I installed a new DMCH alternater PN 110101.

When the engine is at idle with blower on #4, headlights on low beam and AC running the voltage is 14.2 volts but every 18 seconds there is a quick dip to 13.0 volts. It's like the alternator switches off every 18 seconds. I did not notice it with the accessorys turned off but the battery could be holding the voltage without the loads.

Is this normal for this alternator?