Short in the electrical system

I have a real dilema. My 93 with 4.0 OBD 1 was throwing a code 41 and the battery light came on too. Long story dull, I noticed the o2 sensor wires were shorting out my system because they melted onto the exhaust manifold. However, with that fixed, I'm still showing voltage between the negative post and negative cable (unhooked) Could this be a problem that will correct itself? The battery is only a year old. Now the alternator isn't outputting 14 volts like it used to either. Do you think it got fried?

You will always have SOME current flow due to the computer, etc., but it will be low. You should check the amperage being drawn. Some other posts detail what the draw should be, I'm not sure what it is.

You always have a minor micro sized draw on late models to keep the memory active in the radio, run the clock, juice the computer, etc.

Every time I unhook, it takes a week to reprogram for best drivability, repunch the preset radio stations, and finally notice I'm two hours early at work. Hmmm.

does a short like that destroy the PCM or is there a fuse to look for other than that 60a alternator fuse that looks to be o.k.?

What is the alternator output voltage now, measured with a meter, not the dash gauge?

Shorted O2 sensor won't hurt the PCM--just set a code.

Put a voltmeter on the battery with engine off - should be 12-12.5V. Start vehicle and check again - should be 13.5-14.5. If alternator aoutput isn't what it should be, pull alternator and take it in for a free check. Make sure battery connections and grounds are good first.

guess I will take the alternator off this weekend and take it in for testing. I just hope the short didn't fry my PCM. I'm getting under 12 volts now with no change under load at the battery or on the back of the alternator. Anyone know if a fuse could be blown?

It helps to understand the basic setup for the alternator. ASD relay closes and sends battery voltage to the field winding in the alternator (post 90 and pre 98 I think/maybe later) The amount of current through the field winding is regulated by the PCM (ECM, computer) on the ground side. Don't know for sure, but it's likely it is regulated by the amount of voltage supplied to the PCM and not the output of the alternator. If the ASD relay isn't working the Jeep won't usually start or run.
The alternator has three diodes in there, each responsible for about a third of the output voltage (not exactly, but near enough).
80 amp fuse in the PDC (power distribution center, under hood fuse/relay box).
The alternator does have brushes and these can become worn or dirty, iffy or no contact will affect output.
Arching or loose power connections to the PCM, has been known to pulse and scramble the brains for the voltage regulator (TSB). In other words, unhook the battery before attempting to remove the PCM power plugs and try to avoid jiggling anything electrical, from the batter terminals, through the ignition switch to the PCM whenever possible. Sometimes the voltage regulator (driver) built into the PCM, just takes a dump.

8Mud said:

If the ASD relay isn't working the Jeep won't usually start or run.
Sometimes the voltage regulator (driver) built into the PCM, just takes a dump.

Click to expand...

Very good explanations of the electrical charging system, BUT as you say, if it's bad ASD it won't run. AND Voltage regulator in PCM failure is rare.

Daveisdabomb said:

guess I will take the alternator off this weekend and take it in for testing. I just hope the short didn't fry my PCM. I'm getting under 12 volts now with no change under load at the battery or on the back of the alternator. Anyone know if a fuse could be blown?

Click to expand...

FYI:
You don't have to take the alternator/battery out for testing.Advance auto has a portable deal that they wheel out to test your vehicle.
Don't know how your local parts stores are but I'd call and ask before going through the trouble of pulling parts.....

thanks guys,
yeah it still starts on its own, amazingly enough. I did have someone at zone hook up the machine on the jeep and quickly assumed the alternator. He knew nothing of the PCM and couldn't test it. I'll pull the alternator out this weekend and get it bench tested. I'm assuming it's fried. Is there any test to determine if the PCM is still functioning though? Is it normal to still be measuring 12 volts between the negative cable and post?

This is normal. What would be of concern is how many amps it is drawing. If you can leave everything hooked up for a week and still start it, it's probably not too much. But is should be able to withstand 3 weeks and still start (with a good battery to start with).

it started for me this morning. I drove it to and from work for 4 days before I found the fried wires. This morning I started it, with 11.5 volts, then jumpered it to my van which took it up to 13.5 so the battery was accepting the charge from the van. Guess I'll start praying the PCM is o.k. and test the alternator out this weekend.
Is getting 12 volts between the negative post and negative cable normal?

I dug back to the tread about solar panels to find this information from another forum member.

"FYI, I did some testing a while back, and the 'normal' drain on an XJ is about 12-14 mA. About half is through the TCM, and about half through the ECM (including radio memory and KAM)..... "

So don't check the volts between the cable and post, check the AMPS. He did not relate this information to a particular year, but I would think you're still OK up to 30mA.

thanks, unfortunately i don't have an amp meter. I'll have to pull the alternator out I guess. Being that the jeep runs, I'm thinking (hoping) the PCM is o.k.

The code 41 is "Alternator field control circuit open or shorted". The PCM monitors the voltage it sees on the ground side of the field coil (recall that the positive side is provided via the ASD relay, and the PCM controls the ground side). That means the PCM didn't see voltage show up when the ASD relay was engaged. It should see battery voltage when the ASD relay is closed and the PCM isn't grounding the other side.

Its unlikely that the PCM is bad. The code points to either a broken field coil inside the alternator or a wiring issue between the PCM and the alternator. Have a look at the wiring and connector at the back of the alternator and make sure its still plugged in and looking healthy. You can also check if the field coil has voltage across it.

thanks for all the help! the alternator did test bad. I put a new one in and my charging system does seem to be back to normal with no more trouble codes. Looks like no other damage to the system besides the alt.
thanks everyone