I seem to be going through alternators and I could use some help to figure out why. I've been throwing used alternators at it for a while and finally decided to bite the bullet and buy a new (reman'd) Bosch unit. Since I had a bit more money into this alternator I figured I better spend a little more time investigating what the hell is going on. I did a log with the Ranger and then did the same exact drive in my 2002 Jetta. Both vehicles have ~1 year old batteries. The Jetta's alternator is original (311k miles) and the Ranger's is brand new...first miles were driven during these logs.
In case you were interested, here was the drive:
1) Lights on, radio on
2) Pull out of driveway, get into 5th and drive for 4 miles (~60 mph)
3) Park and let engine idle, turn off headlights
4) Turn lights back on, turn back around and head back home.
A couple of observations/comments (please add others you may notice)
1. You can see that the low and high coolant glow plugs were activated at the start of the log on my Jetta. Ranger doesn't have coolant glow plugs. I don't believe combustion glow plugs operated for either vehicle during the logs (checked before I started logging but didn't log those MBs).
2. During steady-state cruising with the GPs off you can see the Jetta's duty cycle (DC) hovered around 25% while the Ranger was in the 60-65% range.
3. I did do a throttle blip on the Jetta (towards the end of the idle period) to make sure the DC dropped with revs. I forgot to do this with the Ranger but I'm not sure I saw the same behavior just looking at the MB without logging.
4. I should note that the battery on the Ranger was somewhat low just prior to logging (remember...bad alternator). Could have something to do with the abnormally high DC?
Here is the same log but with battery voltage added.
Observations/comments:
5. You'll notice that the Jetta's cruising voltage is 14.2 while the Ranger is 14.1.
6. You'll also notice both seem to want to increase voltage during idle. Jetta is able to achieve the very typical 14.4V while the Ranger is barely able to eclipse 14.2V.
7. Oddly, the cruising voltage for the Ranger dropped the longer I drove it. The opposite was true for the Jetta.
So I'm trying to rack my brains why the Ranger has a higher duty cycle. the only additional loads would be the lift pump. Could that possibly be drawing so much power that it is the problem? I'd be surprised because the Ranger originally came with a 95A alternator (120A alternator on both the Ranger and Jetta).
Knowing how high the DC is with no additional loads, I'm pretty sure when I hook up my trailer with its electric brakes and lights I've got to be pushing the alternator even harder...possibly to the point of failure.
I did measure resistance from my negative battery terminal to the accessory mounting bracket on the engine at 0.2ohms. Same as other places on my block and similar to what I measure on VWs.
Thoughts?
TIA!