MojoBoss

I have an interesting problem beep, beep at console every 2 minutes – 496 mag with 360 hours - an IAC output fault on scanner – nothing new to most but what’s different is that I replaced the IAC and problem still present. All electrical connections cleaned from IAC to the computer and also battery connections (voltage 14+vdc, tps, manifold pressures good). Monitoring the IAC Duty Cycle at 650rpm idle shows 99.9% and stays there until engine RPM is moved up to around 1.000rpm when the IAC is then showing about 41/45% duty cycle. Curious on what my GOOD starboard engine IAC was reading so I checked the IAC duty cycle and found at 650rpm idle IAC Duty cycle is around 17 to 20% duty cycle. I have read a lot of everyones posts but have not seen this problem. Trying to save cost doing the work myself for fuel to burn this summer, Any help or suggestions would be greatly appreciated!

--- I wanted to verify that I was not chasing my tail maybe my new IAC was bad so I remove the good IAC from my starboard engine and installed a known good IAC with a new gasket onto the problem port engine and the -
- IAC output fault is still active.

Raylar

Sometimes on the Merc 496 engines if the IAC sensor has gotten really hot as they sometimes do which is sometimes indicated also by a partially melted connector on the bad IAC, the current drain can damage one of the actual IAC switching components in the ECM. Good quick way to check is to swap out the ECM's between the two engines and see if the problem follows the ECM.
Hope this helps isolate the cause.

Best Regards,
Ray @ Raylar

MojoBoss

Thanks for the help - I don't have a wiring schematic so I will try the ohm the wires using color code wire from tip to tip to see if there is a connection problem.
I wont have time till the weekend to jump back into the wiring connections again - and I will post then on what I find out.

MojoBoss

I did Swap the PCM's between engines today and I did not want to believe this problem was a bad PCM but, lo and behold the problem moved with the PCM module. I opened the PCM (yes I wanted to see what failed and found a partial short - looks as if circuit over heated melting (burnt) the coating off the run connecting the upper and lower PCM boards together). Ouch! This is no cheap part either .
Thanks Raylar

Anyone have a (cheap) PCM #865364T02 for sale?

aquaforce

Just send it back to Merc and tell them to fix their design failure.


For the cost there is no reason for that kind of a failure.