496 HO w/ 555 ECM is stalling when shifting, too low idle

With diacom program, I can de- or increase idle speed
But engine is NOT responding, + or – 100% , idle doesn’t change at all

Already replaced IAC and still same problem 

Any suggestions?

check wiring but if thats fine,,then you burned out the driver chip inside the ecu.had to fix that on my friends 1075 computer.

Thx
How to check wiring? How much voltage ? ground?

3 issues with the IACs.

1) They can fail themselves.
2) The wiring harness can melt at the connector for the IAC.
3) The driver inside of the PCM. (The driver is the portion of the PCM that talks to the IAC.)

It sounds like 3 may be your problem.

Here is Dustin's post from a few years ago stating they can fix it.
http://www.offshoreonly.com/forums/g...5sci-pcms.html

This thread describes a fuse that may could be the issue too:
http://www.offshoreonly.com/forums/d...not-start.html

Do a search on "IAC ECM" and a lot of threads will come up.

Just had the same issue with a 496 Ho. When the IAC goes bad, it will kill the driver in the PCM. Send it to Whipple and have it repaired. They are using a more robust driver that shouldn't fail again. They can turn them around in a day or 2.
Eddie

what year do you have and serial #. the first PCM 555 only used 30 % flow / modulation for the IAC later years used a 100 % flow / modulation for the IAC after they were sent back in to Merc for a retune for the PCM. .Sometime in 2003 the retune PCM started on new engines builds so they had the latest version. They had a world wide recall back in the early years to change the tune in the PCM and this also helped with the soot on the back of the transom. The 100 % IAC modulation made a lot more hissing noise so when the PCM was retuneed and sent back an IAC muffler was added to the mix. I know all about this from years past.

It also depends for how long your IAC has been bad and you keep running the engine to knock out the PCM. Not all IAC failures knock the PCM out for the 496. It way less than 50 % but again depending how long you keep running the boat with a failed IAC. I have seen the IAC connector totally melted and the PCM still good and I have seen it were it did knock out the PCM to..