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Random voltage dip when idling when hot (Only happens once)
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 that there are no stored codes on any modules and that the batteries were replaced 6 months ago.
The issue I'm having is that, once warmed up after about 10-15 minutes of driving, when I stop for a prolonged period of time (>5 seconds) the car will idle perfectly for 4-5 seconds but then there is a bit of a shake/wobble. When looking at the voltage I can see it drops from about 14.1 to 11.8 immediately when this happens before quickly recovering. The weird this is this only happens once. I can happily pull away to the next stop light and it won't do it again for the remainder of the journey. It doesn't even do it on the return journey, the only time it will happen again is the following day.
Below are some data logs. The columns are (Time, Voltage, RPM, Fuel Pressure, Throttle position, and Manifold pressure). As you can see manifold pressure, fuel pressure and throttle position are all consistent so I don't think it is a fuelling/vacuum/throttle-body issue.
Although the Secondary Air Injection pump normally comes on at cold startup, it can cause a similar symptom when turning on, and sometimes it will come on after the vehicle is warm (for some reason).
Although the Secondary Air Injection pump normally comes on at cold startup, it can cause a similar symptom when turning on, and sometimes it will come on after the vehicle is warm (for some reason).
Thanks, that should be fairly easy to monitor tomorrow morning. If there is an issue when the secondary air pump switches on then, does that sound like an issue with the pump or the alternator failing to supply the electricity needed by the pump?
Thanks, that should be fairly easy to monitor tomorrow morning. If there is an issue when the secondary air pump switches on then, does that sound like an issue with the pump or the alternator failing to supply the electricity needed by the pump?
I would not think so. When it comes on, the startup of the motor draws a lot of current initially and it then levels out.
Since it quickly recovers, that is what it sounds like. Normally I would not think it would drop into the 11V range, but with the A/C on and blower, radiator fan, etc., I could see how it could. I have an S600, which didn't have a voltage readout stock. I flashed the cluster to get AMG gauges as I wanted to monitor the oil temp and voltage as well, but haven't had it that way long enough (or drive the car enough) to see what mine does when the SAI pump comes on.
*and I never think to enter Service Mode on the cluster before stating the car it seems to monitor voltage there!
Thanks. I think it is called M33 Electric Air Pump on my diagnostic tool so I will keep an eye on its status when this happens and report back tomorrow. Many thanks.
Depending on the tool you are using, you may be able to manually actuate the pump on demand and simulate the environment in which it does it (i.e. A/C on, A/C fan on what setting it is typically on when it occurs, etc.) in your driveway.
It's an Icarsoft MB V2.0. I'm not sure it allows me to manually actuate anything unfortunately. However, I can at least view the current status for various things (the air pump being one) so I can at least see if there is a link. While stationary I have actuated things manually such as AC (which will spin-up the radiator fan) but this had virtually no impact on the RPM and only caused the voltage to drop slightly (from 14.1 to about 13.5/13.6) before it quickly returned to 14.1.
Make sure it is in Drive, and any accessories you have normally running are on. The SAI pump should come on by design about 45 seconds after a cold start, but during a cold start the RPMs are elevated anyway, so you could not determine much from that.
The heated seats and rear window defogger will put the largest load on the system.
Make sure it is in Drive, and any accessories you have normally running are on. The SAI pump should come on by design about 45 seconds after a cold start, but during a cold start the RPMs are elevated anyway, so you could not determine much from that.
The heated seats and rear window defogger will put the largest load on the system.
Well when you said "Sometimes the secondary air injection pump kicks in when warm (for some reason)" it turns out I need to figure out what that reason is.
The problem didn't happen yesterday, I started the car for 60 seconds or so, then had to switch off because I left something In needed for work, switched back on and carried on my journey. No problems at all.
However, this morning it did happen again and I had my diagnostics with the M33 pump data up. The moment the car "stumbles" the air pump is actuated (for less than a second). I don't know if the immediate drop in voltage is detected and it switches off or what but I need to figure out why the pump is trying to kick in when the car is warmed up. First stop will be the vacuum tubes on the valve cover which goes to the two valves (that the air pump routes its air to) as I have read that these vacuum tubes are often culpable. I have pre-emptively ordered some replacement tubing from Mercedes because even if they aren't leaking I'd like to renew them. Another possibility is the swichover valve that lives under the intercooler but this is throwing no errors. Does anyone know what state the valve should be in and when. On my way in this morning it was "open" for most of the journey. I don't know if that means that the pressure in the tubing is atmospheric (and closed means it pulls vacuum) or vice versa.
Well when you said "Sometimes the secondary air injection pump kicks in when warm (for some reason)" it turns out I need to figure out what that reason is.
The problem didn't happen yesterday, I started the car for 60 seconds or so, then had to switch off because I left something In needed for work, switched back on and carried on my journey. No problems at all.
However, this morning it did happen again and I had my diagnostics with the M33 pump data up. The moment the car "stumbles" the air pump is actuated (for less than a second). I don't know if the immediate drop in voltage is detected and it switches off or what but I need to figure out why the pump is trying to kick in when the car is warmed up. First stop will be the vacuum tubes on the valve cover which goes to the two valves (that the air pump routes its air to) as I have read that these vacuum tubes are often culpable. I have pre-emptively ordered some replacement tubing from Mercedes because even if they aren't leaking I'd like to renew them. Another possibility is the swichover valve that lives under the intercooler but this is throwing no errors. Does anyone know what state the valve should be in and when. On my way in this morning it was "open" for most of the journey. I don't know if that means that the pressure in the tubing is atmospheric (and closed means it pulls vacuum) or vice versa.
Thank you Polar Bear for getting me this far.
This behavior is certainly not unique to M275 vehicles - it does it on M112, M113, etc. I am no exactly sure the cause.
This behavior is certainly not unique to M275 vehicles - it does it on M112, M113, etc. I am no exactly sure the cause.
Some closure for anyone who finds this thread after the fact.
So I finally managed to track down some information on this. It seems as part of the OBD2 I/M Readiness tests this is one for secondary air injection which involves this. I have had details of the procedure for running the tests and as I understand it if these tests cannot be completed they will retry on every engine start.
I did not want to wait for these tests to complete so I disabled the secondary air diagnostics on DAS and it no longer attempts to run - and therefore no more rpm dips
Some closure for anyone who finds this thread after the fact.
So I finally managed to track down some information on this. It seems as part of the OBD2 I/M Readiness tests this is one for secondary air injection which involves this. I have had details of the procedure for running the tests and as I understand it if these tests cannot be completed they will retry on every engine start.
I did not want to wait for these tests to complete so I disabled the secondary air diagnostics on DAS and it no longer attempts to run - and therefore no more rpm dips
Thank you all for your help.
I actually correctly diagnosed something!
Now how in the world do you turn off the SAI?
I was going to mention that you could maybe do that as a test - but decided not to on a public forum
Glad you got it sorted. Those things pull quite the load when spinning up!
I think it is possible to disable the SAI in someway but haven't looked into completely disabling it.
I'm happy to have just switched off the diagnostic 'self-test' function. I think if there is a fault in the system that gets detected on cold start it will still throw up error codes and the like.
I think it is possible to disable the SAI in someway but haven't looked into completely disabling it.
I'm happy to have just switched off the diagnostic 'self-test' function. I think if there is a fault in the system that gets detected on cold start it will still throw up error codes and the like.
It is indeed pretty trivial to disable SAI, EGR, rear O2s, etc., as much of that stuff is not required in other countries, therefore they ECU can be adapted for its destination country. The self-test function, if runs and fails, will kill a Readiness Monitor for OBD2 inspection counties/states, and depending on the equipment the state uses if your reside in a state/county that does emission inspections, it may appear to the inspection computer as "not applicable" to that vehicle, or "failed." In my state, it ignores it if it is turned off and does not cause a fail in emission inspection.
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