Rmartin1818

I've done research after research on this battery draining issue that seems to be pretty common in all GC. I read about the #7 fuse, the BCM and all the other common issues including all 5 door switches but my dome light isn't staying on. Here's what I own and here's what I found so far. I just bought it for $2000 with 72,000 original miles and I'm fairly new to Jeeps. It's a 94 Grand Cherokee Laredo with the 4.0 straight 6. Just bought a brand new battery bc the other one was toast. I'm getting over 5 amps of draw when I put a multimeter in between the negative cable and battery. I pulled the very first fuse under the hood and it's a 60A for the alternator. Walla! It dropped the amps to .7 instantly. There isn't much info on where to go from here and I don't wanna go buy a new alternator just for the hell of it. Could it be a bad diode and is there a way to test it? I also noticed the fuse is harder than usual to pull out bc one of the female ends on the fuse box is twisted. The motor does run off just the alternator. Is it possible that it will still run my motor with a short or a bad diode? Any help or info is much appreciated. Thank you!

dave1123

The diode rectifier bridge in the alternator has 6 diodes in it. If one is shorted, it will still put out DC voltage, but not up to full charge rate. When it engine is shut off, that diode will keep the field engergized and draw down the battery. You can find an AC output voltage in the DC if you have a good multimeter.

A good automotive electric shop can fix your alternator for you, or you can buy another one. Possibly one from a junkyard.

Rmartin1818

Thanks for the reply. I tried some testing with my multimeter last night, putting it between the positive cable that hooks to the alternator and the post itself but I couldn't get any reading. I'm new to using a multimeter so I'm sure I've got it on the wrong setting or plugged in the wrong holes. I just bought a $20 one at Advance the other day so I'm sure it's not the best but I'd think it should be suitable for me. I've watched a bunch of videos and did a lot of reading on using multimeters over the last few days but it's still hard to comprehend some of it. Can you tell me what holes I need the leads plugged in to and what setting it needs to be on to do the testing you were talking about above?

Rmartin1818

This is the cheap multimeter I bought. Can you tell me what I need it set to test for a diode short?

http://m.ebay.com/itm/221714289644?nav=SEARCH

dave1123

Looking at that picture, I haven't a clue. If you have the alternator output wire disconnected, set the meter to amps and test between the wire and the post. It shouldn't be drawing anything with the engine off.

Rmartin1818

I still haven't figured out this issue but something new has happened and I hoped it would help to narrow it down. We've been using the kill switch that was installed by the previous owner every time we get out of the Cherokee. Lately though the domes lights have been coming on after we turn the kill switch off. So with all power cut, the fact that the dome lights are coming on and staying on for about 30 seconds should tell me that the short is somewhere in the dome light wiring or door switch wiring correct? Please let me know a good place to start. As always thank you everyone for the help. It's appreciated.

dave1123

Sorry dude. Electrical problems are hard enough to diagnose via a keyboard, but once you start with adding stuff like a "kill switch" to the mess, I'm bowing out.

I had a Japanese multimeter that had a specific function for detecting AC in a DC circuit. It's been many years since I had that meter. AFAIK, the only way to be sure a diode is bad is to take the alternator apart and check each diode separately. A 5 amp draw tells me to suspect the alternator rectifier bridge. It connects the field windings directly to the battery, but allows current in one direction only; toward the battery. If one or more of those diodes is shorted, current can flow back into the windings, keeping them energized with the engine off. Disconnect the alternator main output lead and see what your amp draw is.

Why did the PO install a kill switch anyway?

Rmartin1818

Anyone have any input? I was told that just bc the dome lights are staying on for 30 seconds after all power is cut doesn't necessarily mean the short is in the door switches or anything else that turns on the dome lights. Sound right? I guess at this point I'm leaning towards the alternator.

Rmartin1818

Sorry I never saw your reply. He installed the kill switch bc he couldn't figure out the drain issue and I guess figured it was the easiest thing to do at that point. He was very honest with me but for $2000, I couldn't pass up a 94 with 72,000 original miles, new tires, no rust, even if it did have an electrical problem. I'm fairly good with mechanics so I was pretty confident I could figure it out but that was mainly on Hondas so the Cherokee is new to me. Thanks for the reply. I'll unhook the alternator and see what I got. I never thought of checking it that way.

Bearstream

I do a quick alternator test with multimeter like this: disconnect all the alt wires, set m-meter on ohms X 1, positive test lead to power output lug and negative to ground, continuity near zero ohms, so far so good. Then I switch leads positive to ground, negative to power lug... No continuity, good. Continuity = short. Diodes only allow current travel in one direction so if you're getting continuity in either direction, I'd say at least one of your diodes are smoked and your alternator is a core exchange candidate at your auto parts store.

dave1123

Perfect Bearstream! That is pretty simple, ain't it?

Rmartin1818

I'll give that a try. Thank you very much.

97grand4.0

*First take it to have a free alternator test, but 5 amps is about right for the following:

This is an old and common problem with a simple, yet obscure answer. I had the fortune to talk to a sharp tech at the dealer , who said "no, not too many problems with the BCM but a lot of problems with the door switches on those" after describing the problem. Answer: driver's door key cylinder switch, they wear out and break and are stuck in either the open or closed position. $21 at rockauto, make sure you specify driver's or passengers door they are dif. . I got both at the pick and pull yard for about $4.
Put them in and ..whadda ya think. everything works incl. the remote alarm. Did have to bypass the back door window switch, tho, but can live without it. After putting this right, was able to observe the proper 'going to sleep' shutdown of the BCM current draw on my ammeter. With it all locked up and the alarm on, red light f lashing in the window, the resting current was on the order of 9 milliamps. You can actually watch it time out and wind down, takes about a minute or so. Cabin lights come on with the key in the door turned, and shut down and time out. Perfect.