Help me diagnose this problem

[ColoDmax]

I have a 2003 3500 Dmax, Intake exhaust and Edge W/Attitude. When hauling my 30' 5th wheel coasting down steep grades with tow haul engaged at around 4000 rpm the battery light will come on, volts drop, tach stops in odd places or fluctuates rapidly. When reaching bottom of grade and give it some throttle trans upshifts and everything goes back to normal. Dealer can not find anything wrong. Any thoughts, thanks Steve

[jbplock]

.. at around 4000 rpm the battery light will come on, volts drop, tach stops in odd places or fluctuates rapidly... Any thoughts ..

Alternator failing at high RPMs .. serpentine belt slipping ??

[BlackMax 2001]

Isn't 4000 rpms a little over revved?

[DmaxMaverick]

Isn't 4000 rpms a little over revved?

No. When in T/H mode, the grade braking feature can increase engine speed up to about 4400 RPM's to maintain or decrease ground speed. This is on compression, with no fuel. There is no fuel present or power production above governed engine speed (3200 RPM's). This condition is the reason later models have no redline on the tach. Trust the PCM to do its job. It won't allow an engine overspeed.

This problem sounds like an alternator failure at that RPM. Once the PCM is unable to control the output voltage of the alternator, it shuts it down. The IPC fluctuations could be a result of system voltage fluctuations, or a protection action. The alternator pulley is approximately 1/5 the circumference of the crank pulley, therefore, the alternator spins at approximately 5X the engine speed. At 4,000 engine RPM's, that's an alternator RPM of about 20,000. It should be able to handle this, and gassers can be nearly double that. If the system is able to protect itself, and it works within acceptable limits otherwise, you could dismiss or avoid the condition. I probably would, unless there are other indications of a problem.