Bellkrank

I just replaced my alternator and now the jeep runs fine after start up for about 30 sec then the voltage spikes out at 19 volts.... any thoughts???

Bellkrank

Changed the battery and its still doing it! next step is to rip off the new alternator and take it in....

any one have anything on this!!!! please help!!!

Bellkrank

So I took the "new" alt off and brout it into the auto store to have them test it. and bot alternators new and old were putting out about 11.8-12 volts.

did some research and this is what Im thinking:

1- I got a bad "new" alt.

2- the voltage regulator is bad. but its external and in the PMC (computer)

so I don't know what to do next???

-where is the PMC?

-is the alt only putting out 12 volts on the bench because the voltage reg is external?

PLEASE HELP!!!!!

Bellkrank

come on guys?

anyone?

the_big_h

Like you say the voltage regulator is in the engine control computer, so putting out too much voltage is not likely the alternator's fault. It's possible the signal wire between the computer and the alternator is grounded somewhere, applying full field to the alternator (ie full output - just keep on pushing out the amps. Voltage is unlimited: more than 120 volts into no load).
It's possible the output transistor of the engine control computer is failed. (Transistors usually fail ON). This means you need new engine computer.
It's also possible to have a bad connection in the wiring, so the engine computer doesn't see the alternator outpu voltage. Check the wiring from the battery terminals, and the main connection to the underhood power distribution fuse block, make sure it's all in good shape.

Bellkrank

hey thanks for the response!

I checked all the wires and they look good, I even took that fuse block apart and check that out. all the major wires lead into the big wire bundles so I cant check the full lengths just the ends. but they look ok.

Im about to put in the old alt and run it to see what happens...

do you know where the computer is i cant fine it any where online or in my manual???

also I read a thread about changing the computer and they said you have to "flash" it with the jeeps VIN # . any idea how to do that? or just go to auto zone and see if they can do it?

thanks!

98jeepsrt6

ECU (Engine control unit) is located under your center console. towards back seat.

Diesel

Its not a external regulator, its in the PCM(not pmc). You cant replace just that part, you need to replace the whole PCM. They run from $180 refurbed to $300 new, parts and labor is usually around $600

Bellkrank

Thanks "PCM" so that's the problem!

I found one at a j/y today for $80 off a 2000

Is the swap a simple on/off or will I need to do something at a shop with the computer?

Thanks for the help!

Diesel

Just undo the 5/16 bolt holding the harness to your original pcm, then unplug the harness and plug it into the replacement computer and see if the jeep will start. if it seems fine, then unbolt the original computer and bolt in the new one. don't forget the bolt to hold the harness in place. You can swap out the computer in about 10 minutes.

Don't forget to unhook the battery before you attempt to undo your computer. And don't hook it back up for about 10 minutes after you plug in the new pcm.

Gee oh Dee

Why wait to hook the battery back up?

Diesel

Something with stored current, helps it relearn faster

Gee oh Dee

Are you sure the ECU isn't the only thing that stores current?

I thought thats why you pull the batt cables off to erase the ECU's memory. Pull 'em off and leave 'em off for X amount of time because the ECU stores a bit of power in case the battery is removed.

I can't imagine any sensors having the ability to store juice. But maybe I'm incorrect?

Diesel

Ding. Takes anywhere from 7-23 minutes for it to erase its memory. You need it to be wiped in order for your new pcm to be enjoyable and learn at a decent speed.

Gee oh Dee

Ok, but if you are swapping the ECUs the new ECU will most likely have been off any truck for well over 23 min, so there is no juice inside to worry about.

I still don't get why you need to wait. You agreed the ECU stores the juice, but you are swapping the ECU...so your taking out the part that was currently storing the current...your taking the current out completely...