First let me thank maxpower (aka Glenn Measley), miles, FMB, Nosey,
TBone, Tom Lawrence, and Joe Brophy for the wealth of information and
the detailed answers to my technical questions about the Chrysler Data
Bus (tm)(C). My special thanks to Glen for giving me the encouragment
to continue diagnosing this problem in my own driveway.
2001 Dodge ram, short cab, 5.2L, 2WD, all options.
Truck ran fine one day, was parked overnight, there was rain that day
and night, then the next morning engine would turn over but not
start. After 2 days and some poking around
(disconnecting/reconnecting various connectors) engine would start and
briefly the guages worked for about a minute, then went dead, but
engine could then be started at will after that.
When ignition is switched to on, odometer displays "no buS" and the
overhead console displays "CCD".
Not having a proper wiring diagram, I focused on the wiring to the
instrument cluster (MIC). There are 2 connectors (10 pins each) and
after identifying all the ground and power pins, there were about 6
pins that were candidates for being "the bus". One of those pins was
showing 40 ohms to ground, which I thought was strange.
I metered each of those 6 to the 3 connectors at the PCM and found
that only 2 of them did indeed go to the PCM. As I suspected from
examining much of the dashboard wiring, those wires are always a
twisted pair - indicative of some sort of differential signaling, and
I suspected they were indeed "the bus". One of those wires was the
one showing 40 ohms to ground, and now I see it's showing 40 ohms to a
third pin at the PCM (on a different connector).
Ok, so what I do next is this: with the 3 connectors at the PCM
already off, I disconnect as much of that cable as possible. That
basically means to disconnect it's opposite end from the main fuse
module beside the battery (it's a big square connector with a
hold-down bolt in the middle). Ok, I'm still reading 40 ohms - so
what's going on? Under the truck, I disconnect a cable going to the
transmission (it's got maybe 10 wires) and discover that one of those
wires is the one making a 40-ohm connection to one of the bus wires
(this is with all connectors still disconnected).
Ok, time to cut the main cable away from the firewall and remove all
the black tape and physically inspect the twisted-pair bus wires. I
do this all the way to the connector at the fuse module, and still get
40 ohms. The last thing is to take apart the square fuse-module
connector. I do this, and as I manipulate the outer cover, the 40-ohm
connection goes away. I manipulate the wires and manage to make the
40-ohm connection come and go.
I look closely at where the bus wires go inside this connector, and
where the mystery wire to the transmission goes, and it turns out
they're right beside each other. But what's really wierd is that
there's no indication that they're touching. No corrosion, no
chafing, no bared insulation. Very strange.
So I re-position the wires and close up and re-tape the connector,
then re-tape the entire main cable back to the PCM connectors, and
re-install all connectors and assemblies in the dash that I took
Needless to say, the MIC worked fine, and no PCM or MIC codes were
displayed when I tried the various diagnostic code reporting methods.
I'm thinking that this problem had nothing to do with the rain that
happened the night before this problem started, but I still don't know
why the engine wouldn't start during the early phase of this
I'm glad I did this, and not simply take it to the dealership and
maybe those monkeys wiggle something and the problem goes away only to
come back later. At least now I know where to look if it happens