin 7 with the stock radio connected. Would seem completely backward, but wonder if somehow the radio harness being plugged into the stock radio triggers 12V to pin 7, otherwise like you said, I’m really not sure how the radio is even working.

I was able to find this diagram from an old post of MAVA’s, though I’m not seeing how the N10 portion of the schematic gives me any detail on what the trigger could be, maybe you could help interpret it? I’ll attach once I can get on the computer.
 
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