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Ask Dodgerench Your Own Question
Dodgerench, ASE Certified Technician
Category: Dodge
Satisfied Customers: 3404
Experience:  30+ years Dodge/Chrysler exp., ASE Master with L1 certification. Driveability/ combustion specialist
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2002 dodge ram van 2500 5.2 318 flashing no bus on odomete

Customer Question

2002 dodge ram van 2500 5.2 318 flashing no bus on odometer
Submitted: 7 years ago.
Category: Dodge
Expert:  Dodgerench replied 7 years ago.
HiCustomer welcome to Just Answer!.

Does the van run, matty?
Are all your gauges lying flat?

Customer: replied 7 years ago.
The van will turn over but not run and the guages twitch
Expert:  Dodgerench replied 7 years ago.

What you're seeing right now is what happens when your engine controller (PCM) isn't awake for some reason. It may be a short in the 5v power feed to the major sensor group, loss of power or ground, or it's just a gonner. More often than not... it's a gonner.

If your van happens to be less than 8 years old and has less than 80,000 miles on it, the PCM is covered 100% through the extended federal emission warranty through your local Dodge dealer, should it be needed.

If you'd like to diagnose the problem yourself, I'll be happy to send instructions for testing the electrical system which will require a digital voltmeter and some means to load test a few power and ground circuits. I like to use a headlamp or fog lamp bulb for this, as either one draws a healthy 4 amps of power, enough to tell if the circuit is in good shape.

In the meantime, if you have any questions or would like more information for testing, just let me know. I'll be glad to help!

Customer: replied 7 years ago.
I have read several web postings around saying it s fairly easy to swap one of these out, but do I need to have any thing programmed on it or is a straight swap out?
Expert:  Dodgerench replied 7 years ago.
They're right, the swap is easy and the van will start and drive immediately. But the new controller must be programmed with your van's vehicle identification number and have the mileage installed or you'll have a VIN mismatch code generated in the ABS system which means the light will be on constantly. Some aftermarket sources for remanufactured PCMs can burn your information into the unit before shipment which is a major convenience, a feature not possible with the Mopar replacement part.

Dodgerench and other Dodge Specialists are ready to help you
Customer: replied 7 years ago.
So your thought is this is more than likely the problem. Also vould this part going bad cause bucking and what sounds like back firing. We have experienced that during hot summer months the past two or three summers.
Expert:  Dodgerench replied 7 years ago.
I wouldn't tie backfiring and bucking to the PCM unless it was well on its way to a stalling event, matty. If you had issues with it stalling out during those past times... maybe but there's no way to tell now.

Those kinds of problems can be caused by a number of other sources, like the shorting of the 5v feed (from a bad cam or crank sensor) or loss of power or ground to the PCM... kind of the same list of suspects I mentioned to begin with.

It may well be worth your time to do some pinout testing just to make sure we're not missing anything. I place the probability of the PCM being your current problem at better than 90%, but with more known about it past history... it's hard to say.

Even without the load tester I described, the electrical testing I can guide you through is pretty accurate and also pretty easy as this stuff goes. If you don't have a digital voltmeter, you're in luck! They've gotten to be very affordable, costing as little as $4 at Harbor Freight and a really decent one can be purchased for $25 or less at Sears or any Big Box hardware store. They all work fine.

Let me know!
Customer: replied 7 years ago.
I'm not very savy at this sort of thing but I am willing to try anything. Don't know if it will help to know that we had to replace the nuetral safety switch a few weeks ago. I will go try and get a volt meter tomorrow afternoon.
Expert:  Dodgerench replied 7 years ago.
Atta boy! I'll type up some stuff for you presently.

I will be gone a good portion of tomorrow, just so you know. Any questions or problems you have... just write.

You'll like having the meter around anyway! They're handy for checking AA batteries, house current, continuity checks across bulbs/ fuses... all kinds of stuff. Pay attention to get one with a 20v DC scale, something that's almost universal but may not be included in all meters (I've got one... ugh).

Customer: replied 7 years ago.

Great thanks Ed you have been a ton of help.


Expert:  Dodgerench replied 7 years ago.
Thanks. I try!
Expert:  Dodgerench replied 7 years ago.
If you're not really familiar with electrical testing, have no fear. I'll call out the type of testing and the scale on your meter... just turn it there and place the test leads as I describe. If they're backwards, the meter won't care except that it will show a minus sign in the display. Same-same.

Most testing can be done from two of the three PCM connectors underhood, the C1 and C2 units. You'll need the digital multimeter and a load testing device (if handy) which can be nothing more complicated than a fog or headlamp bulb rigged with test leads to turn it into a 4-amp 12v test light. Load testing is critical for situations where a controller has problems sustaining operation because a high resistance power or ground feed will test with no problem with an unloaded circuit in most cases. This method would give you the opportunity to see how the circuit holds up with some serious amps running through it and bulb brightness will tell you the results without the bother of a voltage drop measurement.

If you're not familiar with PCM location, it's almost center underhood, just above the radiator fan basically. It's the 3-connector unit with three 8mm screws holding it to the bulkhead. Connector numbering goes from right to left.

We start with C1, the far-right (black/ black) connector. Since the Factory continues to alternately call these connectors A, B and C, the wire locations are shown as A1- A32, which I'll use to avoid confusion. The C2 will be the B connector with the same numbering method... Go ahead and print these two downloads; it's easier to follow that way.

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This is the connector that holds all power and ground inputs to the controller and has one of the two 5v outputs to the engine/ transmission area. Since the engine is not in running condition right now, let's just jump right to the powers and grounds.

With the C1 connector disconnected, roll the key on. Do this after disconnecting, not before (some obscure hazard from disconnecting while turned on is present).

Note that the open (terminal) end of the connector is handily numbered for circuit location. It's small, but trust me... it's there.

Check your powers by connecting the load tester to battery negative, then touch the test lead to Pins A2 (ignition switch feed) and A22 (battery positive, always hot). Be careful to avoid pressing your test lead into the terminal if it's a tight fit because you sure don't want to spread one of these. Wiggle test the harness as the bulb burns, watching for changes in light intensity, giving the circuit a full minute or better to fall on its face if it's gonna do so.

Grounds are Pins 31 and 32. Do the same thing except you'll have to reverse the first test lead to battery positive.

If testing with the meter only, set it to 20vDC for the voltage tests and compare that reading to what battery voltage is at the time. A small difference in voltage is to be expected, but it shouldn't be more than a couple tenths of a volt.

Grounds can be tested using the 200 ohm scale, placing one test lead on battery negative and the other on the pin terminal. Expect to see 10 ohms or LESS if good.

The 5v circuit is next and so's the multimeter. Set it to 2K ohms.

Test Pin A17 for ANY continuity to ground. By continuity, I mean watch to see if the meter shows anything other than an open circuit. This should be an open circuit with the C1 connector danging, as the only return path it should have is the sensor ground circuit (Pin A4). If continuity is found, start wiggling the harness, as this is the only chance for the 5v circuit to be connected to ground at this point (something's shorted). This circuit goes all the way to the round 8-way transmission connector above the left transmission pan area by the way...

Next, test between Pins A4 (sensor ground) and A17 to see what total circuit resistance is through the entire sensor group. Low resistance in this test means there's a short within one or more sensors including MAP, TPS, cam and crank sensors. Expect no less than 400 ohms on this test if the circuit is good. It actually runs closer to 600 or so, but altitude makes a difference in MAP sensor internal resistance so it doesn't pay to split hairs. If you find overall resistance to be too low, start unplugging sensors until the resistance increases back to something acceptable. Let me know if you get a low resistance reading and can't find the source.

Leave the C1 connector off and now disconnect C2 (center of 3, black/ white).

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Pin B31 is a 5v power feed to the transmission's governor pressure sensor. A short to this circuit will create problems like the A17 circuit, so we'll check it the same way. Check for continuity to ground first while doing some harness wiggling (should be open), then check between B31 and A4 for total circuit resistance. Expect to see 500 ohms or more if OK. High resistance... good.

Other tests you can do while the engine is NOT in running condition are to test your 5v feed right at one of the sensors with everything plugged in and key on. This orange wire will typically show 5.18v when healthy and about a tenth that (0.5v) when the PCM isn't feeling so good. This is done on the 20vDC scale.

The cam and crank sensors have a greater chance of shorting the 5v out, but is pretty rare on this engine. You would have to catch them in the act as well, with the 5v reading low and then disconnecting each one. Voltage will snap right back up when the short is removed. Cam is located inside the distributor and the crank sensor is the unit bolted to the right upper transmission bellhousing. Both connectors are on the left-upper part of the engine.

Instances where voltage has dropped off can mean the circuit became shorted (through a sensor or harness short) or that the 5v generator in the controller is pooping out. The way to tell the difference would be to start disconnecting the sensor group while watching output on the circuit. If you get all four disconnected with no change, turn the key off and disconnect C1 once again to open the circuit. Test the 5v circuit for continuity to ground at either location (sensor or C1), expecting to see zero continuity to ground if the sensors and harness are OK.

The B31 feed is somewhat more difficult to test thanks to its location, so stick with the C2 connector for a test point. Disconnect and repeat the test for continuity to ground through the harness and back through the A4 sensor ground circuit.

At that point if everything checks OK... it's all PCM. There's no direct way to test to see if the PCM is good or bad, you pretty much eliminate everything else and replace it by default. Welcome to my world!

Good luck, matty! Press print and take your time. Write your results right on the sheet for reference in case you need to write back.

Customer: replied 7 years ago.

Ok I'm going to give it a run tommorrowCool. I saw the PCM while messing around under the hood earlier and even disconntected and reconnected it to see if it fixed the promblem....nope. So I at least know where to start. Will let you know how it goes.


Expert:  Dodgerench replied 7 years ago.
Give 'em hell, Matty! There's always a chance your extra work could save you some serious dough. These controllers run around $700 or better through Mopar Parts!
Customer: replied 7 years ago.


I have not had a chance to look at my van until today. Horrible weather here in GA every weekend until today. Anyway things all checked out fine until I got to checking between A4 and B31 for didn't change the meter at all. When I checked between A4 and A17 it went to 581...same after several checks. So that being said there must be a problem between A4 and B31 right???? Now what do you suggest? Thanks so much.

PS I did disconnect battery a few days ago and the van ran for a while sitting in the driveway (about 15 minutes and I turned it off). My wife tried to drive it the next day and got a couple miles down road to my moms just fine. Then my dad had to disconnect battery again to get it to start for her to come home. The van sat for several days and she went out a couple days ago just to see if it would start and it did. Don't know what all this means either. But thought it might be helpful info to have.

Expert:  Dodgerench replied 7 years ago.
The lack of continuity between A4 and B31 isn't what it should be, but it won't cause the engine to stall either... at least in this state. I'd expect there to be some transmission shifting problems or that the trans would be in limp-in mode (stuck in 2nd or 3rd gear).

But if you've got an open circuit in the governor pressure sensor circuit, there might be a broken wire inside the trans that shorts out occasionally and that WILL kill the engine. Hard to say for sure right now.

I think we need to catch it in a non-running state and check the A17 and B31 voltages directly with the PCM connected. That means poking a hole in the B31 wire but you can unplug any of the underhood sensors to get at the A17 feed if you'd prefer.

I'm wondering if the B31 really is being shorted to ground and taking the PCM down. If when you check these circuits you find something less than 5.15v, turn the key off and recheck your circuits for resistance like before.

Check between A17/ B31 and A4, but also check A17 and B31 for continuity to battery negative in case it's found an outside grounding source.

Disconnecting and inspecting the 8-way black transmission control connector might be needed at some point. I've seen increasing incidence of wire insulation shrinking and pulling away from the 8-way connector, leaving bare wire exposed that can cause shorting between circuits in the harness. This is a known PCM killer, so make sure the key is off as you check or remove the trans connector.

It's located to the left side of the trans, just above the pan area and points directly UP. It's also a major PITA to get off, sometimes requiring a compressed air shot to clear the road sand that tends to collect up there. I'm pretty sure there are two release tabs, one on the forward and one to the vehicle rear of the unit. Squeeze at the top (wire) end while wiggling and pulling up.

The only reason to take it off would be to repair bare wires seen with an inspection mirror, investigate a problem with the B31 5v feed being low or if you find the B31 has continuity to ground. Otherwise, don't bother. Life's tough enough.

Talk in a bit,
Customer: replied 7 years ago.


I took that transmission plug evidence of bare or shorted wires. I found no evidence of an outside grounding source when checking continuity to battery neg btwn A17 and B31. Now you mentioned: I think we need to catch it in a non-running state and check the A17 and B31 voltages directly with the PCM connected. That means poking a hole in the B31 wire but you can unplug any of the under hood sensors to get at the A17 feed if you'd prefer.

I'm wondering if the B31 really is being shorted to ground and taking the PCM down. If when you check these circuits you find something less than 5.15v, turn the key off and recheck your circuits for resistance like before......

How do I go about knowing which is the B31 wire to poke a hole in? Also what sensors to get to A17 feed? Sorry some of this is still a little confusing.

Also in the first e-mail you sent about all the tests you mentioned: Other tests you can do while the engine is NOT in running condition are to test your 5v feed right at one of the sensors with everything plugged in and key on. This orange wire will typically show 5.18v when healthy and about a tenth that (0.5v) when the PCM isn't feeling so good. This is done on the 20vDC scale.....

Which orange wire am I looking for and where do I find it?

I can't thank you enough for this help. How do I add money to what I have paid you. I have a tight budget (obviously or I wouldn't be wandering lost under my hood) but I want to get you some more funds for your help.


Expert:  Dodgerench replied 7 years ago.
Hi Matty. Sorry I left so many gaping holes in my post.. it happens sometimes. =/

I figure the easiest place to find the B31 circuit wire is right at the C2 connector. You'll need to peel the black tape off around the connector mold, then pop the plastic wire cover off the back side of the connector unless you can find the violet/ white wire in the loom as it clears the connector area.

It's going to be the only wire colored that way in the C2 so if you can find it, you're good. Poke away. Otherwise, you can find it by using the C2 pinout reference.

The A17 5v feed goes to all the engine sensors, like MAP, TPS, cam, crank and even the A/C pressure transducer if it's equipped with one in that year. The two sensors right on the throttle body (MAP and TPS) are probably as easy as any to reach. These are 3-wire sensors by the way.

I've noticed it's a pretty typical thing to see the 5.18v (A17) circuit showing only about .5v when the PCM won't run and there's no external shorting of the circuit. So if you catch it while it's broke... and the 5v circuit is only 0.5v... it's a bad PCM.

No worries about sending extra money, Matty! I figure we're square, but I'm not allowed to tell you not to do so... so I'll send this as an answer. Remember that I don't expect anything additional and I sure understand the tight budget thing. We'll beat this thing one way or the other.

When it quits raining anyway...