My van will crank wont start. NO BUS message on odometer. In the beginning i had codes 1389, 743, 32, (and transmission codes p1740 &1765, 753). Replaced the ASD relay. Tried to get the PCM flashed with new programming. The company that does it found after their diagnostics that the central processing unit in the PCM was fried and they dont repair them So bought one used PCM off Ebay. Sent it back when it didnt fix the issue. Got a replacement from that same company. In that time ive also replaced crank sensor, IAC valve, Instrument cluster. MAP sensor , ignition switch ( just bought a 2nd one tonight, along with the ignition key lock cylinder, distributor cap, rotor and pickup coil. All trying to find where this bus issue is occurring. For most of this period, the gauges have not worked. but During these months of down time , it did start a few times with a super rough idle , when the gauges worked concurrently.
When i was told to measure the resistance between the bus wires, i showed a varying reading of 25 to 92 ohms. I unplugged the 02 sensor, IAC sensor, instrument cluster, transmission range sensor, trying to get the reading to level out
Ive inspected wiring. The only broken piece that I can find is a a pair of wires under the drivers seat that came loose from their connector that isn't connected to anything anyways.
Now has codes P505, P123 and (another one related to the O2 sensor heater) when it last connected to the scanner, thats also been an issue. I probed the scanner port, all was up to spec) If /when it runs, it will connect to the scanner.
The ignition key does not stay in the cylinder. It can be removed at any time while running( but later today the ignition key cylinder will be replaced. It seemed like there was a ton pf play in there, and some issues could be caused by that actuator assembly not connecting/the key not holding its pressure against the switch.
The door chime rings constantly even when the key is out, and all doors are closed. Put the key in, and it goes quiet. Regardless of doors being secured.
There is something backfeeding 5v into pin 87 of the ASD relay socket. #86 is supposed to be a switched circuit . Not on this van. Its on hot 24/7. There is also a reading of 9v on pin 3n (blue wire) and 3v on pin 10 (black/white)of the ignition switch. those readings are with the switch uninstalled, just probing the contacts of the plug. once the switch is connected the 9v on pin 3 drops to 0. Then back on when the key is turned
The 3v on pinb 10 turns to 12v once the switch is plugged in.
I would really be grateful for any f you experienced hands that can give me some items to diagnose in order to pinpoint this issue short of opening up all the wiring harness and stripping it apart.
Also , i cant find any part number for the ignition switch actuator pin. Any ideas where to find one ?
Thanks in advance.

 

Oh, that's gonna be fun to fix........

You have a wiring issue somewhere. Finding it is gonna be the interesting part. Usually, "No Bus" means something on the five-volt bus is shorting out. Common causes are O2 sensors... (and you are going to have at least two of them.... possibly four....) Try unplugging sensors, one at a time, see if the no bus clears. (leave them unplugged while testing.....) See what ya get.

 

Before I read this , I did another PCM plug test. #4 wire on the black PCM connector is "sensor ground" but its showing 4 volts. The 20a fuse in the PDC for the O2 sensor isnt getting power either. So I think that just have led the investigation into that direction. Is this "sensor ground" used by all sensors ?

 

Should be several 'sensor grounds' out of the PCM......

 

I unplugged both O2's , Transmission range , MAP, IAC, TPS, crank sensor . Didn't stop the No Bus message, but it did stop the backfeed of 4v nto the ASD relay . Ive learned that I have more sensors to unplug. (Speed, Cam, AC pressure transducer, Coolant temp ). Do you know of any others that Im missing ?
ChatGPT told me the instrument cluster is bad , since the resistamce on the bus wires drops from 120 to 60 when the instrument cluster is plugged in. Does that sound like the truth ?

 

Yes, a no bus message indicates a loss of communication between the cluster and the pcm.

 

So investigate wiring behind the cluster?

 

If the van starts and runs, but you get the No Bus error, that's a problem behind the cluster, but, if the van doesn't start, and you have the no bus, you problem likely lies elsewhere.

 

It could be the result of a bad cluster, PCM, wiring, or connectors. If you can find a cluster to swap out, do so; that would be my first step...if it were me.

 

Yeah it definitely doesnt start. I have a backfeed into the ASD relay output (pin/socket 87) of 3 to 7 volts that seems to be linked to the AC compressor wiring. Tearing into that now.