Wits end....I need help. No bus power now.

Ok vehicle is a 97 XJ 4.0, aw4, 231. What is does.

Occasionally was shutting down while driving. would restart.

Now idling in my driveway shuts down. No CEL on restart at all. Meaning no CEL at all now. Doesn't come on and go off. NO CEL EVER.
No bus power.
Cooling fan runs.
No gauges on dash work

Changed:
CPS
IAC

I have unplugged every sensor I can find and tried to get bus power,
Unplugged the tranny harness, speed sensor.

Swapped computers.

Flipped power relay box and tried wiggling and pulling wires.

Unplugged all A/C items.

I need this truck to run.

Any ideas? I have nothing else I know to try. There is something shorting killing the bus power I think. I tried banging on the dash and door switches.

Help.

Check fuses?

All fuses good. Just pulled the cluster because I read sometimes the connectors fail but that wasn't it.

Stupid Question, I Know You Said The Cel Light Doesnt Come On, But Have You Checked To See If Maybe The Bulb Is Bad And Throwing A Code Anyway? I Guess Im Asking If You Have Tried To Read The Codes anyway

The last two times my CEL went out it was a short in the low voltage supply circuit for many of the sensors. I found the low voltage (2-3 volts) at the sync sensor though it turned out to be a shorted CPS one time and a cooked to the exhaust rear O2 harness the second time. I just tested for voltage at the sync sensor because it was an easy spot to check.

Mine is a 96 export and a transiton model and not a true OBD II, it is mostly a 95. But my experience on my 96, may be worth a look see on your 97 anyway.

Yes, I believe that is the issue as well with a short. When sensors are unplugged I can't get it to go away. What sync sensor?

In my experience, when the cooling fan runs continuously, the CTS is telling the engine its overheating. Even though it may not actually be overheating, there is a fault in the sensor. This sensor will not always trip a CEL or throw a code. This is what I have seen over the years on obd I and obd II, something to look into.

I unplugged the coolant sensor and I still have no CEL and no Bus power with key on. I am thinking a frayed wire at this point.

A computer bus is not a power bus like in an electrical cabinet. It is a communication signal wire.

As for sensors all of them are simple and are controlled directly by the PCM. None of them use the CCD bus wires.

On the back of the gauge cluster the CCD bus wires are in connector #2.
Cavity 1 - D2 20WT/BK - CCD (-)
Cavity 2 - D1 20VT/BR - CCD (+)

On the PCM, connector #3.
Cavity 28 - D2 18WT/BK - CCD (-)
Cavity 30 - D1 18VT/BR - CCD (+)

On the TCM
Cavity 6 - D2B 20WT/BK - CCD (-)
Cavity 7 - D1B 20VT/BR - CCD (+)

If there are frayed wires it is likely there are wires unrelated to the CCD bus issue that are causing the PCM to be nonoperational. The failure of the CCD bus will not necessarily cause the Jeep to not work.

Ok, that is a good place to start. I will trace them.

Re: Re: Wits end....I need help. No bus power now.

CSaddict said:

Yes, I believe that is the issue as well with a short. When sensors are unplugged I can't get it to go away. What sync sensor?

Click to expand...

Sync sensor is the cam position sensor. Located in the distributor.
Sounds to me like there is a problem in the 5v feed to the sensors. Typically, the CPS is the culprit but all of the have the ability to draw the circuit down and cause the PCM to go"braindead" of sorts.
To diagnose, turn the key on, unplug the TPS and check voltages with a DVM. You should see 5v 5v and 0v. On the pins accordingly.
If that is not the result, then find the short on the wire missing the 5 v. (4.8 or 4.9 is fine too). Could be a wire rubbed through shorted to ground or a bad sensor. Don't forget the fuel tank pressure sensor.

Yeah the CCD Buss is how the computers communicate with each other. The "NO BUSS' is typically caused by a direct short in the 5v sensor circuit part of the PCM, or PCM failure (witch you've already tested for). I would look very closely at all the sensor wiring harness for frayed, or melted wiring (the injectors are not part of the 5v circuit). The fuel guage is supplied off the PCM, but I don't think it's part of the 5v circuit. I'd check it anyway. Its wiring runs under the drivers side trim panel alog the floor, and out under the rear seat. The O2 sensors are the same (Fed off the PCM, but I don't believe they are on the 5v circuit.). Don't forget the VSS. It is part of the 5v ciruit.

Can we get a little more info on the issue? Only happening when warm, or seem to happen as its warming up? Does the "No Buss" happen as soon as you turn the key on, or after it dies? You mentioned the cooling fan comes on when it does shut down. Have you replace the temp sensor in the thermostat housing, or tested is to insure it doesn't have an internal fault? FYI all the gauges in the gauge cluster are read off the CCD Buss. None of them are actually hardwired to the cluster itself.

A/C clutch or cooling fan never worked in the truck when I got it. Could be turned on with a snap on scanner but nothing else. I could manually actuate the relays to make the comp clutch lock up or turn the fan on. I don't know if that is related. I was driving it on the highway and it hiccuped. Never stopped and came back to life. Then the next day warming up I came out and it was off. Keyed on, no cel. Weird. Came back later in the day. Fired up drive fine. Gets worse as time goes on. I replaced the CPS on a hunch. We tried with the scanner but no codes stored. kept the scanner on it and it finally died. IAC shorted. Replaced IAC. ends up doing the same thing. So I figure its in the IAC wire to the computer at this point.

Ok, I have 2.5 volts on both bus + and - at the Diag port. I have +5 on both 5 volt pins coming out of the pcm plugs.

FIXED IT!!! A22 on plug C1 on the PCM was getting intermitant power from the ASD. I found a wire in the harness along the firewall with a break in it.