recharging problems

My brothers 89 4.0 HO will not charge the battery. Here's what we have done, New Battery(x2) New Alternator (x2) I have tested the alt out of the XJ but its tests fine so my next thought is to take it and test it on the vehicle. It seams like there is a short between the battery and Alt. What else may cause it to not charge? My other thought is the the plug that goes to the Alt is bad, but I don't know what it is actually for (as in what it connects to besides the Alt) I am going to test the volts out of the Alt and then check the wire that attaches to the Battery to make sure it is the same amount. We are stumped and he really needs this thing to run. If you have some advice I could really use it.

TIA

DuG

I think I tried to post this earlier but I couldn't find my thread.

Don´t trust the voltmeter in the XJ.
The yellow wire going into the back of the alternator, should have battery voltage, with the ignition on. You can pull the plug to test it.
The tan wire is for the gauges.
The big red wire has a fuse built into the wire, near the main distribution lug, at the starter relay. Called a fusible link.
Clean the poles on your battery good and check the ground at the dipstick bracket.
No power to the yellow wire, no charge.
Fusible link burnt out on the big red wire, current can´t get to the battery.
Grease on the battery poles, poor/no contact.
Ground wire broken or poor contact at the dipstick/motor ground, poor charging.
Rare, but oil on the alternator mount or poor ground for the alternator, poor charging.
Take a look at the front of your alternator it should be turning to the right, with the motor running. I´ve seen the serpentine belt, hooked up in the front of an XJ, in stange and wonderous ways.

I'd start by checking voltage at the battery terminals for a referent, then check terminal voltage against voltage between the + post and the engine block, and the + post and the chasses - engine off.

After that, leave the voltmeter connected across the battery terminals and have an assistant start the engine. Voltage should drop to no lower than 10VDC, and recover to 13.5-14.6VDC.

Use a chassis or engine block ground, and check voltage from there to the battery + terminal and to the B+ terminal (large wire on screwpost) on the back of the alternator. Both should be roughly equal.

Troubleshooting -
If the voltage at the engine block is low or nonexistant, suspect the primary ground lead from the battery to the engine block (attaches to machined pad above starter.)
If the voltage at the block is good but poor at the chassis, replace the engine ground strap (left rear of cylinder head to firewall. Probably should change it by now anyhow, if it's original...)
If the running voltage at battery + and alternator B+ are not nearly equal, suspect the fuzible link in the charge lead has popped and needs to be replaced. Confirm as follows:
Locate start relay near battery. The large screwpost will have several leads coming off of it with leads where the first segment is different from the rest. These are the fuzible links.
Find the lead from the alternator B+ and feel the fuzible link part. If it bends very easily, the link has melted out and needs replacement. Fuze link wire is available at well-stocked parts houses, or you can replace the fuzible linke by adding a MAXI fuze block and rerouting through that. While the fuze link is not, strictly speaking, necessary (I deleted mine when I upgraded my alternator most don't have the confidence in their work to perform the deletion and feel comfortable driving. Look up the stock capacity of your alternator, and use that as a guideline for selection of the MAXI fuze.

Another common problem is the alternator ground itself. Being mounted low, it's subject to getting all cruddy and it can lose its ground. No ground, no charge. This can be cured by adding a heavy-gage lead (at least as large as the B+ lead!) running from the alternator to a convenient (and higher!) ground that is easier to keep clean. There is a threaded boss on the back of the alternator case that accepts either an M8x1.0 or M10x1.0 screw, use that for the alternator grounding point. After connection, coat both points liberally with Ox-Gard or similar - check the electrical department of your local hardware store for the stuff.

Another small thing - be sure to check the "location" section of the replies you get to look for a RENIX XJ-er in your area. We're usually quite willing to help out (simple jobs only cost a couple beers or equivalent) and probably have the parts and stuff on hand anyhow...

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