Another Mack Question!

[KR69IDI]

OK guys, I've been getting great info here so I'm going to keep asking.

Brothers mack (04 AC400) for those that don't know, been running real good, but today he calls and tells me that it suddenly started low idling at 500rpm. Normally he said its always been at 650-700. The other strange thing is that he said the volt meter is showing around 13v when this happens. He said it reads 14 to 14.5 normally. Neither one sounds bad to me. Just wondering why these would be related. It goes right back up to 14-14.5v while running.


Anyone think that the alternator on the truck doesn't get "excited" at that low of an rpm? I've seen this on ag stuff before only reason I ask.

Been bustin' my head around this all day and I keep coming up blank

 

[Ak HDM]

The alternator should be very similar of the same as what is on Ag equipment.
As far as the low idle it has to be controlled by the engine computer. He might have to have someone talk to it.

 

[NJKen]

The alternator can not generate enough current (amperage) to keep up with the electrical demand so the voltage drops. Once you bring the RPM up this overcomes the problem. What you need to do is find out why the idle droped.
Ken

 

[KR69IDI]

I'm on track with both of you. Here's the next question. My brother is tight on funds right now due to the previous trans problem. I told him to just bump up the idle manually with the cruise switch for now to avoid the issue while idling. Anyone see anything wrong with this for a temp. fix to avoid the issue? BTW, he spends lots of time idling in lines waiting to dump grain.

Any ideas why the idle would drop? Think this may be related to a sensor failure and the ECM went to a "limp mode"?

I also told him to disconnect the battery's for a while and then reconnect and see if the ECM would reboot. Any chance this would work with trucks?

Thanks!!!!!!

 

[NJKen]

ECM reboot is a long shot but worth a try anyway. As long as the voltage stays above 13 there is not a problem. If it falls below 12 then you have a problem with any extended idiling.
If you know there is a sensor problem then you should get that corrected before you spend too much time looking for other problems.
Ken

 

[Gary79F250]

It seems with all of the problems that he is having with the truck that there is an intermitent loss of ground to the ECM, or a ground/hot with high resistance to the ECM. just my .02

 

[Ak HDM]

Keith, where has the truck spent most of its life? I'm thinking as Gary is that there are some ground/hot resistance issues, maybe corrosion at connections or where grounds are bolted to the frame.
Mike

 

[KR69IDI]

The truck was purchased from northern Illinois, but I don't know the history of it. I'll check the grounds and try some dialectric grease in the ECM connections and see what happens. Thanks