jturuk

Hi all

99 Ram with Turbo diesel

The charging system in this truck is just not putting out like it's supposed to (about 12.5 volts at idle, at highway speed voltage varies from 1/3 to the mid-point on the guage). Already replaced the batteries. I suspect that the alternator is tired but I want to make sure. I want to test the voltage regulator portion of the system is. Was wondering if i could disconnect the field harness at the alternator and jumper the two terminals with a test light and then turn the ignition switch on. The way I see it if the voltage regulator is good the light should illuminate.

Thanks

John

Lil Dog

The regulator for the alternator is part of the PCM. The alternator only has field voltage terminals that are controlled by the PCM. You could test these terminals with a DVM to see what the voltage is at each of those terminals and the differential across them. IIRC I saw around 10V across those terminals when it was charging. Either way, 12.5V at idle is not enough..

jturuk

Thanks lil dog for the reply

Apparently the field current is controlled by "cycling the ground path" but the service manual isn't clear (at least to me as to whether this is done before the field current is sent to the field terminals on the alternator or whether it is done after the field current returns from the alternator to the PCM. If it is done after I'm thinking that you could have a screwed EVR but still have normal voltage readings at both alternator field terminals. Thats why I figured if one used a test light between the two harness terminals at the alternator it would show current flow and indicate that the EVR was alright

thanks

John

nickg

almost always low alt output is a sign of failing alt brushes...
I highly doubt you will find any issues with the field (PCM) as when a volt req goes its usually full field or no field.

if you have a denso alt remove it and pull the rear cover off, the brush holder can be removed (3 screws) and you can see the brush condition as well as the slip rings. I'd suggest having a spare set in the glove box (like 10-12$) it coul;d get you out of a jam should you loose an alt far from home

infidel

Alternator rebuild parts> http://www.fostertruck.com/alternators.htm

Lil Dog

I am going from memory here but I do remember the voltage at the terminals straight from ground didn't make sense to me so measuring across the field gave more of an idea of load to the field. After initial start up, the delta voltage dropped to 3V or something to maintain 14.3 V on the output.

I had a GEN lite (older truck) come on and the alternator was just skipping on the slip rings. I pulled the brushes, cleaned the contact area with scotchbrite pad and then thoroughly cleaned the brushes with electronics cleaner. It looked like one of the brushes was just getting stuck in the holder and not contacting with full force.

Point is, the alternator on these trucks is pretty hard to kill. Check the diodes on the bridge rectifier and replace the brushes, good to go for another 100,000.

Junk Man

I had a parts store diagnosis on mine that showed bad diodes. I left it alone since it was charging. It suddenly quit charging on me 100 mi from home. Only 1 store had a replacement & I couldn't find a shop to install. I wasn't going to rent a hotel so borrowed tools & changed out of my fancy go to meetin clothes (business suit). An hr later & $100 + later I was back on the road. My point: do whatever is necessary while you have the luxury of dealing with it at home.