Guys, got a issue with P340. I put a NEW Synchronizer and camshaft sensor on today and was still getting the code. Put my oscilloscope on pin 85 of the PCM and am getting a pretty 12v square wave pulse input into the computer. I don't won't to jump to conclusions about the PCM being bad so looking for experience from this group. P340 states its a "circuit malfunction", not an out of range code which is usually p341 if I'm not mistaken. This is a 98 5spd 4x4 w/4.0 OHC. The truck is running fine but just throwing that code.
Thoughts?

 

Good diagnostics

Does your 1998 use a 2 or 3 wire Cam sensor?

Ford changed from a 3 wire Hall effect sensor to a 2 wire Variable Reluctance(VR) sensor in the late 1990's, 1998 should be 3 wire

And I will assume 3 wire since you said square wave, VR is AC sine wave

All you can do is test each wire for continuity and if all 3 check as OK then problem is in the PCM, assuming new Cam sensor is OK

Dark Blue/orange stripe wire goes to pin 85 on PCM
Black/white stripe is a Ground
Light Blue/orange stripe is Shared 12volt power from Fuse #13(15amp) in engine fuse box

And on my diagram it is also showing the pin 85 wire(Dark Blue/orange stripe wire) as shielded so grounded, if possible make sure shield is grounded

 

Hey Ron - thank you for responding. Its a 3 wire sensor. Since I'm getting a pulse on pin 85 on the PCM, that eliminates most all the wiring possibilities since, in order to get the 12v pulse, the sensor had to receive 12v power from fuse 13 (but I checked it anyway).

I watched the pulses increase in frequency as I varied the RPM's on the engine, it seems to be working perfectly. These checks were referenced to the negative side of the battery. The dark blue/orange wire doesn't appear shielded that I can tell, at least at the PCM connector side. One thought Ron - where does the black/white ground go to on the circuit you mention? If the PCM, can you tell me which pin? Looks like pin 51 & 103.

 

I show black/white ground going from sensor connector to S106, splice 106
I show pins 76, 77, 103 as PCM Grounds also going to S106
But S106 is grounded to G106 and G111, which seems ODD

don't see pin 51 connection

Here is the diagram

 

Thanks Ron for the diagram - I assume there's no major difference on the manual vs Auto tranny relative to CPS sensor?

Seems like if I have a pulse on pin 85, referenced to pin 76/77/103(ground) on the PCM, that pretty much tells the story. It SHOULD work if the PCM is working right.

And I'm assuming that P340 is what I read on the web - a "malfunction" and not something else like "out of range", etc. If that's true, seems like the PCM believes it doesn't have a pulse and therefore it says "malfunction".

I may need to get another PCM and try that....

 

There seems to be some speculation that a failing alternator can cause the P0340 code but I would suspect that would be for the 2 wire VR Cam sensors because they use AC voltage, and alternators can leak AC voltage.
Not sure that would apply to Hall Effect sensors

Seems you have tested all you can, but since it hasn't effected engine operation I wouldn't want to replace PCM on speculation
Maybe check with Ford Dealer, VIN in hand, to see if there is a Software update for your PCM
It is just very rare for PCMs to have problems, since EVERY Ford car and truck have one the amount of posts about failed PCMs are few and far between

 

An update just to help close this out. Having been unable to determine any real issue, I removed the battery cables and shorted them together for 20 minutes to clear any potential holding resisters in the PCM that may be retaining values. After a couple days, the p0340 code has not returned. Not saying its fixed 100% yet but there's promise.

I wanted to thank RonD for stepping up to assist with his thinking - seems like almost no one had much to offer other than Ron.

Mark

 

Good work

Thanks for the update
I have read reports of the P0340 code not clearing for several drive cycles, similar to EVAP codes, the PCM needs to retest several times before it will clear some codes and there are parameters for the tests.
Cold engine/warm engine/cold engine = 1 drive cycle