I tried running a search and didn't quite find any related results to my issue.

I have a 120K on the clock (new to me at 115k) - not sure if the alternator was ever replaced...

I've recently replaced water pump / thermostat housing / tensioners and belts (within a couple hundred miles) - everything runs smoothly until yesterday.

The motor is idling a touch rough and it coincides with the pulsing of the dash lights / nav screen / and incandecent bulbs for the running lights. As the engine RPM increases the pulsing remains constant.

I'm thinking the alternator's rectifier is starting to go out but I'm not sure if this is the case or not. when I measure static (no load) battery voltage I get 12.5v across the terminals. When I start the truck and test the same voltage across the battery I'm getting a cyclical voltage ranging from 13.2-13.5 or there abouts...so this seems to indicate that the alternator is performing correctly.

I tried the usual - inspected and tightened ground and power leads to and from the battery as well as chassis mounts and alternator connections - all seemed to be pretty clean and for the most part tight...was able to tighten the alternator positive cable a bit though...remeasured voltages and it's the same - the pulsing of the dash lights and such appear to be a little less intense though but still doing it. I'm planning on taking off the alternator and having it tested but my guess is that this isn't the problem...does anyone have any other ideas for me to check?

Oh and when the engine is off the lights don't pulsate and act normal...

Thanks in advance,

Tony

 

My guess would be a loose power or bad ground , even though u checked them

 

Thanks Tom! I ended up taking the alternator out and having it tested. They determined it was in fact the rectifier - so I'll update this as soon as I swap in the new alternator when I get it. My guess is that it just didn't go all at once - also the fan was turning on randomly and apparently that is an indicator that is common with alternator failure with certain types of viscous coupled fans. Good to know. Hope I never need that one again. Lol

 

Yup loosing voltage does weird stuff on these

 

Yeah you definitely answered your own question with this one - the only variable in this scenario is the alternator w/ internal regulator, etc..

The ONLY component in the entire vehicle capable of varying anything impactful relative to the RPMs is the alternator. Period. Rpms converting to current. Simple as that.

Your failure mode is ideal honestly - as I was forced to deal with a few variables (random) combined w/ other things like load-shedding and low-power mode on stereo, etc.. then finally (after I was paranoid) I smelled something on the way home, and shut all electronics off, tested voltage at home and found it wasn't charging. Turned on HVAC @ max and voltage dropped to 11ish. etc.

I already had a new alternator with me though, due to the signals the vehicle was giving me - but yeah, mine was about 112k miles at the time and THAT was ideal - it sounds like you got lucky.

 

The PWM signal which controls the fan is also induced by signals from the alternator - they're definitely interlinked and I noticed more activity from my clutch fan after I replaced the alternator.

 

UPDATE:

As noted by others...it was in fact the alternator...and you guys are right - this was the best scenario I could imagine as well..kind of a slow death with no real critical components acting up...a bit tougher to know for sure what it was but it all makes sense after the fact....

I can't believe how much better the truck runs with the new alternator...so buttery smooth LOL...(that may have been the tune-up too though but it's all good).

thanks for the help and I hope this thread helps someone else...