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Mercedes Benz SLK (FIXED)
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Technician
- TwoWaves
- Posts: 707
- Joined: Thu Apr 24, 2014 8:32 pm
Mercedes Benz SLK (FIXED)
I towed this car into the workshop as a none starter/runner. As it is a 1997/8 model petrol 4 cylinder the diagnostics are either blink codes or the old scanners of age. Using a solus pro with 14.4 software I read out any fault codes and only one was present, which referred to the ABS system. I can see underneath the car the wiring is corroded and damaged to the nearside rear ABS sensor so I'm not concerned with that at the moment.
My boss spoke to an auto-electrician over the phone who advised him that these SLK's suffer from the main relay control unit failing. The auto-electrician advised me that he had changed loads of them when I rang him back to advise that I could not see anything wrong with it physically, i.e. burned or corroded.
£200 later the car is still the same. I did manage to get the car to start and run but it cuts out when warm. Another code read and the scanner now shows a fault code for the camshaft position sensor circuit.
Today I put the scope on the sensor terminals and got the attached live reading. The sensor is a Hall Effect so I'd expect a good square wave pattern, however I'm not seeing one. Although I'm confident that the camshaft sensor is poor in operation, even when the engine will not start, the camshaft sensor is still trying to trigger, which suggests to me that it is not the primary cause of the engine not starting.
I'm thinking a fuel volume flow test before the engine cuts out and one afterwards to see if a fuel problem like a partially blocked filter could the be cause?
Anyone got any experience with these Merc's?
My boss spoke to an auto-electrician over the phone who advised him that these SLK's suffer from the main relay control unit failing. The auto-electrician advised me that he had changed loads of them when I rang him back to advise that I could not see anything wrong with it physically, i.e. burned or corroded.
£200 later the car is still the same. I did manage to get the car to start and run but it cuts out when warm. Another code read and the scanner now shows a fault code for the camshaft position sensor circuit.
Today I put the scope on the sensor terminals and got the attached live reading. The sensor is a Hall Effect so I'd expect a good square wave pattern, however I'm not seeing one. Although I'm confident that the camshaft sensor is poor in operation, even when the engine will not start, the camshaft sensor is still trying to trigger, which suggests to me that it is not the primary cause of the engine not starting.
I'm thinking a fuel volume flow test before the engine cuts out and one afterwards to see if a fuel problem like a partially blocked filter could the be cause?
Anyone got any experience with these Merc's?
- Attachments
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- Mercedes-Benz_SLK-Class_1998_Petrol_Hall Effect camshaft sensor operation (poor).psdata
- (16.7 MiB) Downloaded 316 times
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- Mercedes-Benz_SLK-Class_1998_Petrol_camshaft sensor operation during cranking (poor).psdata
- (16.78 MiB) Downloaded 310 times
Last edited by Technician on Fri Jul 31, 2020 7:10 am, edited 1 time in total.
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Technician
- TwoWaves
- Posts: 707
- Joined: Thu Apr 24, 2014 8:32 pm
Re: Mercedes Benz SLK
Just an update to this car problem...
I've not got to the root cause of the fault at the moment. I have Mercedes-Benz diagnostic data for this car and this evening tested the operation of the cam and crank sensors, followed by the ignition and fuel pressure/volume flow rate.
I'll upload the wave forms tomorrow evening when I bring them home on the dongle. What I've noticed is that sometimes when I turn on the ignition switch the fuel pump does not initially prime, and therefore initial fuel pressure is not there, and the engine does not start during cranking. Today with the scope attached the engine started straight away so I was not in a position until the engine failed to start to scope any signatures from the engine sensors or ignition system. When the fuel pump primes the pressure builds to about 45 psi. The engine then started and ran. I then increased the engine speed to around 2800 rpm and maintained that speed until the engine was hot. I looked at the cam/crank signatures using different voltage and time base settings to try and establish the best scope traces. The cam sensor signature looks better when measured at the ECM terminals. Maybe I was picking up interference previously! The crank sensor operation is looking good on the scope. The engine does have a slight misfire that could previously be heard. I've now established that the misfire is caused by the crankshaft teeth, were the scope is showing a couple of teeth damaged, roughly around the TDC position.
When the engine stops running I did do some cranking tests using various voltages and time base settings to see what scope traces I could produce from the cam/crank and ignition. In practice the only sensor that produced anything was the cam sensor, but again the signature is poor. The fuel pressure never missed a beat maintaining around 45 psi.The evidence then shows that the crank sensor and ignition system shut down when the engine is hot and thus preventing the engine from starting. The crank sensor is an inductive type which has left me wondering why the voltage shuts off every time the engine gains normal temperature.
So my question becomes, if it were the crank sensor at fault, when the engine is running, should it not continue running on the cam sensor signature?
I've had experience with other makes like Vauxhall where the cam/crank sensors used to fail just for the fun of it, and we used to change both sensors together. I'm not sure about the exact make of vehicle with this next example, but seem to think it was a VW, the crank sensor failed on that one and ran on the cam sensor, but once the fault code was cleared, the engine would not start again until the crank sensor was replaced.
The Mercedes does not seen to behave the same!
I've not got to the root cause of the fault at the moment. I have Mercedes-Benz diagnostic data for this car and this evening tested the operation of the cam and crank sensors, followed by the ignition and fuel pressure/volume flow rate.
I'll upload the wave forms tomorrow evening when I bring them home on the dongle. What I've noticed is that sometimes when I turn on the ignition switch the fuel pump does not initially prime, and therefore initial fuel pressure is not there, and the engine does not start during cranking. Today with the scope attached the engine started straight away so I was not in a position until the engine failed to start to scope any signatures from the engine sensors or ignition system. When the fuel pump primes the pressure builds to about 45 psi. The engine then started and ran. I then increased the engine speed to around 2800 rpm and maintained that speed until the engine was hot. I looked at the cam/crank signatures using different voltage and time base settings to try and establish the best scope traces. The cam sensor signature looks better when measured at the ECM terminals. Maybe I was picking up interference previously! The crank sensor operation is looking good on the scope. The engine does have a slight misfire that could previously be heard. I've now established that the misfire is caused by the crankshaft teeth, were the scope is showing a couple of teeth damaged, roughly around the TDC position.
When the engine stops running I did do some cranking tests using various voltages and time base settings to see what scope traces I could produce from the cam/crank and ignition. In practice the only sensor that produced anything was the cam sensor, but again the signature is poor. The fuel pressure never missed a beat maintaining around 45 psi.The evidence then shows that the crank sensor and ignition system shut down when the engine is hot and thus preventing the engine from starting. The crank sensor is an inductive type which has left me wondering why the voltage shuts off every time the engine gains normal temperature.
So my question becomes, if it were the crank sensor at fault, when the engine is running, should it not continue running on the cam sensor signature?
I've had experience with other makes like Vauxhall where the cam/crank sensors used to fail just for the fun of it, and we used to change both sensors together. I'm not sure about the exact make of vehicle with this next example, but seem to think it was a VW, the crank sensor failed on that one and ran on the cam sensor, but once the fault code was cleared, the engine would not start again until the crank sensor was replaced.
The Mercedes does not seen to behave the same!
Re: Mercedes Benz SLK
I'm sure this will have a 38 pin diagnostic port under the bonneteither blink codes or the old scanners of age.
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Technician
- TwoWaves
- Posts: 707
- Joined: Thu Apr 24, 2014 8:32 pm
Re: Mercedes Benz SLK
Yes your correct it does have the 38 pin connector, which I connected the snap on solus pro into and pulled the code for the ABS fault. No other codes initially were recorded. The engine did start afterwards and during running I scanned the engine again and found another code appeared for the camshaft sensor circuit. No other codes are recorded.
I've not managed to get back to it at the moment but tomorrow is another day. I'm thinking of tracing the crankshaft sensor wiring through the harnesses back to the ECM, opening up the harness this time as I can't find a connector plug for the crank sensor, hence I'm thinking Mercedes have run the crankshaft sensor harness complete from the sensor and connected directly into the E block ECM connector.
So I'll splice into the wiring harness and run the engine until it stops again, and then disconnect the crank sensor from the ECM and while connected to the scope crank the engine to see if the crank sensor can create a voltage signature, and if so, then something is pulling the crank sensor down.
I've not managed to get back to it at the moment but tomorrow is another day. I'm thinking of tracing the crankshaft sensor wiring through the harnesses back to the ECM, opening up the harness this time as I can't find a connector plug for the crank sensor, hence I'm thinking Mercedes have run the crankshaft sensor harness complete from the sensor and connected directly into the E block ECM connector.
So I'll splice into the wiring harness and run the engine until it stops again, and then disconnect the crank sensor from the ECM and while connected to the scope crank the engine to see if the crank sensor can create a voltage signature, and if so, then something is pulling the crank sensor down.
-
Technician
- TwoWaves
- Posts: 707
- Joined: Thu Apr 24, 2014 8:32 pm
Re: Mercedes Benz SLK (FIXED)
Why did the crankshaft sensor circuit not show up as a fault? The crank sensor stopped producing a signature when the engine gained normal temperature. The parts suppliers advised today that there are two different types of crank sensor for this car. The one fitted has a top cap that unplugs from the sensor, which I can't remember seeing one like that before, anyway installed a new sensor given that previously the sensor would not produce a voltage when the engine was hot. Ran the engine which started straight away and it has kept running and not cut out at all. Conclusion, faulty crank sensor.