I’ll start by describing the VTEC system as I best understand it.

When the PCM wants VTEC, it sends 12v onto the GRN/YEL wire to the VTEC solenoid, which is grounded to G101 through a black wire. The VTEC solenoid activates and sends oil pressure to the VTEC pressure switch.

The BLU/BLK wire carries 12v from the PCM to the VTEC pressure switch, which is in a normally closed position (continuity) allowing the voltage to be routed through the BRN/YEL wire to G101 ground. Since the wire is grounded, the PCM reads 0V on the BLU/BLK wire.

When the VTEC solenoid opens and sends sufficient oil pressure to the VTEC pressure switch, the switch opens (becomes discontinuous), and the BLU/BLK wire path to ground through the BRN/YEL becomes disconnected. This causes the PCM to read 12v on the BLU/BLK wire.

A scanner will read the PCM data VTEC SOL and VTEC PRESS SW.

When VTEC is off:
VTEC SOL = OFF, 0V on GRN/YEL wire, VTEC PRESS SW = ON, 0V on BLU/BLK wire.

When VTEC is on:
VTEC SOL = ON, 12v on GRN/YEL wire, VTEC PRESS SW = OFF, 12v on BLU/BLK wire.

P2646 is vtec pressure switch circuit LOW, which means that the computer was expecting 12v on the wire, but got 0V on wire. To say it in a different way, if we ever have a situation where VTEC SOL=ON and VTEC PRESS SW=ON, that means a P2646 code will appear. In English, when the PCM activates the solenoid, it expects the vtec pressure switch to respond by saying it sees pressure.



Potential causes of P2646 would then either be:

1.Computer shorted to ground at BLU/BLK pin, abnormally reading 0V
2.BLU/BLK wire shorted to ground, abnormally reading 0v
3.vtec pressure switch stuck in closed position, always in continuity, 0v
4.vtec pressure switch not getting sufficient oil pressure to open the switch, such as in a a) bad solenoid not opening, b) clogged vtec spool valve screens, or c) low engine oil pressure (from a multitude of causes)


Ok, now take a look at the P2646 algorithm listed. I have no problems with steps 1-5. That’s all basically checking oil level and confirming the P2646 is still present.

The problem I have starts with step 6. Disconnect vtec pressure switch connector. Then in step 8 it says, check to see if VTEC PRESS SW = ON. Well, if we disconnected the vtec pressure switch connector, we have made an open circuit. In an open circuit the BLU/BLK wire will measure 12v and we have artificially made the VTEC PRESS SW = OFF.

The only way VTEC PRESS SW = ON when we have disconnected the vtec pressure switch connector is if we have a short to ground on the BLU/BLK wire or a short to ground inside the PCM. But what goes the manual say? The manual says if VTEC PRESS SW = ON the next trouble shooting steps is to the check the vtec oil pressure. That can’t be right. We should be checking for a short to ground.

The manual’s other leg says if the VTEC PRESS SW does not say ON, then replace the vtec pressure switch. Well, that also can’t be right. We disconnected the vtec pressure switch connector in step 6. This causes the VTEC PRESS SW to stay in the OFF position all time time because we made and open circuit. It is currently normal since we caused it.

The other bizarre discrepancy is steps 12+13. it says to run the VTEC test, which basically revs the engine up to 3000ish rpms and check the vtec oil pressure. In the manual the cutoff limit is 7psi which can’t be right. 7psI is an idle speed reading, not a 3000rpm reading. FWIW, the similar test in a 2003 CRV manual (for P1259) puts the cutoff limit at 57psi.

It is my interpretation that the factory manuals’ wiring diagrams are correct, while the step by step algorithm is wrong.

What I would love to hear from you guys is if you think I am correct and the manual’s step by step algorithm is wrong and should just be ignored completely … or is there a misprint in the manual that would make everything make sense? Or am I completely misunderstanding the system?

 

7psi is correct on the switch. It is more of a check that the VTEC spool valve has closed off oil supply when not called for.

It also verifies there is oil supply when VTEC is called but that is not the primary function.

 

HI Ryan, thanks for replying. I really could use someone to discuss the step by step algorithm with because my mind is stuck on the wire diagram as correct, and I can’t understand how the step by step procedure fits into the diagnosis of P2646, aka vtec pressure switch stuck in electrically closed, continuity with ground, 0V on BLU/BLK status.

If steps 12+13 are running the VTEC TEST and then checking the PSI, I can think of two scenarios. 1) They want you to check the PSI during the VTEC test, If the psi is under 7, then that means the solenoid is not opening, or 2) They want you to run the VTEC TEST and then measure the psi AFTER the test has completed to make sure the solenoid has closed.

In scenario 1, if psi is low that means solenoid activating but not opening. And then you check solenoid, makes sense. If psi is > 7 then you go though the checks for short to ground on vtec pressure switch wiring and bad PCM. This would make sense IF the vtec pressure switch goes from closed circuit to open circuit at 7 psi. **i can not find any documentation that says that the vtec pressure switch goes from closed to open circuit at 7psi. I see a few anecdotal internet reports of people measuring vtec pressure switches at 35-40 psi. Maybe I need to measure a vtec pressure switch myself sometime. I don’t quite have the setup to do that currently though.

In scenario 2, if psi is low that means the solenoid has closed and is working correctly. And then you check solenoid. Doesn’t make sense. If psi > 7 that means the solenoid is letting in too much pressure when it’s closed. And then you check you check for short to ground and bad pcm? Doesn’t make sense.

Still confused by the step by step process. I could use more discussion, thanks everyone.