so I've been having more frequent clunky / rough shifting lately, especially uphill. at times, it will throw the flashing M+S and sometimes go into limp mode. also noticed that when it's really struggling uphill, the lights will dim, more strain=less lights. any ideas? TIA

edit- thanks for the replies, re: alternator. not much good at electrical. wasn't sure if it was battery / battery connects (just now cleaned them), or alt. dimming lights points to alt. getting 14.3- 14.4 volts reading on UG, that matter?

 

Your alternator isn't charging.

 

Re: Alternator. If lights dim, voltage could be low. If voltage is low, spark is weak. If spark is weak, truck struggles. Any OBD codes? Low voltage can set lots of codes, leads to limp mode.

 

About electricity...you will not have the same voltage everywhere. If you lights are dimming, you have lower voltage at the bulb. If at the same time your lights are dimming, you see solid ~14.3V at your UG, then your OBD connector is getting good voltage. That means that your alternator is likely fine. You get different voltages at run time when you have bad cables or connections. A bad ground strap connection can cause this sort of behavior. Check both ends of all ground cables/straps and ensure that they are clean and tight.

 

Have your battery tested as well.