Random RPM dip? Voltage drop.

[pillow]

Hi all,

2007 CL600 M275 5.5

I'm having an issue which I am struggling to diagnose. First thing I should point out is there are no codes stored on any of the cars modules. Both batteries in the car are only 6 months old.

The issue is that, once warmed up, and coming to a stop the car will return to idle, after about 4-5 seconds of perfect idling the voltage suddenly drops to 11.8v before immediately recovering. This causes a noticeable change in the engine tone and sometimes a little shake. Once it happens everything seems to return to normal and it won't happen again on the journey or subsequent journey on a given day.

Attached are some data logs. The columns are (Time, Voltage, RPM, Fuel Pressure, Throttle Position, Manifold Pressure). As you can see the fuel pressure, throttle position and manifold pressure all remain stable so I don't think it is a fuelling issue, vacuum leak or throttle body issue.

Any assistance very much appreciated.

 

[ajlsl600]

iffy regulator ?

 

[Tony Dyson]

I think first of all you're witnessing the fluctuating charging algorithm set for your car and battery combination, possibly exacerbated by other current consumers switching on/off or even gear changes? If you monitor over time with no changes in demand on the engine or battery, you will see a pattern developing, this is normal.

 

[pillow]

Thank you both for the replies.

I am leaning towards swapping out the regulator or even to get the alternator bench tested to see if there' something wrong with it. The fact it only happens once is what puzzles me.

It almost appears to disengage and then re-engage straight away.

I'm not sure this is normal charging because of the sudden disconnection and the drop down to 11.8v which suggests everything is loaded onto the battery for that moment. This happens at idle where there is no discernible change in power requirements. I appreciate the power supply is variable and adjusts depending on varying factors such as battery charge level and load but this does not seem normal to me. If any S600 or CL600 owners want to chime in please do.

 

[ajlsl600]

Thank you both for the replies.

I am leaning towards swapping out the regulator or even to get the alternator bench tested to see if there' something wrong with it. The fact it only happens once is what puzzles me.

It almost appears to disengage and then re-engage straight away.

I'm not sure this is normal charging because of the sudden disconnection and the drop down to 11.8v which suggests everything is loaded onto the battery for that moment. This happens at idle where there is no discernible change in power requirements. I appreciate the power supply is variable and adjusts depending on varying factors such as battery charge level and load but this does not seem normal to me. If any S600 or CL600 owners want to chime in please do.

if you can have the alt bench run for a length of time ,try that, prior to expenditure, maybe the brushes are at the end of their tether as well ? at least its limited expenditure to rule that out...

 

[NBurns]

Need to have a look at your data on a desktop before I give any feedback.



Sent from my iPhone using Tapatalk

 

[pillow]

Some closure for anyone that finds this thread with similar symptoms at a later date.

The dip was caused by the secondary air injection pump starting temporarily as part of a self test. I believe the test is part of the OBD2 I/M readiness tests. If these tests fail to complete in a drive cycle they retry on every cold engine start.

I have managed to resolve this by disabling the "Secondary Air System Diagnostics" on DAS (Development mode). The secondary air function still happens on cold start but this randomly testing the system whilst idling happens no more.