We are having issues with rough idle in general on my son's 1991 500SL. While studying the Diagnostics section of the factory electrical manual I noticed that step 22 of the N3 Module diagnostics states "Right camshaft timing adjust signal output - Engine at idle, insert bridge from Pin 1 to Y49/2 Pin 1 (max 10 sec.) engine runs roughly". There is a similar instruction for the Left camshaft To me this means insert a jumper between 12 VDC source and Pin 1 at the magnet. To me, this implies that 12 VDC energizes the magnet and advances the cam. The reason for this question is that I am reading 12 VDC at the adjuster at all times. This is leading me to believe the rough idle is caused by the cam being advanced at all time, nut just the 2000/4700 RPM range. Can someone confirm that at idle I should be seeing 0 VDC. This is occurring on both intake cams.
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Voltage to camshaft adjusting magnets on M119.960 CIS-E engine
- Thread starter vcanepa
- Start date
For the later LH-SFI cars I believe the system provides +12V to the cam solenoid at all times. The ECU switches the second pin to ground to energize the solenoid. So, reading +12v would be normal.
I do not know if your older CIS-E system works the same way, but I suspect that it does.
To test the solenoid function: With the engine at operating temp and idle RPM, disconnect the cam solenoid from the engine wire harness. Apply +12v directly to the solenoid. The idle should get rough (or, rougher) when the solenoid is energized, and smooth out when not energized.
I do not know if your older CIS-E system works the same way, but I suspect that it does.
To test the solenoid function: With the engine at operating temp and idle RPM, disconnect the cam solenoid from the engine wire harness. Apply +12v directly to the solenoid. The idle should get rough (or, rougher) when the solenoid is energized, and smooth out when not energized.
As near as I can tell from the above referenced manual the LH cars are indeed different than the earlier cars. We suspected our issue might be caused by the ECU sending out a signal to advance the intake cams all the time, not just in the specified RPM range. However, to add some confusion, we tried disconnecting the leads to the solenoids, but there was no difference in the behavior of the engine.
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