no bus, no start, and p0700 - lots of testing already done

1999 XJ Sport - 4.0L - I6 - AW4 - 242
Ok so here's the story. For a week or so the gauges started acting funny, namely the speedometer.
This started happening after I swapped the Transfer case.
Figured it was just a bad speedo sending unit on the donor t-case.
Before I had a chance to do anything about it, and after a week of the jeep sitting in the garage (its not a DD)
I went to start it, and no fire. Cranks but no fire.
I spent the rest of the day and into the evening troubleshooting everything I can...
From going thru the voltage and resistance testing procedures of various sensors and components...
To actual removal and inspection of said sensors and components.
Thru my investigation I see this:
(easy to overlook when most of what yur doing is under the hood or under the jeep)


So after a slurry of curse words that I'm sure offended the entire neighborhood, I go online and start digging around to find an answer,
most are telling me it's just the instrument cluster, some are telling me its the PCM.
I try to hook up a code reader to find out more information.....
(hows about some "NOPE" to go along with that fail....)


So there's a guy on my local chapter that is parting out his late model,
I hit him up for the tranny cooler he's been saving for me, and ask if he's still got the PCM...
Sure enough, he does!...and both were acquired for a fair price.
(old and new-to-me)


The PCM was swapped out and low and behold......
"NO BUS"
:bawl:
Well at that point all I wanted to do was end this misery...


So i did what any sane, level headed, law abiding citizen would do in a situation like this....
I tore the dash apart:


This was of course to get at the instrument cluster and not out of sheer frustration.
I took it inside (once out of the jeep) and proceeded to clean the contacts, using a small file.


After a good scouring on the leads (which SEEMED to be fairly clean/clear of corrosion/debris/oxidation to begin with)
....I took the instrument cluster back out to he Jeep and installed it.
"no bus"
:flamemad:

So at this point I was about ready to set the damn thing on fire.
I had up to this point:
-tested the TPS according to the procedure outlined in the FSM
-tested the Crank Position Sensor (CPS)
-continuity test done on connector for CPS
-tested the ASD relay (passed)
-tested the fuel pump relay (passed)
-bypassed the fuel pump relay
-bypassed the ASD relay
>>> neither of these options worked other than I DID manage to get the fuel pump to turn on <<<
-tested the CKS (CAM position Sensor / aka pick up coil / aka Distributor Ignition Pickup / aka stator / aka distributor switch plate)
-checked every fuse in the jeep (three times) - using a logic probe
-swapped the CPS for a new one (I had two spares)
-swapped the coolant temperature sending unit.
-tried to pull codes - unsuccessfully
-reset the computer (twice)
-cleaned contacts on back of inst. cluster
-blew out female connectors in dash where inst. cluster plugs in.
-swapped relays where I could
-cussed a LOT
-bled a little
-drank more than I should
-did countless hours of research online and thru my FSM (Factory Service Manual)

We know its NOT the PCM,
We know its NOT a faulty CPS
And before any of you mention Dielectric grease, do your research, its a NON CONDUCTIVE lubricant/water repellent....

NOTE: Most of the engine bay grounds have already been gone thru.
Last year I upgraded the wiring for the charging/starting system, and replaced the grounds from the bat to the fenderwell,
from the bat to the block, and from the block to the firewall.

Thru poking around and just random acts of "why not try this" ... I find this happening:

With the ignition on and pulling fuse 21 in the PDC...it tripped the cooling fan ON... and also trips the ASD relay, the fuel pump relay,
(i can hear and feel them click - confirmed this by pulling each one separately/individually)
It trips the AC clutch relay and the AC clutch itself, as well as the cruise control diaphragm thingy on the PS fender wall.


Sometimes, when i go out to the garage to begin work, I will turn the ignition on and the ASD relay FREAKS THE "F" OUT,
and will buzz like its going 1000 cycles a min. then settle down to about 1-2 cycles/sec.
This thing is also clicking at the exact same time at the exact same interval.


When the no bus issue DOES go away (on its own) ....and I'm actually able to pull codes, I get this:


From my research i am seeing MOST people who encounter this code also have codes that cover "Range selector" failures
(meaning the NSS is going bad or needs to be cleaned/refurbished)...
To solenoid failures.... i'm not getting those additional codes... JUST this one.
And up till this point the Jeep drove just fine, no transmission issues at all.

So that's where I'm at.... A no starting - no bus showing - intermittent issue - bout to go to the shredder Jeep Cherokee in my garage that is starting to get on my last nerve.
If any of you can help me trouble shoot this further, that would be great.
Obviously the next step is going to be to track down a donor Transmission Control Module....
but I really would rather not continue to blindly throw money at this by chucking more parts at it.
As always, any help / advice is greatly appreciated. Thank you.

The PCM needs to be flashed to your VIN and specific XJ. I don't believe you can simply swap them without doing this. I paid $260 for a fully flashed, lifetime warranty PCM flashed to my VIN and that solved my crank, no start problem on my 98 after much diagnosis.

Did you replace the CPS with a genuine Jeep part? Aftermarket CPSs have a spotty reputation and have been known to be bad out of the box.

Something is dragging down your network, hence the "no bus." I would try pulling sensors one at a time to see if your network returns. When I was diagnosing my PCM, I put an ammeter in series and found the PCM was pulling 1.5 amps when the key was off. It was also warm to the touch near connector A. Is yours warm? Mine shorted internally, taking down my network, but anything on the 5v line could be shorted and take down your network. I also suffered from a loud and rapid clicking ASD relay. Since the PCM was draining the battery, I had a low voltage situation freaking the ASD relay out. You're pointing to your purge control solenoid for the EVAP system, and I suspect you may also have a low voltage issue due to a short somewhere.

Since you mentioned the speedo was acting wonky, I would pull the VSS connector and see what happens. That is on the 5v line provided by the PCM and could be backfeeding the system. There is also a TSB on the CPS wiring harness causing an intermittent short. Since your problem is intermittent you may have some chaffing on the harness causing the short and subsequent network failure.

Here's my thread on my diagnostic process for my eventual PCM failure:

http://www.naxja.org/forum/showthread.php?t=1106580

98NWCherokee said:

The PCM needs to be flashed to your VIN and specific XJ. ]

Click to expand...

This is incorrect.

If the PCM has the same part number, meaning it came from a suitable donor (Year, engine, transmison same ), the PCM will work just fine as-is.

If you have a No-Bus and the engine runs, there is an instrument cluster issue. If you have a No-Bus and it doesn't run, you have a short circuit on the Bus circuit that connects the 5 volt sensors.

The rapid and loud clicking should have you looking for a short circuit or a shorted sensor.

Tim_MN said:

This is incorrect.

If the PCM has the same part number, meaning it came from a suitable donor (Year, engine, transmison same ), the PCM will work just fine as-is.

Click to expand...

The OP only refers to the PCM as being from a late model XJ. I contacted Jeep when I was price shopping the PCM and they told me it would indeed need to be flashed. This was also confirmed by the company that I eventually purchased the unit from.

I'm not saying you're incorrect, just there is conflicting information out there and swapping a used part for a used part is not always concrete diagnosis.

I have swapped compatible PCM's, and it works, but only if the part numbers match.

If one is trying to mate a 97 PCM to a 99 XJ, or some other mismatch of years, the PCM will need to re-flashed to the correct programming.

Tim_MN said:

I have done it, and it works, but only if the part numbers match.

If one is trying to mate a 97 PCM to a 99 XJ, or some other mismatch of years, the PCM will need to re-flashed to the correct programming.

Click to expand...

That makes a lot more sense. I suppose the company I purchased mine from could have used a 97-01 PCM and flashed it accordingly. Thanks for the clarification. Hopefully this gives the OP some additional routes to consider.

Do you have SKIM? thick grey key. I to am having similar problems. thought it was a bare wire that shorted and put the PCM in thermal protect but think not now. Have you rest the computer. If I dont get mine started it wont start at all and the key on the dash illuminates. runs fine after I reset it. the longer it sits harder to start, think I have the check valve problem on mine, '01 xj. try putting your foot to the floor after restting the computer when you start it. use the procedure of grounding the positive terminal to the AC compressor for 30 seconds then reconnect. turn key to on posistion then headlights on for 30 sec. then off and key off. this is the only way mine starts. Think the SKIM locks up the PCM if not started soon enough. Have not had enough time to find the problem. something electrical for sure. Just which one of the 400 you have will be the fun of finding it

SKIM issues will illuminate the SKIM symbol on the dash. When equipped, SKIM does not prevent starting, but will kill the engine 3 seconds after starting.

No-bus, and the Trouble Code reader not linking, point directly to an issue with the communications BUS, usually a short circuit, or perhaps a faulty PCM.

Thanks guys, good stuff so far, and definitely has me thinking about a couple things.
Gonna swap out the VSS right now, and I'll report back on the change (if any)

Im ready to torch mine as well. 98 cherokee sport. p0700, p0463, p0705. Had them all off today to get it tested and they came back on when I had to restart in the middle of the test. Shoot me in the face! No driveability issues, no shifting or starting issues. Replaced NSS and cranks fine in park and neutral.

Woodk61000 said:

Im ready to torch mine as well. 98 cherokee sport. p0700, p0463, p0705. Had them all off today to get it tested and they came back on when I had to restart in the middle of the test. Shoot me in the face! No driveability issues, no shifting or starting issues. Replaced NSS and cranks fine in park and neutral.

Click to expand...

Here's what I learned thru all this (and sorry I forgot to report back on this sooner)
There are multiple systems in the 5 volt sensor array, that ALL have to be functioning properly for the bus system to work properly. Granted some of these components can be a little off and the Jeep can still run... for instance an o2 sensor can be going out, and the Jeep will still run and drive...albeit a little crappy, but still.... One of the components that I didn't think was even part of the bus system was the VSS ... no thats not a typo.... The Vehicle Speed Sensor was the culprit in my situation. The VSS is a lesser known part of the 5 volt sensor family that controls the ASD (Automatic Shutdown Relay.) During startup, the PCM does a check of all 5v system sensors, and if all is how its supposed to be, the computer sends the signal thru the ASD to allow the fuel pump and other sensors (like the CKS and CPS) to do what they're supposed to do. (this is my interpretation of the start up procedures for the late model XJ 4.0L)


FROM MY BUILD THREAD:

So as you may have guessed, I HAVE FIXED THE PROBLEM!!!
After getting tons of good advice from XJTalk members, Jeepforum, and alike, I was able to piece together a solution....
Oddly enough, the one component I was going to swap out shortly after I swapped in the donor transfer case was the culprit and cause of all these issues.
The VSS (Vehicle Speed Sensor) was to blame. Apparently it is in the same 5volt family of sensors as the Crank Position Sensor (CPS) ... o2 sensors.... and TPS (Throttle Position Sensor)...as well as all the others...
I HAD NO IDEA.
I figured, "why, during the startup sequence does the PCM need to check the condition of the VSS?" ...and "why would the SPEED of the vehicle have any bearing on the PCM to begin with"..... well i guess it does, and swapping out the VSS from the donor T-case for the one out of my old one solved my problem. One little sensor took down the entire system.
The VSS is located on the side of the transfer case, regardless of the year of XJ:

(picture courtesy of: www.stu-offroad.com)

It's very easily removed by a single bolt and a retaining clip.

(picture courtesy of: www.4wdmechanix.com)

Pulling the VSS straight out removes it, this is not a serviceable part (for now - i may change that here very soon) ...so its just a matter of replacement.
The one I pulled out had an orange gear with the number "1" embossed on it,
and the one that came out of my old case was green, and had the number "4" embossed on it.


The stock one (green) is a 35 tooth, the one out of the donor case (orange) is a 36 tooth.
According to the info over at Morris 4x4 Center ...the orange one is for 33's and 4:10's.

With the two VSS units out of their respective t-cases, I merely switched the gears between the two (to keep my mental speedo-adjustment the same)
And installed the old one into the donor t-case.


Once done, I reset the computer, turned the key on and VOILA!
No more "no bus" ...all the gauges came to life, and the jeep fired right up.

Click to expand...

FYI for anyone interested...

A P0700 code is an indicator code set in the PCM when there are additional codes in the transmission control module (TCM) This is done to prompt you to scan the correct module.

Digger87xj said:

FYI for anyone interested...

A P0700 code is an indicator code set in the PCM when there are additional codes in the transmission control module (TCM) This is done to prompt you to scan the correct module.

Click to expand...

^ This is correct.
Thanks for posting up.... i forgot to mention that with the pic of the scanner.

Tim_MN said:

This is incorrect.

If the PCM has the same part number, meaning it came from a suitable donor (Year, engine, transmison same ), the PCM will work just fine as-is.

If you have a No-Bus and the engine runs, there is an instrument cluster issue. If you have a No-Bus and it doesn't run, you have a short circuit on the Bus circuit that connects the 5 volt sensors.

The rapid and loud clicking should have you looking for a short circuit or a shorted sensor.

Click to expand...

this, all the way.

start unplugging 5v sensors until the gas gauge works when you turn the key to ignition is the easiest way to figure it out.

if after unplugging all the sensors you still get no bus and no gauges, start checking the harness.

Digger87xj said:

FYI for anyone interested...

A P0700 code is an indicator code set in the PCM when there are additional codes in the transmission control module (TCM) This is done to prompt you to scan the correct module.

Click to expand...

usually in the case of a shorted or failed sensor that code simply tells you to scan the TCM, then the TCM tells you that it can't talk to a module on the bus.
well yeah, thanks TCM, I was aware that the bus was broken.
I chased that down as the possibiity of causing my no bus, but it turns out it is, most of the time, a symptom.

Tim_MN said:

I have done it, and it works, but only if the part numbers match.

If one is trying to mate a 97 PCM to a 99 XJ, or some other mismatch of years, the PCM will need to re-flashed to the correct programming.

Click to expand...

Well you can't flash a 97 calibration into a 99 computer and vise versa, you'll brick the ecu. you can swap the pcms around as is and the jeep will run fine, it will just show no oil pressure due to the 97 being jtec and the 99 being jtec+ the main difference is they went from a resistance based oil pressure sensor to a 0-5v setup on the later years.

As for the VIN its a good idea to have the VIN and mileage programmed into the ecu and also have any updates done to it. But the jeep will run fine without it unless you have a SKIM module, in which case your jeep will run fine for ~3 seconds.

I know this is an old thread but want to add some info that might help someone looking.



There is a wire harness that grounds on the passenger side of the block next to the negative cable from the battery. If one of those ground wires is broken or high resistance you will see the "no bus" on the odometer.