I have a 2002 jeep liberty. The other day it died and seemed like it was the alternator but had it tested twice and passed. The battery did the same. All connections are tight and look good. It starts and runs fine when jumped but after removing the cables it dies after a few minutes because the alternator is only putting out 6 volts. My buddy says it might be the PCM. Any help would be great! Thanks
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- Thread starter EvilSnipa
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renegade 04
Full Access Member
You can test the alternator by first seeing what the jeeps resting voltage is BEFORE you start the jeep. Then start the jeep and see where the voltage is. Running voltage should be between about 13.8 and 14.3 or so. Best of luck. If all else fails you can switch to a external voltage regulator like me.
Billwill
Full Access Member
As stated with the engine OFF the reading across the battery should be +12.5 volts approx.
Then move the meter Red lead to the alternator thick-lead terminal...the voltage should also be +12.5 volts with reference to ground.
If this voltage is not there at the alternator then there could be two problems:
This thick lead to the alternator could have a dirty connection where it joins onto the positive lead of the battery or the Power Distribution Module (Fuse/Relay box)
Or it could be that the "Fusable Link" inside this thick lead has blown so the voltage present at the battery is not fed back to the alternator output.
This Fusable Link is located near to the battery end I believe and is like a fuse...it will blow if the output from the Alternator shorts to chassis or to a shorted-out battery or if someone has tried to jump-start the Jeep and has connected the battery the wrong way around...this usually causes other damage as well.
So if it seems like the Fusable Link may be blown...disconnect both ends of this cable and measure the continuity with an Ohm Meter to verify....you need to buy a new thick cable with the Fusable Link inside...for fire/safety reasons.
Or else check that the small connector on the alternator is firmly in place, the pins have not been pushed back and are not corroded. Also disconnect the battery and re-plug the connectors on the PCM a few times in case there are dirty connecting pins there.
Check that the grounding strap that grounds the engine to the chassis is clean and tight both ends...the Alternator needs this ground to be able to supply current. If you are only getting 6 volts it could mean that you are dropping the other 6 volts across a bad ground connection.
I believe the 6 cylinder 3.7 has two grounding cables. You can temporarily take a thick cable such as the black cable of the jumper cables and clamp one end onto the Alternator body and the other end to a clean chassis point or negative post of the battery.:shrug:
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Billwill
Full Access Member
Yes the voltage is controlled by the PCM/ECM.
The PCM senses the DC voltage somewhere near the ignition switch.
The PCM then increases the voltage pulse duration on the field coil by using Pulse Width Modulation (PCM). This means that a square-wave voltage is applied to the field coil ie. each pulse is OFF for a certain amount of time and ON for a certain amount of time....if the voltage sensed is too low then the PCM increases the length of the ON time of the pulse until the PCM is happy that the DC voltage across the battery is correct.:emotions34:
