I have been stalking this forum since I got my 1997 Dodge Ram 1500 5.2L RWD Ext Cab over a year ago and have come across an issue I have not been able to find mentioned. My truck has recently started to idle low when the blower motor is on for heat and the truck is in gear. Runs fine when moving but if I let off the gas to coast or stop at a light my RPMs drop to around 580 and the alternator output drops to 11.9v-12.1v... any other time the alternator has a steady 14v output and RPMs stay around 625. The problem I am having in diagnosing the issue is I dont know if I should be looking at the alternator, the engine, or the transmission? What would be the most likely cause for all of these symptoms? Any suggestions?

 

I’d start with battery. Sounds like a bad cell

 

The A/C is also on once you turn any amount of defrost on. Try it in the floor or dash positions and see if it still does it.

 

The truck has dual battery set up (connected in 12v series) to run an inverter. Both batteries are less than 2 years old and problem persists even when disconnecting either battery.

The vacuum control switch for the vents quit working last year so I have the vacuum lines tied together under the quick release dash with an adapter 😅