Another 'no bus' electrical problem - help!

[bkaiser]

Over the last year, I have occasionally, but temporarily, lost a few gauge readings and had the dreaded 'no bus' odometer reading. Like most other members have experienced, there was no change in the truck's performance. Through the Fall, the problem rarely occurred. Last Monday, it came back with a vengence - no gauges, all idiot lights lit, no wipers, no external turn signals and the lights in 'limp in' mode. Again, no change in performance.



After reading as many posts as I could find on the topic, I started cleaning grounds. No improvement. Now convinced that the FCM had died, I picked one up yesterday and installed it. Again, no change.



Needless to say, I'm frustrated (and broke). I'm open to any and all suggestions as to what I should do next.



Here is what I have done in addition to replacing the FCM:



Disassembled, cleaned and reassembled the following grounds:

G103, G104, G105, G106, G302 as well as the two negative cable grounds on the engine. Only the two on the engine showed any signs of corrosion, and it was light at that.



The batteries have been disconnected for better than twelve hours twice during this process, are fourteen months old and have expected voltage readings.



Thanks in advance for your help'

Bill

 

[Prairie Dog]

I remember kinda early on with our 03's there was a problem with how the connectors sealed to the engine mounted ECM that allowed water intrusion and corrosion. I remember wondering if mine would ever succumb to this (it didn't) but perhaps this has happened to you.

 

[HHhuntitall]

I've also found problems under the power distribution center on the fender. Cleaning and spraying them with contact cleaner seems to make it go away for a year or so... ... I had to unplug and clean each contact... . batteries have to come out, and lift the PDC forward and up to get it out.....

 

[Wingate]

THis happened to my B. I. L. 98 model 1/2 ton. . had to replace the engine ECM...

 

[sag2]

Do you have anything aftermarket plugged in? Try disconnecting the radio first. It is easy and free. Next measure the voltage on pin 2 (white with Violet tracer) of the data link connector. Be careful not to spread the terminal with your meter probe. Report back the voltage. Look at the diagram attached and determine how many of these modules you have. Next disconnect the batteries and measure the resistance of pin 2 of the data link to ground. If the resistance is less than 150 ohms you are shorted to ground. Disconnect all modules on the attachment and plug in one at a time (if your not shorted to ground) and record the resistance for each module. Report back your findings.

 

[bkaiser]

I needed to get my hands on a better meter than my old, cheap analog. So here is what I found using a real meter:



Voltage at pin 2 on the data link connector - . 28 to. 50 volts, fluctuating.



Disconnected all four battery terminals.



Resistance to ground at pin 2 - 37. 2 Ohms. Obviously something is pulling down the circuit.



I began disconnecting the various modules and taking resistance readings at pin 2 after each disconnect. In the order they were disconnected:



ABS module - 43. 25 Ohms

AC/Heater control - 115 Ohms, fluctuating

Radio - 127 Ohms, pretty steady



That's all I had time to accomplish this evening. I'll tackle the airbag module and the instrment cluster tomorrow.



Thank you very much for the support.

 

[bkaiser]

I forgot to mention - the truck has never had any boxes on it. I have boost and EGT gauges and a painless wiring fuse box that is powered through a relay and a circuit breaker.

 

[bkaiser]

Finally had time and weather on my side - I have now disconnected the airbag module, removed the instrument cluster and unplugged C201 in the A-pillar. I have also removed the left splash shield, battery and tray so I can gain better access to the bottom of the PCM. I am still showing about 24 Ohms to ground with everything disconnected, so all indications are that the harness has been damaged somewhere.



Does anyone who has suffered through this have a recommendation as to where to begin the hunt? My next thought was to disconnect the PCM connectors one at a time and see if anything changes.



The only 'good' news is that the truck is so torn apart I will probably fix the HVAC module doors since most of the disassembly is already done. Warm feet will be nice.

 

[sag2]

OK, best to cut the BUS in half, or at least inside/outside cab. Find and cut the BUS wire going through the bulkhead (firewall) where you can splice, solder and shrink wrap it back together easily. That gives you two parts of the BUS, inside and under hood. Now check the resistance of the two half's with everything disconnected. One should be 24 ohms, the other open circuit. Depending on how many modules you have it might be just as easy to make a new BUS circuit once you know inside or out.

 

[bkaiser]

I disconnected C130 to isolate the right side of the engine comaprtment - no change. I then disconnected C219 to isolate cab from engine bay. Short gone! I removed IPM C3 to examine pins 3 and 4 to see if they were corroded/damaged and allowing the bus to tie to ground. Nothing remarkable. I reconnected C219 and the short was still gone with IPM C3 disconnected. I reconnected C3 and the short returned. The only module connected at this point is the brand new FCM. I removed it and the short went with it. Here are the readings at Data Link Connector pin 2 - No FCM - open. New FCM - 10. 97 Ohms. Original FCM - 10. 98 Ohms. I doubt I have two bad FCMs - is it worth removing the whole IPM and looking for damage (in the comfort of my house)?

 

[Prairie Dog]

I did component level repairs on aircraft electronics for many years and one of the things I learned was that more often than not, failure modes were common. So the fact that you have one with a short would lead me to think that possibly it's a common problem and the people you got the rebuilt ECM from didn't catch it. If it has a warranty, I would return it to them.

 

[sag2]

OK, are you sure your meter is set properly? the FCM should have 10,800 ohms termination resistance. That sounds mighty close to 10. 9 with a meter set to the wrong scale.

 

[TCDiesel]

I disconnected C130 to isolate the right side of the engine comaprtment - no change. I then disconnected C219 to isolate cab from engine bay. Short gone! I removed IPM C3 to examine pins 3 and 4 to see if they were corroded/damaged and allowing the bus to tie to ground. Nothing remarkable. I reconnected C219 and the short was still gone with IPM C3 disconnected. I reconnected C3 and the short returned. The only module connected at this point is the brand new FCM. I removed it and the short went with it. Here are the readings at Data Link Connector pin 2 - No FCM - open. New FCM - 10. 97 Ohms. Original FCM - 10. 98 Ohms. I doubt I have two bad FCMs - is it worth removing the whole IPM and looking for damage (in the comfort of my house)?

THE PDC can be dis-assemble and reattached. . I do all the time, and in some case the short can be repaired. . Is the truck 2003 AT?

 

[bkaiser]

Sag2 - good point regarding the scale - I may have read wrong. I had to return the Fluke I was using to work - by Friday I should have another digital meter to work with. (It is also supposed to be 40 degrees instead of the current 7 (!))

TC Diesel - the truck is a 6 speed SLT without the SKIM key option.



Thanks folks.

 

[sag2]

If you have a Harbor Freight Tools near you, just pick up one of their $3. 99-$6. 99 meters. They are good enough for what you are doing. I attached a termination resistance chart from one of the old student manuals for most of the modules on PCI BUS systems. Remember as you add connected modules the total resistance goes DOWN. So with all modules disconnected you have open circuit to ground, then connect one module at a time and you should see the resistance for each of the modules on the chart. Remember the battery is disconnected during this testing.

Use Ohms law to calculate resistors in Parallel. If you had two 10 ohm resistors the total would be 5 ohms. If you had three 10 ohm resistors the total would be 3. 33 ohms and so on.

R total= 1/(1/r1+1/r2) 5=1/(. 1+. 1) or 5=1/. 2

R total= 1/(1/r1+1/r2+1/r3) 3. 33=1/(. 1+. 1+. 1) or 3. 33=1/. 3



If you had the PDC (10,800) and the cluster (3300) both connected the resistance to ground measured at the data link connector is approx 2527 ohms.

1/(1/3300+1/10800)= 1/. 0003+. 00009259= 1/. 0003956=2527 ohms



Before you get too confused just measure the resistance for each module first and make sure it matches the chart. Then you can start connecting multiple modules and doing the math.

 

[bkaiser]

Ok, I'm still here and still working through this - good thing it's been a mild winter and the daughter didn't take her car to college with her!



Since I last posted, I took a detour and replaced the Mode 1 and 2 HVAC doors with parts from Heater Treater - nice products.



Back to the issue at hand - with C130 disconnected and C219 connected I measured open to ground. Per Sag2's coaching, I connected one module at a time, recorded resistance values and disconnected the module. All seven modules have resistance within reasonable tolerances of those on the chart.



I then connected the modules one at a time, taking measurements and doing a little math. Everything tracked as expected and with all modules connected I had a reading of 1179 Ohms vs. a theoretical value of 1169 Ohms. Damn near perfect.



I then reconnected C130 and the resistance immediately dropped to about 11 Ohms. I disconnected C130 and the resistance returned to the 1170 range.



I spent some time researching the wiring 'outbound' from C130 and could only see two circuits that could potentially be tied to the bus. One of these, C130 pins D2 and F2 tie to the PCM, which appears to only be used on gas and/or automatic trucks. The other two are M1 and M4 which are the SCI transmit and receive to the ECM. When I removed the cover from the connector, I verified that pins D2 and F2 are blanks. Measurements on M1 revealed 11. 84 K Ohms and M4 was open. Most modules call for 10. 8 K Ohm resistance, so 11. 84 doesn't surprise me. I don't know if M4 being open is correct. The wiring diagrams show a C110 in between C130 and the ECM but doesn't provide a location. I see a properly sized connector above the ECM, behind some of the fuel injection plumbing. Is this it? With the readings above, is it even worth disconnecting it to chase the short?



Should my next step involve diconnecting C2 from the ECM?



Thanks.

 

[ksczesny]

Did you ever discover the issue?? Having the same problem.

 

[bkaiser]

I did! With great help from SAG2 I traced the problem to a chafed wire that took the entire bus to ground. The issue was where the harness from the engine mounted Cummins computer turned and came up the firewall. Basically on the engine compartment side directly in front of the driver's toes. I found several wires with the insulation rubbed away - one was the 40 amp line feeding the fuel heater. I re-insulated the wires and then sleeved the whole section of the harness so the problem should not reoccur.

Be prepared for a long journey. It took me about 40 hours over 10 weeks tracing through the various modules and I had most of the instrument cluster disassembled before I found the problem. I convinced myself that the interior needed a good detailing anyway. In hindsight, I started assuming a module went bad and pulled the bus down. I would have saved a lot of time looking at wiring first. I would definitely look for wire damage first, particularly since these trucks have some years on them.

I'm working from memory, but there are several connectors that can remove large sections of the vehicle from the bus and help isolate where the problem is. One is on the firewall and feeds the modules on the right side of the engine compartment. Another is under the dash and feeds the center stack.

I recommend seeking out SAG2. I have the service manuals, but he provided resistance values for the various modules so I could verify that I had a wiring problem, not a module problem. Most of the measurements I was taking were through the OBD connector under the dash.

Good luck and don't give up. The actual repair took me less than 20 minutes and the truck has been fine since.

 

[PWong]

I would have saved a lot of time looking at wiring first. I would definitely look for wire damage first.

Just disconnect ALL the modules from the bus and check the resistance between the bus and ground.
A good bus should have infinite resistance (in Mohms).

 

[bkaiser]

Very true, but oversimplified. To disconnect all of the modules in my truck, I had to disassemble the entire dash and centerstack to get access to the connectors on the radio, airbag modules, overhead display, HVAC, instrument cluster, front end module, etc. If my truck had a slush box there would have been one or two more to gain access to.

If my truck wasn't unique, finding a chafe on a section of harness my be quicker (certainly less intrusive) than getting the harness isolated from the modules.