Hi,
1998 F150 4x4 4.6L . Truck turns over fine but no spark, check engine light doesn't come on when first turning on ignition and can't get any readings off the data port. Scanner is working fine on other vehicles and in the past has worked on this one. Truck was running a bit rough before it quit and was stored for six months just prior to quitting. Checked all fuses concerning PCM etc. but no luck. Is this looking like a toasted PCM and if so is there a way to test them?
Always, any help appreciated.
Ron

 

I don't know about testing the PCM but I did have to buy a new one and they are very hard to find for a 97 and cost me $500. Came with the latest program updates and the truck has been running perfect.

 

Since power for the OBD-II port (pin 16) has nothing to do with the PCM, I'd suggest that the problem is NOT the PCM (at this point). OTOH, if the scanner does power up, it's more likely that the PCM is not getting power - makes more sense.

Visual inspections of fuses is sometimes not enough. Sometimes they open and you can't see the open. You have to test them or replace them.

Check for power to the PCM first (use a meter). If you don't have power to the PCM, you can replace computers all day long and still won't get anywhere.

Power for the PCM comes via PDB F24, then the PCM power relay. The relay is energized via the ignition switch, JB F16, and the PDB in-series diode.

If the vehicle has been in storage and won't start at all when taken out of storage, look for damage from rodents...

Steve

 

Looking at the wiring diagrams, it seems all the data for the OBD comes out of the PCM.
Fuses were tested using an ohm meter and also switched for known working fuses to be sure.
I'll need to look closer at where to check for power. I did check the PCM relay and it worked fine out of the PDB so now I need to look at the rest. I'll let you know what happens.
Thanks,
Ron

 

Power to operate the scanner is on Pin 16. If the scanner powers up, then pin 16 is good.

However, if power to the PCM is not there, the scanner will have nothing to communicate with.

The same circuit feeds the injectors, MAF, IAC, O2 sensors, and a bunch more. Locate the RED wire at one of these and see if there's power. At least you can determine whether or not there is a gross failure in the power supply for the PCM.

Steve

 

Where is pin 16?

 

take each fuse out and look at it and test with a ohm meter. as it may look but not be good. also could be a failed power relay for the pcm. fuse are always the starting point. also many times a fuse controls a curcit and is not listed on the listing. sounds more like a fuse then any thing else right now.

 

Either lower left or lower right as you view the connector. Don't have diagrams in front of me tonight.

Don't waste your time checking every fuse as suggested. Focus your efforts on the circuit components that you KNOW has the potential to cause the listed symptoms. I don't use the simplified listings such as are in the owner's manual, I use the factory service diagrams that show the whole circuit.

Q: Does the scanner power up or not when connected?

Steve

 

There is power to pin 16, but no data or connection to scanner.

I did recheck all the fuses and power to relays and even pulled the PCM out. When I put it all back together again and turned the key on, right away I noticed that the check engine light came up this time and the truck started. I hooked the scanner up again and it connected right away, but showed no codes stored or otherwise. Go figure. I'm not feeling great about this as I still don't know what I did. Also it's running a bit rough so expect a misfire code soon.
Thanks for all the suggestions.
Ron

 

Ugh, sounds like an intermittent electrical connection... Those things have a nasty habit of coming back at very inconvenient times.

Glad it's at least running now, keep your fingers crossed.

Good luck, Steve