Voltage tested at battery is insufficient while at full load

Tiny
MURRLYN1000
  • MEMBER
  • 1999 CHRYSLER TOWN AND COUNTRY
  • 3.8L
  • V6
  • FWD
  • AUTOMATIC
  • 145,000 MILES
I recently put in a new ECU due to my battery discharging over a week or so. I also installed a re-manufactured ECU. I cleaned the battery terminals and battery, cleaned the wire connections at the power distribution box, had the alternator pulled apart diodes and rotor short test performed which test fine. I had new brushes installed for the heck of it, I had the van scoped with the first re-manufactured ECU and everything tested fine including voltage drop tests etc. Except for the ecu driver was reported bad. The ecu had a lifetime limited warranty so the seller all computers inc. Sent a second ECU. After installation same problems. I tested the field wire on the ECU and it tested with engine running from cold no load 5.8v with engine revved it went down to 2.5v or close to it. The voltage measured 13.85v. Once at full load the field wire tested at 11.5 to 12.20v at idle and stayed there with the engine revved. I then tested the positive wire that runs to the alternator (the small one not the big one) at the ECU and it measured exactly what I got at the battery with the engine running under all loads and conditions. Also measured the negative ground into the ECU and read a 0.04v drop between what was at the battery and what was at the ECU. I also hooked a jump starting cable directly to the alternator case and the battery negative post with no changes in voltage both at the ECU wires and the voltage at the battery. I also removed the alternator wire from the power distribution box and left the battery cable hooked up thus running the engine on only battery power and eliminating the risk of blowing electronics. I started the engine and got 29v at idle and 50v with engine revved. I cannot figure out why the voltage output is not at least 13.8v under full load. Does anyone have any ideas or is it just a bad driver again? What is a ECU driver anyway? Is that the software program it uses or something else?
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Wednesday, July 6th, 2016 AT 10:06 PM

10 Replies

Tiny
HMAC300
  • EXPERT
So in short your battery is not charging from the story I just read. It is probably not the ECU. Check items in picture, like for corrosion and wiggle wires around fusible link as it may only be connected by a couple of strands. If it is still messing up, check for connection problems at PCM as well as alternator. Finally have a local professional scan for codes to see if the body module is the problem. Or a BUS problem within dash or body module.
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Thursday, July 7th, 2016 AT 5:53 AM
Tiny
MURRLYN1000
  • MEMBER
Thanks for your help. I am not seeing the picture your speaking of in your answer. Do you know where the main ground points are located So I can assure they are not corroded?
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Friday, July 8th, 2016 AT 3:46 PM
Tiny
HMAC300
  • EXPERT
Sorry I goofed the grounds are at left front frame rail and rear of engine. Here is picture. Check these fuses and fusible link it may only have a few strands connected causing the problem. Then have it scanned because it may be a can bus problem or body module which only a pro scanner can pick up. I in inadvertently marked the asd relay which should not be the problem. It can also be corrosion on fuses so if they are clean it and use dielectric compound which are in small tubes on a lot of parts counters about $2.00.
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Friday, July 8th, 2016 AT 4:53 PM
Tiny
MURRLYN1000
  • MEMBER
Before I go out and buy a alternator charge wire can I just place an alligator clip testing wire on the charge post and attach the out end to my volt meter then start the vehicle and see if there is an appreciable difference in the voltage at the alternator and at the battery to tell if the charge wire is weak or damaged?
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Saturday, July 9th, 2016 AT 12:12 PM
Tiny
MURRLYN1000
  • MEMBER
In addition to last reply: I meant to say(see if theres an appreciable diffence at full load at the alternator and at the battery)
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Saturday, July 9th, 2016 AT 12:15 PM
Tiny
KEN
  • ADMIN
Sorry to but in here, have you tried to install a new battery? I have seen where the battery would cause this kind of problem.

Please let us know.

Best, Ken
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Tuesday, July 12th, 2016 AT 4:32 PM
Tiny
MURRLYN1000
  • MEMBER
I actually believe I have found the issue my vehicle by doing a load test with the vehicle running and no accessories on and reading the current on the negative cable and then running all accessories full blast with the engine off but in the run position and taking that reading and adding them together. I found out my vehicle is asking for 130 amps at full load with an aftermarket stereo system with two amps. After that I measured the charge wire current off the alternator and found it to read fully loaded with engine running and engine revved up 103 amps sooooo not near a high enough amperage alternator. One question though would this make the voltage reading drop as well to the 12.3 I stated above?
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Thursday, July 14th, 2016 AT 9:39 AM
Tiny
MURRLYN1000
  • MEMBER
Problem with reply function test.
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Thursday, July 14th, 2016 AT 10:04 AM
Tiny
HMAC300
  • EXPERT
If you added new stereo equipment like a new amp you probably did not see how many amps it takes which requires a higher amp alternator because if it is a Town and Country it has rear window defog and all the goodies so that is why it came with a 103 amp alternator, sufficient enough to go happily along with what is on it, but not enough when you start adding stuff like amps for stereo.
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Thursday, July 14th, 2016 AT 10:46 AM
Tiny
RENEE
  • ADMIN
Hi Murrlyn1000,

Please respond to this thread.

Thank you,

Renee
Admin
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Thursday, July 14th, 2016 AT 3:11 PM

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