Can anyone work this out ? MAP sensor fault code

[clk2000]

Hello everyone,
After much tinkering and replacement of the main ECU on my 2000 CLK 230 to the correct one for the car, I have one of the original error codes coming up - P0108.
I have disconnected both the MAP sensors and done voltage checks and I find that I have a 4.5V positive on the earth wire static with no sensors connected ? This rules out the actual sensors themselves and I know that the same error isn't on both ECU's by reading the codes on the replacement ECU at time of fitment, with no such error code.
Logic tells me that there must be another 5V sensor somewhere within the engine circuit that is short circuited and sending the voltage back through the line to the MAP sensor earth. Can anyone shed any light as to where or what this could be ? .. I have unplugged the MAF sensor and it makes no difference.
I await your responses good people and any input appreciated.
Kind Regards

 

[ajlsl600]

Pinched wires seems most likely to me?

 

[clk2000]

Hello ajls,
You have a valid possibility, but for one small thing. The earth is supplying positive, rather than any break of either polarity. Just to make the issue more interesting, Earth has continuity at the same time as being live, so what I have is that the MAP sensors are receiving both positive and negative supply at the same time down the earth wire, without a break of one polarity or the other.
To my way of understanding of DC voltage, it's either one or the other, positive or negative, but this shows continuity of negative to the engine block and also on a voltmeter it's showing 4.5v DC positive .. very strange. This makes me think that it's some other sensor or unit that is shorted and sending a positive down through the negative line.
If I knew what else is on the 5v line, I could isolate each one in turn until the positive disappears, then I know what is at fault, if that makes sense.
My last option would be to cut the earth wires to the sensors and make them separate lines by link wires, but I would rather get to the bottom of this, as I don't know what other sensors are receiving the same issue. Cure rather than remedy.
Kind Regards,
Paul

 

[Oldspanners]

Hello ajls,
You have a valid possibility, but for one small thing. The earth is supplying positive, rather than any break of either polarity. Just to make the issue more interesting, Earth has continuity at the same time as being live, so what I have is that the MAP sensors are receiving both positive and negative supply at the same time down the earth wire, without a break of one polarity or the other.
To my way of understanding of DC voltage, it's either one or the other, positive or negative, but this shows continuity of negative to the engine block and also on a voltmeter it's showing 4.5v DC positive .. very strange. This makes me think that it's some other sensor or unit that is shorted and sending a positive down through the negative line.
If I knew what else is on the 5v line, I could isolate each one in turn until the positive disappears, then I know what is at fault, if that makes sense.
My last option would be to cut the earth wires to the sensors and make them separate lines by link wires, but I would rather get to the bottom of this, as I don't know what other sensors are receiving the same issue. Cure rather than remedy.
Kind Regards,
Paul

Don't just check with a volt meter, check it under load with a bulb or a power probe you may have a high resistance which will allow a voltage reading but not current to flow.
P.S. You'll need a wiring diagram to show the routing and what's connected to the 5V reference. Hopefully someone here with access to them can help otherwise you are only going to end up more confused and with a lot of wires to repair.

 

[clk2000]

Hello Oldspanners,
I understand what you mean in regards checking with something like a bulb, but the issue I'm having which made me check is my EML is on and even after reset, comes back immediately I turn the ignition off and on again. I'm not at my diag computer right now to give the full wording, but it says "High pressure sensor short/ open circuit to positive" which tallies with the readings I am getting. I know it's not actually the sensors as they have been substituted for known good ones and still the fault code remains. This is the only code on the ME system, so I'm guessing whatever is causing this issue is on the 5v rail, but not actually monitored by the ECU for fault codes for some reason.
I agree, to get hold of the 5v circuit diagram is the way to go, but finding one for the W208 230 is a complete nightmare.

 

[clk2000]

View media item 3425

Ok everyone, I have found a wiring diagram for this W208

It seems that the 5v rail supplies 3 units. That is the Pressure sensor (MAP), the Altitude Sensor (which I take to be the sensor for the secondary air supply - same part no on the vacuum pipe) and the Recirculated air flap actuator (I assume the MAF ?).
They all share the same 5v positive supply, but it seems that the negative supply for the circuit is independent for each sensor, supplied via the ECU.
At least at this point, I can narrow the issues to 3 units and associated wiring. I was half expecting the 5v supply to take in just about every sensor up to and including the Lambda sensors ... phew!!
I will attach a screenshot of the circuit for your perusal

 

[Oldspanners]

View media item 3425

Ok everyone, I have found a wiring diagram for this W208

It seems that the 5v rail supplies 3 units. That is the Pressure sensor (MAP), the Altitude Sensor (which I take to be the sensor for the secondary air supply - same part no on the vacuum pipe) and the Recirculated air flap actuator (I assume the MAF ?).
They all share the same 5v positive supply, but it seems that the negative supply for the circuit is independent for each sensor, supplied via the ECU.
At least at this point, I can narrow the issues to 3 units and associated wiring. I was half expecting the 5v supply to take in just about every sensor up to and including the Lambda sensors ... phew!!
I will attach a screenshot of the circuit for your perusal

When the circuit that reports the faults is down you won't get any more information than it's not functioning. The common point is the ECU, check the connector for corrosion and if none try back pinning at testing the ECU o/p with and without the connector.
Sorry can't read the diagram as it's too small on my computor, I'll try with my reading glasses

 

[mattseabrook]

an interesting defect.

what I have read above, you state you have -5VDC on the ground lines from sensors B28 and B18 (using your diagram above)

With M16/7, B28 and B18 disconnected, and the ECU unplugged, what do you have on the B28 and B18 ground lines ?

With M16/7, B28 and B18 disconnected, and the ECU unplugged, do you have any continuity between the wires that feed B28 and B18?

With the ECU plug to B28 and B18 disconnected, are you measuring a ground now on ECU pins 49 & 18?

 

[clk2000]

ok .. now I am totally confused.
I thought I would run the carsoft on the ME system after disconnecting and let the computer tell me the differences. I disconnected the MAF and the error I got was exactly the same as the MAP sensor (short to positive). I guess this could mean that the MAP sensor is not connecting rather than a short.
The errors I got when I disconnected the ECU had a "Pressure sensor" and the "High Pressure Sensor" which is the one that is causing issues. Now I'm able to access the diag PC, I can give the full message.
P2004-001 High pressure sensor Signal - Short circuit to plus/open circuit (P0108)

So, is the High pressure sensor the Altitude Sensor or the MAP sensor ? .. because if it is, I have been trying to sort out the wrong sensor.

Many thanks for your input Matt. On your observation concerning the -5vDC on the ground lines, I wish that was the case, but it's actually +4.5v. I can confirm in your question 1, there is 0v, but unfortunately rain has stopped play for now (the car is outside) and I cant find anything fine enough to get into the MAP sensor plug .. paper clips don't cut it

 

[Oldspanners]

ok .. now I am totally confused.
I thought I would run the carsoft on the ME system after disconnecting and let the computer tell me the differences. I disconnected the MAF and the error I got was exactly the same as the MAP sensor (short to positive). I guess this could mean that the MAP sensor is not connecting rather than a short.
The errors I got when I disconnected the ECU had a "Pressure sensor" and the "High Pressure Sensor" which is the one that is causing issues. Now I'm able to access the diag PC, I can give the full message.
P2004-001 High pressure sensor Signal - Short circuit to plus/open circuit (P0108)

So, is the High pressure sensor the Altitude Sensor or the MAP sensor ? .. because if it is, I have been trying to sort out the wrong sensor.

Many thanks for your input Matt. On your observation concerning the -5vDC on the ground lines, I wish that was the case, but it's actually +4.5v. I can confirm in your question 1, there is 0v, but unfortunately rain has stopped play for now (the car is outside) and I cant find anything fine enough to get into the MAP sensor plug .. paper clips don't cut it

Having put my best reading glasses on I still can't make out all the wiring diagram details but from your description of the fault I'd be looking at the wire from where the two sensors join the air flap actuator (Z7/Q7?) teeing point back to the ECU as if this is broken you are getting a 4.5v feed from the air flap actuator and not an earth.

 

[clk2000]

View media item 3426
I appreciate your time on this oldspanners, I really do.
From the wiring diagram (which I have enlarged for you) .. the teeing point you reference is indeed the positive supply for the 4.5v feed to the MAP (B28) and Altitude sensor (B18). The issue seems to be on the negative side either from the sensors themselves (ruled out by substitution) the MAF (again, ruled out by substitution), the wiring ( that seems intact) or heaven forbid .. the ECU which has not had this problem stored until it was fitted to this car. Even the replacement sensors came from the same car as the ECU, without fault.

 

[Oldspanners]

View media item 3426
I appreciate your time on this oldspanners, I really do.
From the wiring diagram (which I have enlarged for you) .. the teeing point you reference is indeed the positive supply for the 4.5v feed to the MAP (B28) and Altitude sensor (B18). The issue seems to be on the negative side either from the sensors themselves (ruled out by substitution) the MAF (again, ruled out by substitution), the wiring ( that seems intact) or heaven forbid .. the ECU which has not had this problem stored until it was fitted to this car. Even the replacement sensors came from the same car as the ECU, without fault.

Check the o/p from the ECU at the pins indicated on the diagram and compare with the other ECU in case you have a compatibility problem with the ECU ie not quite the right one.

 

[clk2000]

This is where I really start considering scrapping this car ..
Based on my knowledge and experience, I thought it was high time I applied some logic to this situation.
Firstly, in answer to your point oldspanners, I have checked the EPC and indeed, the ECU as replaced is the same one it left the factory with, although, the one supplied with the car at time of purchase was incorrect and actually for a 2002 C230. The one I obtained was from a CLK 2001 cabrio, which is the same as my facelift from 2000 to 2002.
Back to my tests so far ..
I have 5v on the supply to the altitude sensor and 0.89v on the output without the car running, so this seems correct. I have continuity from the 5v positive to map sensor from the altitude sensor (hence, it all seems in order on the positive supply).
The negative on the MAP sensor reads -5v when taken from the terminals, so again, based on this, no issues on supply to MAP sensor. The only thing I can't check is the output voltage of the MAP sensor as I cant get to the plug(as it is right behind pipework on the vacuum line for the brake servo), but I have checked by substitution of the MAP sensor with no difference.
This leads me to believe that the ECU is throwing a false error for some reason, and yet, it had never thrown this before fitment to this car, based on the codes read at time of replacement, which only showed a misfire from cylinder 1 on the old car that the ECU came from.

This is absolutely insane GRRRR

Oh .. I should add that M16/7 on the diagram is not as I thought, the MAF, but is actually the Recirculating Air Flap on the top of the Air Filter housing, based upon research. Again, this seems operational and without issues.

 

[Oldspanners]

P2004-001 B18 (high pressure sensor) signal,positive/wire open cause short [P0108]
P0108 Manifold Absolute Pressure/Barometric Pressure Circuit High Input
If I understand you correctly. The original ECU doesn't report this fault but the new one does. So try back pinning the connectors 18, 30, 31 at the ECU and noting the values with the ignition off and on. Do this with both ECU's and see if there is a difference.

 

[clk2000]

The original ECU did record this fault oldspanners, which tells me there is an issue with this car itself. Backpinning the ECU as fitted is fine, but as this has the type which locks to the ignition barrel and steering lock, means I can't turn on the ignition with the wrong ECU fitted. The key will not recognise unless I swap out all 3 units.
Much as the same fault is recorded, my thinking is surely, if the sensors are receiving the correct voltage and polarities, then why is the ECU and EML saying short or open circuit ? .. that is the frustrating part. My tests state that the ECU is live and sending voltage to the sensors.
The scenario is .. Fault comes up and EML comes on. Reset the fault codes, start car, no EML. Turn car off, restart and EML comes back immediately. If I could be sure the fueling is right, I would just remove the EML bulb and be done with it, which incidentally, the previous owner had done.

 

[Oldspanners]

The original ECU did record this fault oldspanners, which tells me there is an issue with this car itself. Backpinning the ECU as fitted is fine, but as this has the type which locks to the ignition barrel and steering lock, means I can't turn on the ignition with the wrong ECU fitted. The key will not recognise unless I swap out all 3 units.
Much as the same fault is recorded, my thinking is surely, if the sensors are receiving the correct voltage and polarities, then why is the ECU and EML saying short or open circuit ? .. that is the frustrating part. My tests state that the ECU is live and sending voltage to the sensors.
The scenario is .. Fault comes up and EML comes on. Reset the fault codes, start car, no EML. Turn car off, restart and EML comes back immediately. If I could be sure the fueling is right, I would just remove the EML bulb and be done with it, which incidentally, the previous owner had done.

That the previous owner went to the length of removing the EML he may well have done a mod to the wiring as well. Worth a look around.
More interesting is the fault only comes back after a restart. If this is when the car is actually restarted and not just with the ignition on it may point towards a voltage problem to that circuit. Check battery and alternator voltage and a good "wiggle test" on the associated wiring.
Don't give up if it's running without fault when cleared and started we are not looking in the right place

 

[clk2000]

I have got to say oldspanners, I love your determination .. very inspiring

Ok, where we are at with this. The battery is about 8 days old (I had to replace it as it died and wouldn't accept a charge after the car had sat for a year out of disgust). The gear selector also died, (but that's a different story haha). The alternator is pushing 14.3v running.
I'm not sure if you are with me on this one, just a thought.

Maybe it's time to revisit the recirculating air flap ? .. it's the only other thing on the circuit. I'm beginning to wonder if there is a momentary short either within it, or maybe there is a momentary break in the 5v supply wiring when starting. I have checked the sensor wiring extensively when pulling the connectors around during testing and also the ECU wiring. I can see nothing left in line only the 1 thing I haven't checked and that is the recirculating air flap. Maybe this needs "relearning" with the change in ECU ? or possibly something else is going on there. I haven't thought to check this before as no error codes are coming up for it.

 

[Oldspanners]

I have got to say oldspanners, I love your determination .. very inspiring

Ok, where we are at with this. The battery is about 8 days old (I had to replace it as it died and wouldn't accept a charge after the car had sat for a year out of disgust). The gear selector also died, (but that's a different story haha). The alternator is pushing 14.3v running.
I'm not sure if you are with me on this one, just a thought.

Maybe it's time to revisit the recirculating air flap ? .. it's the only other thing on the circuit. I'm beginning to wonder if there is a momentary short either within it, or maybe there is a momentary break in the 5v supply wiring when starting. I have checked the sensor wiring extensively when pulling the connectors around during testing and also the ECU wiring. I can see nothing left in line only the 1 thing I haven't checked and that is the recirculating air flap. Maybe this needs "relearning" with the change in ECU ? or possibly something else is going on there. I haven't thought to check this before as no error codes are coming up for it.

Yes, I'd go with that as if you have a spurious voltage on that 5v reference common to all 3 components on start up it could cause the problem. Again that teeing point Z7/27 back to pin 30 in the ECU. I think the teeing points are usually hidden in the harness just to make it easy

 

[Flyinspanner]

The fault codes you have - are they from a generic code reader?

it may be worth getting faults checked on STAR? It may be better interrogating the faults to a deeper level.

Disassemble clean & check the Earth points on all common components in that ‘circuit’.

 

[clk2000]

Hello Flyinspanner, welcome to the discussion.
The fault codes I am getting is from Carsoft 7.4. This seems to be able to diagnose all faults on this car and the only thing it doesn't do is codings. I have spoke to MercMedic here in Surrey previously and they basically tell me the sky is the limit when it comes to STAR Diagnostic, depending on what they find. It could be an hour, it could be 10 dependent upon the scenario. As this car is getting quite old and in the last 500 miles or so, this car has had over £700 spent on it and is more a battle of wills at this point, it is not cost effective to go full on with possibly 10 hours of chargeable diagnostics.

As can be seen from the wiring diagram, the circuits rely on 5v positive supplied in parallel with the earths for the sensors coming straight from the ECU. These have all been checked and I have even used an earth from the engine block to bypass the earths to each sensor, just in case maybe there was no earth supply from the ECU without difference.

I'm now thinking maybe there is a momentary issue with the recirculating air flap that is possibly triggering or interupting the supply of positive to the sensors, as there seems to be no "permanent" issue so to speak. My guess at this point is that something happens which is momentary enough to not interrupt the circuit long term, but just enough to trigger a fault in the ECU.