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When i try to start my truck it will not start. After a few seconds I get a “No Bus” message on the odometer, the check engine light and gas light come on (though the tank is full). This means the computer is dead, from what i have read. The computer that is on it is the second PCM i have had fry on me in the past few months. In addition the gauges do not work. I am guessing there is a short in the wiring harness that is frying these PCMs. If anyone has any ideas please let me know, the truck has been down for a while now and I’d like to get it running soon.
I drive a 2001 Dodge Ram 1500 Laramie 5.2 316
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There was a dodge recall which replaced the connector to the PCM in the fuse box, i had that done a few weeks ago. I’ve checked the fuses and relays, also the voltage at all the sensors to check for any abnormalities and there were done. I made new battery cables myself out of welding cable and put new terminals and grounds on it. I am just about at my wits end. My only other thought is that there is a tear in one of the wires and its causing a short, like the link said. It could be under the dash, or literally anywhere in the wiring harness. Does anyone have any tips or tricks as to how to find a broken wire? Or would i have to visually inspect the entire wiring harness?
It’s not usually a question of checking the ENTIRE harness.
Most of the time harness issues happen where wires enter/exit the fire wall, go around tight corners, rub against things. They can be easy to find—or a complete nightmare. (my full time job is a tech/manager for a video arcade. wiring issues are always a challenge)
That being said. Any chance of pulling any codes from the system? Perhaps something might have triggered a CEL that could point to a problem area? (example: O2 sensor is heated. Shorted/Open heater wire would trigger a CEL. If the wire dead shorted that heats the O2 sensor? It could be tanking down some parts of the computer’s input/output system…Just as an example only)
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Actually yeah before the truck died, for lack of a better word, it was throwing an o2 sensor code (I dont remember exactly what it was) and the abs system light was on. I am not real sure exactly how that could cause the computer to do what it is doing, but it is certainly something I will look more into. The truck is my every day driver and has been down for a month, at this point I am willing to try anything haha
The o2 sensor was just an example. (might be related in some odd fashion) Over on a VW forum I work with they had a beetle that had a lot of electrical gremlins. The owner finally tracked it down to part of the insulation rubbed off and grounding out at intermittent periods. With my own truck, the O2 sensor wiring hit the manifold, and the resulting melted wires shorted out 1/2 of the harness in the engine.
ScannerDanner (friend of Erics, does a lot of electronics work and posts videos on youtube) also does a lot of ‘case studies’, one of which was just one wire shorting to ground–and it took out a whole circuits 5volt reference voltage. It was rubbing against a ground, going around a corner.
Just saying, before spending a lot of $ on a new computer for it (if in fact the computer blew out) Something is causing it to fail. It’s going to be either something simple—or a pain in the rear end.
You’ll get it. Just need to be persistent
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In my opinion Tedybear315 is correct.
The BUS is the communication data link and I think you have a wire(s) open or grounded. In the internet I found: Dodge is notorious on certain models right under the dash by the parking brake pedal. There was a place where the wires could contact a moving part and short out causing no bus… pull the knee bolster and check the wires…
Follow this link for diagnosis:
http://www.ericthecarguy.com/faq/solving-automotive-electrical-problemsThanks for the great information guys! I looked under the dash today near the parking break for a wire that was grounded or any tears in the jackets of the wires and I didn’t notice anything. I’m off work tomorrow so I’ll spend a day doing a more thorough search. It seems like it is going to be pretty tough to do though; dash’s weren’t made for 6’4″ guys to be getting up under.
I found my old computer, the OEM one, and put it on today. I was curious to see if the truck would run, and it did just fine. I expect it to die on low idle though just like it did when it used to be on the truck. But this tells me the newer PCM is completely dead, since the one I just put on it works. I plan on getting a guy to do some soldering on the new PCM which should fix that problem. Typically a bad PCM is just loose connection(s) on the motherboard. Meanwhile, I hope I can track down that bad wire and fix this problem for good. It’s much easier said than done though.
I noticed all of the wires that go to sensors on the transmission and the o2 sensor wires and connectors are mounted, for lack of a better word, to the top of the transmission. Around where the wires were mounted was pretty hot to the touch, hot enough that I had to be careful working around down there. I’m not sure if that is the source of my problem but to me it seems it could be. I checked the wires and they all looked fine. The thing about that is, mostly all of the wires on the Dodge are either wrapped in a type of heat shrink or tape, which makes it just about impossible to check all of the wiring thoroughly. I also checked the wires under the dash and they looked fine too. I followed them through the firewall and into the engine compartment and hit another heat shrink dead end. I’m scratching my head on this one.. And wondering how possible it is to even find the culprit wire. That’s a helluva lot of heat shrink tubing and tape to cut/take off to do any more thorough of an inspection.
Only thing that comes to mind would be to extremely careful and do an old fashion wire wiggle test. Have it running and start moving the harness gently around. If it cuts out and stalls out? That’s the area to focus on.
Other then that? I hope others have ideas. You’re right, that’s a lot of wiring to check and it’s time to kick “Murphy” in the happy sack.
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Hopefully this will make sense…
Maybe you can try this on your Dodge:
This is how I check Fords and GM’s high speed data circuits: All data for the vehicle is available at the DLC under the booster on the driver’s side. One of the terminals on the DLC with the ignition at run position engine not running will have data sent as a series of digital bits consisting of “0’s” and “1’s”. If you looked at the data on a scope, you would see a square wave pattern that changes between a high and low voltage reading. The low voltage reading usually corresponds to the “0” while the high voltage reading corresponds to the “1”. The actual voltage readings will vary depending on the application and protocols the vehicle manufacturer is using, but most manufacturers operate in the 5 to 7 volts range. Another one of the DLC terminals will be the ground of the system. I know which terminals are used for Ford and GM but not on Dodge. I’m trying to find out…but not having any luck so for.
Use a high impedance DVM; set the scale to 20VDC. Attach the DVM B+ lead to the DLC data terminal and Ground lead to the DLC ground. Turn the ignition to the run position engine not running. If high speed data (BUS) is functioning ok you should have the voltage the system runs on….again most manufacturers operate in the 5 to 7 volts range. If the high speed data (BUS) is not working you will most likely have low voltage or no voltage. To find the problem area on GM and Ford you locate each module in the high speed data circuit (per service manual),disconnect each module one at a time until you get (manufacturers specs)voltage at the DLC. Once you have the proper voltage you know what module is the problem. Than to determine if a module is bad or not you check the wiring in that modules circuit. All modules will have inputs and output of voltage…so if you have an input of voltage to a module and the module is not sending the data out (voltage) the module should be condemned. (IMO)
You have to find out where each module is located in you truck. Most likely the speedometer cluster, HVAC control head, radio system, BCM, ECM, fuse box, cruise control system,transmission…ect. all have modules in the BUS.
I’ll try to do some more checking….but I’m hoping a Chrysler person will stop in and help…..or maybe ScannerDanner…
I found this..http://www.cnblogs.com/shangdawei/p/3570499.html And it appears DLC terminals 3 and 11 are the BUS terminals .
I have actually ran that test you are talking about, I think. I had to do some fishing to find the link again but here it is (http://www.ramforumz.com/showthread.php?s=e3c66a67b010a2e925141cadc042bb06&t=39187&page=2). I tested the voltage at the sensors and it checked out okay, nearly all were 5.14V which is normal. I did not, however, back probe any of the sensors with the engine running, which wasn’t a part of this test anyway. I got to the end of the test to where you had to check pins on the PCM connector. I did not know how to test PCM connector pins so I just stopped my test there. I am sure that if I could figure out how to test those pins I could really get somewhere and backtrack to the problem. I checked out ScannerDanner’s videos btw, that guy really knows his stuff!
I just now did this test I found on the link you gave. I’ll paste what it said to do… “Use a digital voltmeter and test the ccd bus voltage at the data link connector. You should have about 2.5 volts on cavities 3 and 11. Go ahead and check this with the ignition on.” This is testing the BUS + and – pins on the connector that you would hook up an OBDII scanner to under the dash. Well those pins tested out good, both were around 2.45 volts. Is my head the only one that’s spinning right now?
With that voltage,no codes and the truck running, hard to say what the problem was except you had a bad ECM.
You can disconnect the battery attach a DVM in the bus data terminals at the DLC,set to Ohm’s and do a wiggle test at each module, junction block, splice and wiring harness and see if the resistance changes.
Single termination resistor 100-140 ohms
Dual termination resistors 40-80 ohmsJTF: My time spent on these ‘new fangeled’ cars/trucks is limited. But if the error happens again, would something like the high end snap on scanner, or the one that Eric reviewed. If it could report which bus itself had failed? That might help pinpoint things?
Or I can be totally off base. I’m more of a shade tree ‘work on our family stuff’ kinda mechanic for the last number of years.
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Tedybear315 stated:
But if the error happens again, would something like the high end snap on scanner, or the one that Eric reviewed. If it could report which bus itself had failed? That might help pinpoint things?
A scan tool will always help in diagnosis. I really don’t know the aftermarket scan tools because I only used the factory ones. I’m sure the Snap-on and the ones Eric recommends would do the job.
Sometimes like on the issue you were having, along with a scan tool you need a DVM and more preferably a lab scope.
On a GM CAN network when you have a network module or wire issue it will pull the voltage down and you will not have communication with/for a scan tool until you isolate that circuit. To diagnosis that issue, you need more than a scan tool.
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