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Unread 07-09-2011, 01:23 PM   #1
monoski13
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ECM - where does 8Volts come from??

Okay, new post to same old problem

93 jeep wrangler. No spark and new ECM (re-build).
Tracing voltages to check the ECM and as far as i'v read and been told by the ECM rebuilder I need;

- 8V supply into the ECM, pin 7
- 5V supply into the ECM

the 5V is for MAP and throttle position, so where is that 5V generated?
the 8V is for Camshaft position sensor, VSP and CKPS

all of these sensor have NO VOLTAGE on their connectors now.
a) AM I correct that the voltages do not come from ECM
b) where do these voltages get generated? what circuit do I replace/fix

thhanks

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Unread 07-09-2011, 02:11 PM   #2
SteveJcoYJ
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I would check for a voltage signal going to the primary coil, an check the resistance of the coil and the ground. check continuity from the secondary coil to the rotor. voltage drop across the wire.

As far as the CKPS, and the CPS they are coils that send a dc signal. A signal voltage is sent to a coil, the coil is energized an de energized by a trigger wheel spinning by it ( like the teeth of the fly wheel spinning by the CKPS, which is sometimes also used as a VSS) . This makes an breaks a magnetic field created by a coil, a positive lead runs to the ECM after carrying the pulsating current message. You wont have a signal off the CKPS or CPS unless the motor is turning over, it needs that relative motion to create the magnetic field to send voltage back to the ECM. if the starter engages and the motor turns over and your fuel pump is operating, i would investigate the ignition system.
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Unread 07-09-2011, 02:31 PM   #3
monoski13
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Thanks Steve. there are 3 pins going to CKPS and CPS. one ground, one "signal" as you indicate should pulse when engine turns.. and the 3rd?? as far as you know that 3rd is not supposed to have DC 5V or 8V supplied? Cause I have been told it does need that & I saw some diags that say 5V supply for instance?

I have replaced the primary coil, the AutoShutdown relay and CKPS. I will check the lines to secondary coil/camshaft sensor - pickup as you say.. but, still wondering about these "reference" voltages ..
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Unread 07-09-2011, 02:42 PM   #4
SteveJcoYJ
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the reference voltage is the wire going from the sensor with the pulsating voltage, the signal is power going to the sensor. You wont have a reference voltage without the motor turning. The reference voltage is giving an input to the ECU. they sell these spark plug testers for a few dollars, its a clear chamber with an input for a plug wire boot and it has a plug wire with a boot that comes out of it. You can check to wire from the coil secondary to the distributor with it, an the other wires for that matter. You see a filament light up when voltage goes through. the cps is built into the distributor housing, thats the two small wires coming out of the side of it. You wont have a reference off that without the motor turning over. just inspect the ignition system an make sure you have fuel. the coil has to have a good ground for the secondary to energize. check to make sure you get a resistance reading across the coil (the positive in to the ground side), if you have an out of limits reading there is a break in the coil winding. you can also put your voltmeter leads on a frame ground and the secondary while its cranking to verify it has voltage coming from the coil. This can also be done with the spark plug wire tester. Im not sure what the specs are for resistance and voltage, but you said you have the diagrams.
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Unread 07-09-2011, 02:47 PM   #5
JWELK
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I have a '95. On mine the 8VDC comes from the Power control module pin 7, the 5VDC from pin 6. If you have no voltage there are two and 1/2 possibilities. The 1 and 1/2 is the PCM has no input voltage or is defective. The second is the output(s) are shorted to ground. I would suspect the first as it is common to both of your problems.
Do you know the rebuilder? If so ask him. You said you didn't like electrical, but do you have a voltmeter and can use it. First disconnect the + batt term. Unplug the CPS and stick the red lead in pin 3 this is the 8VDC source. On mine it is an orange wire with the pins facing you and the release tab on top it is right hand pin.
Reconnect the battery and turn the key on, do not crank. Check the voltage should be 8VDC.
Let me know how you're doing, I'm digging out more info and will get back to you in an hour or so.

Jim W
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Unread 07-09-2011, 03:27 PM   #6
JWELK
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The PCM input is Pin 3. It is 12VDC from the Auto Shutdown Relay.

Jim W
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Unread 07-09-2011, 03:30 PM   #7
monoski13
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Thanks guys.

Jim.. your wiring sounds like what I have.
I spoke with the re-builder this morning - he indicated that pin 7 (Orange wire) GETS voltage from outside of PCM and that's why I'm confused. I did find a diagram that says pin 7 of PCM is an "input".

Anyway, i decided to follow the re-builder directions, cut the orange wire/pin7 going into the PCM. .
results w/PCM and all sensors connected, ignition on;

- no voltage coming out of PCM pin 7 wire. PCM not functioning or is it really an input?
- no voltage coming from orange wire - sensor side, so if it's a reference generated under hood - i'm missing that & PCM is good?

also, checked the cut end of orange wire going to the 3 sensors CKPS, cam sens, VSP. NO short to ground. so that's clean.

I previously checked all 12V and ground pins to the PCM - which seemed all good with module plug out. I'll go back and check all to make sure at least 12V & grnd are still good.
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Unread 07-09-2011, 06:56 PM   #8
JWELK
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Well that has me stumped. My schematic clearly states the PCM supplies both 5 and 8 VDC. The camshaft position sensor print definitely says 8 VDC input and the crankshaft position sensor states splice K7 provides 8 VDC to the crank position sensor and the vehicle speed sensor.
In addition the sensors show no input voltage like from fused 12 volts.
What year is the print you found? It is possible that some years had an external regulator. But my print shows Pin 7 to the sensor with no ext regulator.
Sooooo, I think the rebuilder wrong.
If the rebuilder took this apart, so can you. Guaranteed the voltage supply circuit will be something like a 7812 or 7805 these are 3 legged transistor looking items that take 12VDC and turn it into 8 or 5 VDC.
Ask the rebuilder if he has seen something like that, if not open it up and send me a picture.

One of the fellow posters has stated "if it has tits or tires it is going to cause problems." It should read if it has tits, tires or transistors it will have problems.
The one who mentioned the coil input also sounds quite knowledgeable.

In general on all three of these sensors Pin 1 is the sensor signal (output from sensor) Pin 2 is ground and Pin 3 is the supply voltage.
I'm looking at the Powertrain Control Module to see if I can find another input other than pin 3. That input comes from the Auto shut down relay. The shut down relay is controlled by pin 52 of the PCM. My problem is how does the ASR turn on if the PCM gets its power from the ASR. I'm might be missing something here.

Jim W
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Unread 07-09-2011, 09:44 PM   #9
monoski13
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Steve, i ran some of the tests you mentioned on coils. I think those are okay.
Jim, sorry I sucked you into the "stumped" world :-)

Back to the 8V supply - i too am convinced that the PCM generates that reference. So much that I hit craigslist and found a guy parting out 94 jeep cherokee and sold me the PCM for $20. calling it a day, but will check the wiring/connector in the AM to see if I can plug the cherkee PCM into my 93 wrangler harness - then check for the 8V and 5V ref. Hope they are close enough.

here is the link to the diagram i mentioned showing pin 7 - an 8V input to PCM.

http://www.jeepforum.com/forum/f12/1...agram-1170004/

Also, i liked your idea to check for some pwr/voltage transistors. pealed the top off the one I just bought. unfortunately, you can't see crap bcs it's filled over with protective goop. Anyway hope I can plug in and be on my way.

Thanks again guys

Mike W
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Unread 07-10-2011, 02:10 AM   #10
JWELK
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Nice print, I think 8 volt supply means this supplies 8 volts as opposed to a voltage that makes 8 volts.
How does the guy rebuild them if they are filled with goo? Does it look like epoxy or silicone?

Let us know how the replacement works?

Jim W
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Unread 07-10-2011, 09:07 AM   #11
monoski13
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Problem fixed! engine started with the cherokee PCM - yes!
thanks Steve and John :-)


Jim, the goo filled in these modules looks like silicon based protective. It's really soft & I guess they have a way to dissolve when repairing.

Couple more items to close;

There are a number of slight differences in the wiring diag / connectors for my 93 Jeep Wrangler 4.0 standard vs. 94 Cherokee 4.0 automatic. However it works sofar. mainly slight naming differences in connections - same function. I tracked everything down and verified you can plug it in directly with out risk. I will get it flashed or my original fixed for free by the re-builder - one or the other, just to make it right for the next guy.

Where does the 5V or 8V reference for CKPS, cam senor, VSP in my orginal post come from? - THE PCM!

So, the little dotted line at top of this PCM diagram (link below) that divides pin 28 & 34 - marked "inputs" and "outputs" is correct for everything accept 8V supply (pin7) and 5V (pin6) supply should be on the OUTPUT!!

http://www.jeepforum.com/forum/f12/1...agram-1170004/


Lastly, I found this nice writeup covering PCMs in this yr range is nice. However it only mentions 5V source from PCM - there is also an 8V source out of Pin 7 of PCM.

http://www.scribd.com/doc/13051431/J...s-of-Operation

best of luck to all

Mike W
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Unread 07-10-2011, 05:34 PM   #12
SteveJcoYJ
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glad it worked out man
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