SV Forty Two

jordan96xj

I just checked the diagrams. The oil pressure sensor is on the same ground as all of those other important sensors. So include that in the list of thing to disconnect/check. As are both the oxygen sensors, vehicle speed sensor, transmission control module, and intake air temperature sensor.


Also while in the cabin, ensure that turning your headlights on/off doesn't change the behavior of what you see in the gauges (also spin the **** for cabin lamp brightness up and down).

SV Forty Two

jordan96xj

Going back to this important clue and thinking.


"When running in this manner, the volotmeter drops to just above 9"


Have you tried starting the vehicle while being jump started from another vehicle? I ask because after rereading your thread, I don't think we ever ruled out battery/alternator problems. Something is dragging the system voltage way down. That is normally because something is shorting to ground (which would typically blow a fuse) or because the battery and alternator are not capable of keeping the voltage up when the ignition system starts drawing against them.


I don't like trusting the in-cabin volt gauge for testing. Have you put the multimeter on the battery to check voltage when running/not-running and also when jumping the ASD relay?

SV Forty Two

jordan96xj

That has all the hallmarks of a low voltage problem during startup. The PCM has a real hard time managing things when startup drops the voltage like that. Having to give it gas to keep it chugging is classic sign that the computer is experiencing a voltage issue.


Your ground connections got cleaned up, but your battery cables need to be inspected for condition. Also, have you tried jump starting from another vehicle or battery-pack yet?


How do the connections on the back of your alternator look? As soon as the battery turns the starter, it is also turning the alternator, which immediately starts helping out with the electrical load on the system. If the alternator is not working or the connections aren't allowing electricity to get into the overall system, then the battery can be overwhelmed, and system-wide voltage drop will occur.


At this point, I'm pretty convinced that you have a general electrical (starting/charging system) problem. Everything up to this point was probably chasing symptoms.

jordan96xj

Youtube "testing for voltage drop". It is easy to learn, and is probably going to help you here.


I have a sneaking feeling that there is a large voltage drop between your battery and the PDC. There are basically two positive paths from your battery, one goes to the starter directly, and the other goes to the PDC through the fusible link. Do this:


1. Test battery voltage (with everything off).
2. Turn ignition to "on" (not cranking) and turn some high load item like the heater fan on full blast (should pull around 11-13 amps).
3. While this is on test the battery voltage again (should be close to where it was with everything off).
4. Then starting testing the top of the fuses in the PDC for voltage (red lead on fuse top, black lead on good ground or battery negative).


Compare the voltages you see in the PDC with the voltage seen at the battery. Big difference at all? More than 1v lower should be noted and reported back here.


To do the same test above in true "voltage drop test" fashion, you could:


1. Turn ignition to "on" (not cranking) and turn some high load item like the heater fan on full blast (should pull around 11-13 amps).
2. Place red lead on battery positive terminal
3. Place black lead on individual fuses in the PDC
4. The number shown on the multimeter will be the voltage dropped between the two (anything greater than 1 should be noted).


Voltage drop will only show up when a load is present. That is why we have the blower fan on.


(Keep in mind that depending on which leads are used the voltage drop can be displayed positive or negative. It doesn't matter, just treat the number it shows as an absolute value...so negative 1.3 and postive 1.3 on the multimeter display would both mean 1.3 volts dropped).

awg

the above guy give a quality answer!

a couple more things to check

an alternator can short one or more diodes internally, and will cause mysterious problems, (shortly before failing)..it requires comparing known resistance across various points in the alt

slightly easier to diagnose, a crook battery

any battery over 3yrs is suspect, especially if it has been let run flat a few times, and extra special if left flat

The simplest way to tell if a battery is bad, is with a multimeter

If it cannot hold charge...once charged to +12.66V, and had the float charge dissipated with a few minutes draw, it is no good

each 0.1V (under 12.66V) represents ~15% charge..if the battery will not hold above 12.50V for a few days untouched, it is on its way out

cold weather affects the readings a bit, but make it extra critical for a good battery. Anything under 12.25V represents a 25% charged battery

SV Forty Two

I have not yet tried to jump it with another vehicle. It's a bit of a logistical issue. Jeep is pulled into garage currently, garage backs out onto street. I will see what I can do late this evening when traffic dies down! Will see if I can follow steps to check for voltage drop as outlined...

jordan96xj

Doesn't need to be another vehicle. You could try a battery pack as well. All auto parts stores have them now, or walmart, or whatever. (the small ones).


On the front of your PDC (towards the front of the vehicle) is black plastic cover over the fusible link. Pop that open and inspect the condition of that connection. Clean it up if there is any doubt. If it is in bad shape it can lead to the symptoms we are discussing. It is the source of + voltage to everything in the PDC.

BlueRidgeMark

Yep. You need to check all FOUR battery cables. The battery + to starter, battery + to PDC, the battery - to block (near the distributor, below the coil), and the battery - to fender.

I'm with Jordan - this sounds like a low voltage issue. Corrosion in a connector or cable can allow good voltage readings, but not allow enough current to flow to properly operate things.

You might want to read this.

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Still no luck with this... any more ideas?