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97 Caravan charging problem

11K views 14 replies 8 participants last post by  pxl1835 
#1 ·
I have a 97 Caravan 3.0L (135,000 miles) with an odd charging problem. The most noticable symptom is the headlights "pulse" brighter about once per second. The interior lights also pulse when they are on.

I put a voltmeter on the battery with the engine off, & read around 12V. With the engine running, the battery voltage stays around 12V, but spikes to 14+ volts about once per second. I cannot determine how high the voltage spikes, because the digital meter I use does not respond fast enough. Bringing the engine up to about 2000 RPM does not change the base voltage, nor the frequency of the spikes.

While driving, it is apparent that the charging system is barely keeping up with the electrical demand. With high electrical load (lights, heater fan, rear defrost) in low speed driving, the lights get progressively dimmer until the instrument cluster and radio start to cycle on & off. When the electrical load is light and the engine speed is higher, the charging system seems to stay ahead of the demand.

I have already changed the alternator, which did not make any difference. I cleaned the battery cable clamps and ring connector on the alternator. I have two batteries which fit the van, & have been carrying one charged as a backup, exchanging them as required (both were in good condition when this problem started), so the problem is not battery-related.

I searched the forum for this problem a couple of weeks ago & I understand the voltage regulator is in the PCM. Do these symptoms indicate the PCM has gone bad, or indicate there is another problem that is causing the alternator to charge only in pulses?

The van is currently residing out of town with my son (17). I hope to get a day to go over it again in the coming week if he has time to come home. I plan to carefully inspect or just replace the battery cables, but would appreciate any advice about other things I should check or replace.

Cheers,
Eric
 
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#2 ·
I have had the van back home since Saturday (son Nate is cabbing to work and/or mooching rides from friends). I still have not been able to fix the problem.

The repeated bench charge - discharge cycle killed both old batteries, so Nate bought a brand new battery last week.

I replaced the battery cable cable clamps and made new 4 gauge cables for battery + to starter, - to chassis, and - to engine/trans. I soldered new copper lugs on the cables (the new battery terminals are marine-style with the stud & wing nut) & insulated with adhesive-lined heat shrink tubing. I soldered new lugs on the battery + to the power distribution/fuse box & the battery - wire to the harness. EVERYTHING that comes to the battery. I sanded all the new lugs & contact points & coated with a conductive copper anti-seize. None of the old wiring looked corroded or suspicious, but I replaced it anyway.

The ground strap from the rear of the intake plenum to the firewall was weak, so I made a new ground cable for that location. The power cable from the output stud of the alternator to the power distribution box looked pristine, so I left it alone.

The van started without any trouble at all, but the battery light came on immediately. I checked the battery voltage & now it is not charging at all - not even in pulses. I checked the trouble codes using the key on-off method & found 12, 41 & 55. 12 & 55 are meaningless in this situation, but my Haynes manual lists 41 as indicating an open or short in the alternator field wiring system.

I traced the field wiring back to the PCM and neither is open nor shorted (to ground). With the field plug disconnected from the alternator, I measured battery voltage on the dark green/orange wire (correct per the wiring diagram), and 0.2V on the dark green, which is sensible if the PCM's voltage regulator is commanding alternator output, but is not seeing the battery voltage increase. I did not think to check the field voltage with the engine running and the alternator field connected - I will make up a test harness and check that tonight.

I took the new alternator out again & checked it - the field wiring shows about 4 Ohms, which is the same reading as the original alternator. Since the original alternator was still charging in pulses when I took it out, I suspect it is still OK. I cleaned up the mating surfaces on the case and mounting bracket & reinstalled it - still no charging.

This looks like my problem lies in the PCM itself, but the value of the van does not justify the expense of a new PCM. Can a junkyard PCM be swapped in (from another 1997 3.0L), or are there calibration issues to deal with?

Can anybody think of anything else I need to check?

Cheers,
Eric
 
#3 ·
I would check several things before you suspect the PCM.

Check Alternator
You can take it to autoparts store and they can verify it works, however, I have not always had good results from do this.

The PCM adjusts the voltage/current through generator field to control voltage induced in stator. You can ground the PCM side of generator field (I believe it is the dark green wire connector). The dark green/orange is providing the positive voltage (don't ground this) thru ASD relay. With car running (+2000 rpm) you should see 14+ voltage. If not I would suspect alternator.

Next test the resistance between the wire to the alternator and to the PCM with the voltage generated by the alternator. There is a fusible link in there and it is possible there is problems with this.

I think you already verified that there is voltage on the generator field plus side, but if you have not then this should be done. There is a fuse (#12 25A) that protects this circuit.

Do a search on PCMs. I believe others have replaced PCMs, but there are some gotchas I believe. There might be place to get them reprogramed too.

dlm4ut
 
#4 ·
I tried grounding the dark green (low side of the alternator field winding). The alternator works fine (the original is in there now, so it was never the problem). At idle, grounding the field caused the engine RPM to drop (stumble) and the voltage measured at the battery jumped to 14-15 V. With a brief rev of the engine (no tach handy) the voltage jumped to 16-18 V.

With the field connector plugged into the alternator and the engine idling, I measured battery voltage on the dark green/orange (high side) and almost battery voltage on the dark green (low side). "Almost" means about .02 V less than battery voltage - since the system isn't charging, the battery voltage slowly drops as the car runs. Holding the engine at a higher RPM increased the battery voltage from about 11.90 to about 11.95 and the voltage on the dark green stays mostly near battery voltage, but regular blipped down to 10-11 V, an effect with similar timing to the voltage/headlight pulsing I noticed when all of this started.

I skinned the dark green wire at the PCM (pin #4 - conveniently near the top) & measured the same result. I then cut the wire near the PCM and ran a jumper to the field connection on the alternator. Again, the low side is almost at battery voltage. Increasing RPM does not make a significant difference.

I duplicated these tests with the field high side (DG/O) connected directly to the battery + terminal - no difference. Also, before I cut the wire at the PCM, I checked the voltage at the alternator output stud. It was the same as battery voltage & increasing the RPM made no difference. If there was a problem in the wire and/or fuse link leading to the power distribution box, I should still have measured increased voltage at the output stud.

Basically, I have concluded that the PCM is simply not demanding any output from the alternator. So now I need to either find a used PCM, splice in an external voltage regulator (late 70's - early 80's Ford comes to mind) or scrap the van. Dang.

Thanks for your help.

Cheers,
Eric
 
#5 ·
Most pcms are available from so-called "rebuilders" for about $199 on eBay. I'm not sure what can be done to rebuild a pcm, but whatever, as long as it works.

You should be able to find a used/junkyard pcm for your van, but as said above search on the issue to see what PCM you need to find-- on the Durango, for example, if you don't have factory security, you have to find a PCM from a truck without factory security. The first thing of course is to find one from a van with the same size engine.
 
#6 ·
I have just replaced the pcm in my van. I ordered a rebuilt one from auto computer exchange out of Florida. I found them on the computer(ebay). The first computer I got from them was bad but they sent me a new one. It takes about 3 to 5 business days to get one from them. They also programed it to my vehicle so I didn't have to take it to the dealership. My van is a 2003 and the pcm cost $275 plus shipping. If I had it to do over again I would find a local rebuilder. I called the dealership and all they offered was a rebuild. They didn't even offer a new pcm. Their price was $582 plus $110 for programming. I asked the parts guy who they used to rebuild theirs and he told me. It was a local company and I took my pcm to him and he rebuilt it the next day with a lifetime warranty. $250 bucks and included programming. If you have yours rebuilt it will run without reprogramming but they will flash any updates into your pcm. I watched the guy program it. I thought for $110 at the dealership this would be some fancy work. It took the guy less than 5 minutes and all he did was plug it into his laptop computer.

Good luck.

Travis
 
#7 ·
If you go to the junkyard and get a 97 3.0 PCm why would you need it flashed and why wouldn't it work? you need to remove the battery cable anyways .If you touch the 2 cables together and wait 30sec before replacing both cables this will reset the computer.So why wouldn't a junkyard one just pop in and work thinking the PCM is good in the 1st place form the yard:))
 
#8 ·
I am no expert, only a shade tree, but what they told me when I ordered my pcm whas that I had to give them my VIN # and mileage. They reprogram the pcm to match the vehicle. I think it has to do with an anti-theft device perhaps someone with a little more knowledge can help you or give them a call at auto computer exhange. They will answer any questions before you buy one. 1-888-8787. Perhaps your vehicle is old enough that it would not have to be reprogramed. Here is a link to their webpage and the model you need. It is $225 plus shipping.

www.autocomputerexchange.com/Search/Dodge/Caravan, Grand Caravan/1997/1g.
 
#9 ·
Oh, happy day!

I've had a very good day today.

I picked up a PCM at the local wreckers Friday for $75.00. The guy at the parts desk told me that for vans like mine (97, 3.0L, no anti-theft, basic, basic vehicle) the ONLY reason anybody replaces the PCM is because the voltage regulator craps out. I wish I'd known that 2 weeks ago - replacing all the battery cables, ground straps & re-lugging everything else was a royal pain. Although I have a garage, it's full of *stuff* & I was working in the driveway, well below freezing & often after dark.

Anyway, I just put in the used PCM & the van started right up. I wrote down the odometer reading before the old PCM came out, but I needn't have bothered - the same reading came up with the replacement PCM. No more battery light on the cluster. The voltage measured at the battery now sits at 13.8 at idle & increases to 14.0 with increased RPM. With the high beams and fog lights on (all nice & bright now), the readings are 13.75 & 13.8.

After a brief test drive, the battery voltage sits at 13.15 with the engine off.

So, all is well. I hope it stays that way...

Cheers & thanks for your help,
Eric
 
#10 ·
Very Cool. Guess you did some Extra routine maintenance on the van .it helps a bunch. I can get a PCM here on 1/2 price days for like 30$ but i guess ya have to make sure its very close to what your running or Wipe the mem of the comp by removing the battery for 30 sec then touching the +- together before reinstalling.
 
#11 ·
I bought a 2005 Dodge Magnum SXT about 2 weeks ago. I was driving it home in the dark and noticed that all of the lights pulsed when I selected headlights on high beam. They would pulse for about 20 seconds after selecting high beam and work normally when low beam was selected. They got progressively better each time I selected high beam but the best they got was pulsing for about 3 seconds on high beam. Testing later showed that they would pulse even if the engine wasn't running so that eliminated the alternator as the fault. I bought 4 new HID headlight bulbs on eBay for $9 plus $10 shipping and handling. I replaced the high beam bulbs only and that corrected the problem. The lights no longer pulse. There must have been a short in the old bulb(s) and/or somehow they were drawing more current than the system could handle? I don't know if this problem is related to your problem or not.
 
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