I have a 2001 XJ and the battery voltage started dropping when I was at idle this weekend.
The RPMs drop with it as well to about 250. It sometimes even stalls out.
I have no engine mods.
I am not sure if it is the battery or the alternator. Any ideas before I start randomly replacing stuff?
There's a few possibilities:
- Serpentine belt tension / condition. If it's not adjusted to spec and/or at the end of its service life, it's possible that things may be OK at higher RPMS but not idle.
- Battery terminals, cables, and grounds. Give them all a good once-over, cleaning and replacing as necessary. The braided ground straps from the engine to firewall and chassis are particularly prone to flaking out.
- Battery condition. Don't know how old it is, but cold weather will kill a failing battery pretty quickly. Pull battery, have tested, replace if necessary.
- Alternator condition. Pull alternator, have tested, replace if necessary.
- Idle air sensor and idle air controller. If the IAS has never been cleaned or replaced, this is worth a shot. IACs are better to just replace outright IMHO, but a good throttle body cleaning can help.
- Crank position sensor. This is probably less-likely in this instance, so go through the steps above before replacing it.
Don't be surprised if this is a compound problem - it may just be a failing battery, but when they go it's possible that other components that were getting onto their last legs let go around the same time. And given that you've got decade-old components in there (assuming no prior replacements), it's not impossible that this may be the case.
