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battery drain troubleshooting - is it the Powertrain Control Module?

br1anstorm

NAXJA Forum User
Location
United Kingdom
I have a 1993 XJ Sport, 4.0 engine, auto transmission, standard spec (no major electrical mods or accessories). I need help in tracing a battery drain problem.

I replaced my previous battery a month ago. It was 7 years old, wasn't holding charge, struggled to turn the starter, and the little indicator showed "charge/replace". I assumed it was worn out, and replaced it with an Optima Red Top (expensive and hard to find in UK!).

This weekend, after sitting for only 3 days, the jeep wouldn't start. Battery flat - dome light barely glowing. I checked all the obvious things: I hadn't left the lights on, the interior courtesy, glovebox, and underhood lights all went off properly, and the battery connections all seemed clean and tight.

So I charged the battery overnight, and did some more research (I'm not an engineer or auto electrician). I have now just done the "parasitic drain test". Disconnected negative cable, and put my analogue meter between neg battery post and cable, with ignition and all lights etc off (including underhood light, which I removed). Here's the results:
  • When connected, meter reads 48 (or is it 4.8) mA
  • I pulled each of the fuses in turn on the interior fuseblock in driver's footwell. No change in meter readings except when I pulled fuse no 9. Meter dropped very slightly (to 42 mA). I figured this was normal - fuse 9 controls the radio/memory as well as interior courtesy lights etc, so I reckoned there would be a very tiny current draw there anyway
  • So then I pulled each of the fuses in the relay/fuse block - aka the Power Distribution Centre - under the hood. No change in readings until I pulled fuse 2, the 30A one which is labelled 'Engine Cntrl', and on the wiring diagram connects to, and protects, the Powertrain Control Module (PCM). The meter reading dropped significantly - to 10mA.
And that's where I need help. Would it be normal for this reading to drop, or does it indeed show a current drain/bad circuit in this area? And if so, how do I now proceed? The PCM is a black box which I don't understand and am reluctant to mess with. According to the manual and wiring diagram, it controls most of the major electrical functions (fuel pump, injectors, sensors, coil, etc etc). Is there any way of doing further more targeted tests? I can't simply throw parts at the problem experimentally, or swap out components - replacement parts are hard to find here in UK.

Can anyone suggest a step-by-step sequence of further checks that would enable me to locate, or narrow down, the problem? I hesitate to go to a dealer, as apart from the cost, local workshops rarely see US-spec vehicles like mine; if I need something done, it helps if I can at least tell them what and where the problem is!

Expert advice would be appreciated...

br1anstorm
 
You should read ( .03 Miliamps ) ....By experience , I will start disconnecting units that will draw current like an Amplifier unit , especially if it is Aftermarket, check interior lights like Glove Box light , console light if equipped ...
 
Re: battery drain troubleshooting - more clues, and a code 41

More clues to add to my first post above (and, Alacran, I've checked all the obvious like dome light, glovebox etc. Can't try running with PCM disconnected, for obvious reasons: nothing will work!).

The additional clue is that after doing the drain test and posting my question, I started the engine. It struggled a bit then fired up - as if battery was still weak. Then the "Check Engine" light came on. I looked at the codes using the ignition key trick (OBD1). Got a 12, of course, and then a 41 (Alternator field switch not operating properly - check charging system).

I checked the voltages (though analogue meter isn't very precise). Engine off, meter shows battery at 12.6v. Engine running, meter across battery terminals shows 14v. Gauge on dash has needle steady just below the 14v centre mark.

I've read in the forum that the '93 XJ alternator has its regulator built into the PCM (also referred to by some as the ECM...). Is this where the field switch is?? have I got a charging problem AND a current drain when the engine is off? Or - from another post I've read - is it normal for the PCM/ECM to take current when everything is switched off? Or is it not powering down (going to sleep) as it should when the ignition is off?

I'm getting more perplexed. Can someone make sense of all the evidence I've gathered so far?

br1anstorm
 
I don't think 48ma--.048 amps--is a significant drain for computer controlled vehicle; unfortunately, I can't remember exactly what mine is. I did go through the same problem your having--turned out to be a bad battery, that I had just recently installed new. The best test is to insure the battery is fully charged, meaning, after setting disconnected for couple of hours, the voltage, with a digital volt meter, reads between 12.6 (old battery) and 12.8 (new battery). Then let it sit over night and measure again; the voltage should not be below 12.6. Lower voltage would indicate an internal short, or calcified plates, or the battery was not, in fact, fully charged. It probably would be best to leave it on a charger overnight to insure it's fully charged. The problem I had was the supplier (Sears Die Hard) tested the battery at the store and stated it was good. After a long and sometimes heated argument, they replaced the battery--that was three years ago, and all is well. But I've had lots of experience with lead-acid batteries, and new the battery was bad. In the initial, I didn't know what a "normal" draw from on-board electronics was; I think yours is OK.
 
Thanks xjbubba. I'm charging the battery overnight and will then let it sit, and see if it holds charge. But as it's an Optima (sealed, spiral) not a regular lead-acid battery, I'm not sure how far I can verify if it's good or bad.

As for the overall problem, I'm thinking that it's basically one of three things:
(i) the battery is good, but draining because of a fault/short circuit/parasitic load. I can't find one (and I have very few electric or electronic gizmos which might need power when the ignition is off). If my milliamp test-readings are normal, then there's no unusual drain of power. So that must mean ...
(ii) the battery is bad, and can't hold charge. But it's new and has worked OK for the past month. Or...
(iii) the battery is good, but there's a fault in the charging system. This would tally with the code 41 I'm getting. And perhaps the battery, being a Red Top, has been tough enough to start the car over the past month since I bought it, and the car's been relying on alternator power while running, but now the battery has just run down to near-nothing and can't crank the engine. I presume I can't check this but will have to take it into a specialist workshop for more detailed testing?

Item (iii) is now looking like the most credible scenario. If I'm mistaken, I'm sure someone will tell me!

br1anstorm
 
A little known fact about the gel cell batteries: If you flatten them once, they are toast. Unlike a wet cell bat., when the gel cell batteries are totaly discharged, they are hurt bad. If they will take a charge, it won't be a full one,and the lifespan is significently shortened.
Have the optima tested, It probably won't pass, and they might warrenty it out. Don't put the gel cell back in until you solve the problem
 
There are tests that are more definitive than just reading the voltage accross the battery terminals, but a reading of 14v indicates the alternator output is good. The voltage regulator, whether in the ECM or in the alternator does the same thing, it holds voltage constant until it senses equilibrium between battery voltage and alternator output, then reduces voltage to a level that will insure continued charging, until the battery voltage reaches ~14v. Then the regulator will reduce voltage to ~13.2v. These voltages result in the appropriate amperage flow to charge the battery as fast as possible, without over heating it. The charging system I had on my sailboat had temperature sensors on each battery to control overheating, which would resulted in gassing, something that doesn't happen with a sealed battery. I know charging characteristics of a gel cell is different than a wet-cell, but I don't recall exactly what the difference is.
 
Re: battery drain troubleshooting - latest news

Thanks to those who recently posted. Got a bit worried by the warning about gel-cells, but I think I'm OK. I haven't solved the problem, but I think I've made some progress...

After charging-up the battery overnight, I took the Jeep for a 50-mile drive with a few stops and starts. Seemed to run fine (and voltage gauge on dash hovered from 14 down to around 12...). Went to see a friend who has a workshop and some fancy Bosch meters and diagnostic tools. According to his tests, the battery is fully charged and in good health. And when running, the alternator delivers the right voltage(s) and amps: readings vary between just a few amps (since battery is 100% charged) up to peak of 35-40 amps flowing with all the headlights etc on.

So in a way we're back to square 1. Was there some odd and unusual short circuit or fault which drained down the battery last weekend - like a corroded connection or bare wire that only shorted when it was moved or touching something? Or is there an intermittent fault with the alternator or PCM - like a failing diode - that only plays up occasionally but is enough to cause a code 41, and might have caused a current drain? I'm still not clear exactly what or where the "field switch" is for the alternator that the code 41 notes refer to - is it in the PCM? Is it related to the diode?

I've managed to lay hands on a digital multimeter, so next step is to do the parasitic test again and see if I can get some more definitive readings as I pull each fuse. Then if I can wipe the code (how do I do that?) I could take the Jeep out on the road and see if it throws a code 41 again - which would reinforce the suspicion of a fault of some kind with the alternator.

Thanks again for advice so far. The search continues!

br1anstorm
 
Re: battery question

Not sure how your question relates to the problem described in my posts and the others above, Blaine B... but as I understand it the AC Delco maintenance free battery (I used to have one, it's standard Jeep OEM) is a sealed lead acid wet-cell battery with a built-in hydrometer (the little indicator window). The Optima Red Top - which I have now - is a sealed "spiral wound" gel-cell battery.

Hope that helps

br1anstorm
 
A Field Service Manual (FSM) for your Jeep would tell you if the voltage regulator is in the alternator, or not. You didn't indicate whether you let the battery set over night, unconnected, before checking battery voltage again. Doing so is a very good test regarding the battery's ability to hold a charge. Right after being charged, a battery will indicate a higher voltage than normal, possibly giving a false impression that it can hold that charge; hence, the need to let it set to "normalise" the voltage before checking.
Good luck, and please, when you resolve this, post up the results.
 
Re: battery question

br1anstorm said:
Not sure how your question relates to the problem described in my posts and the others above, Blaine B... but as I understand it the AC Delco maintenance free battery (I used to have one, it's standard Jeep OEM) is a sealed lead acid wet-cell battery with a built-in hydrometer (the little indicator window). The Optima Red Top - which I have now - is a sealed "spiral wound" gel-cell battery.

Hope that helps

br1anstorm

TY
 
xjbubba said:
A Field Service Manual (FSM) for your Jeep would tell you if the voltage regulator is in the alternator, or not. You didn't indicate whether you let the battery set over night, unconnected, before checking battery voltage again. Doing so is a very good test regarding the battery's ability to hold a charge. Right after being charged, a battery will indicate a higher voltage than normal, possibly giving a false impression that it can hold that charge; hence, the need to let it set to "normalise" the voltage before checking.
Good luck, and please, when you resolve this, post up the results.
Thanks for these latest thoughts. On the first point, the manuals I've seen - and several forum posts - suggest that the voltage regulator is indeed within the PCM, rather than in, or fixed to the back of, the alternator.

Second - I haven't (yet) let the battery sit unconnected for 24 hours to see if it keeps its charge. But I'll do that this weekend, and report what I find. Meanwhile I've been double-checking a few other things, and thanks to a digital multimeter I can at least now be totally sure about the numbers. Today's story runs like this...

I tested with a digital multimeter just to get over the issue of not knowing exactly what readings I was getting in the parasitic drain test. Result: I'm getting readings of 6.6mA and 7.2mA - so the analogue meter was indeed showing 4.8mA, not 48mA, when I first tested!! Apologies for being an idiot and suggesting that the meter was showing 48mA - one of the hazards of not being sure what scale an analogue meter was working on...

And when I pull fuse #2, the reading drops to 1.0mA. So I'm well within normal for the Ignition-Off current draw, which suggests that there wasn't (or to be precise, there isn't now) a fault or short circuit draining my battery.

Just to be sure, I ran a test with the alternator itself totally disconnected. Same readings... so the alternator isn't allowing current to leak when the engine is switched off.

So I'm left with a mystery. Battery is good. Alternator is good, delivers appropriate charge, and isn't leaking current or misbehaving (so diode is OK, I guess). Doesn't seem to be any fault or short in any other wiring circuit that might be causing a drain.

I cleared the code 41 from the check-engine light, did a 30 mile drive, and checked for codes again. Nothing except 12 and 55! It seems that my Jeep is in rude health...

Which all leaves me totally puzzled as to why last weekend, after being parked for 4 days, the battery was totally flat. Maybe some phantom gremlin got at the electrics when we weren't watching? I reckon there is now nothing more I can do but watch and wait - and if the battery suddenly goes feeble in future, start testing all over again.

Heh, at least life with a Jeep is never boring. Stand by for the next instalment.

br1anstorm
 
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