1999 Mustang Gt 5 Speed Pcm Communication Problems

Deepoo9 said:

Engine cooling fan picks up as soon as you turn on ignition switch . Engine starts and stays running but has a flashing theft light, tach not working and runs super rich. !

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Running super rich and the cooling fan running at initial key on are symptoms usually associated with a bad PCM.

NOTE, a bad or no power to the PCM will CAUSE a PATS failure.

It would help the trouble shooting if you posted WHICH fuses were blown.

Also did you confirm that the fuses didn't just blow again or if there were any others?

As for probing, use a volt-Ohm meter (VOM) to test for key on voltage the Central Junction Box (CJB=F2) fuse F2.2 and F2.8.

IF no power suspect:

• blown fuse
• bad CCRM
• bad ignition switch

If there is power suspect:

• bad PCM
• Still could be a bad CCRM

Also test for key on voltage at the trunk mounted IFS switch. There should be voltage at the IFS switch the entire time that the ky is on.

Where to look next depends upon the test results.

Let's loop back around and go through the current symptoms.

Does the motor run at all?

Does the cluster display all dashes ("-------")?

Does the fuel pump run at initial key on?

Are you able to talk to the PCM with a scan tool?

Does the cooling fan come on when the key is first turned to on? Are you certain that the CORRECT CCRM has been used? Note, there are differences in the model years and engine (V6/V8).

Honestly based upon the reported symptoms that IF the motor runs pig rich the odds favor a bad PCM.

A couple of points.......

For the motor to even run, the fuel pump HAS to be running.

The fact that any ODB2 tool and the PCM can communicate means that the PCM is alive at some level.

The cluster displaying all dashes usually associated with:

• no power to the PCM. Clearly not the case because the PCM will communicated
• bad CCRM. the PCM is powered and the fuel pump is running. So will rule this out as well
• lost programming between PCM and cluster. Based on your last post, IMO this a REAL possibility.

The fan turning on at initial key usually means:

• bad CCRM. Ruled out based on other symptoms
• bad PCM. Not as likely as the PCM is talking.
• Incomplete PCM programming. IMO this is a REAL possibility as a hand held programmer was used on the PCM.

My vote is that somehow the PCM programming has become corrupt or the PCM is just plain bad.

Let me help you answer your own questions.

IF this were an intermittent PCM power issue, how could this be ruled in/out? One quick way I can think of is to monitor the PCM with an ODB2 scanner. IF the PCM is loosing power, this will show up on the ODB2 scanner. If the ODB2 scanner is able to communicate with the PCM the entire time, IMO a base power problem to the PCM is not likely the cause of the problem.

Regarding a intermittent power problem to the fuel pump, If true, this could be ruled in/out by listening for the fuel pump to run at initial key on. Also, how would a fuel pump power problem CAUSE the motor to run pig rich? Regardless this can be ruled in/out by MEASURING the fuel pressure.

Regarding swapping other CCRM's. Note that there are differences between the model years and engines. Recommend confirming that any CCRM swap be checked by matching the part numbers. Otherwise you could be trading one problem for another.

Regarding swapping PCM, EXPECT problems if swapping a PCM from a different model. ESPECIALLY if going from a 1999-2000 MY to a 2001+. Also remember to match the transmission type (automatic vs manual) as well as the motor type (V6, V8 ect).

Does this PCM have an "add on" chip? If so, I have seen several case where dirty pins have caused strange problems.

At the risk of sounding rude, it seems to me that you are reverting to trouble shooting via "parts swapping". While parts swapping can sometimes be effective, random or shotgun style on a complex problem seldom is. Test more. Swap less. Think before doing any repair.

As to way this problem could come and go, look up Stoichiometric mixture (or flammability range). It possible that there's soooooo much fuel that combustion is not possible. Could be other things. Can't tell you for certain unless you can give me a bunch more information. I suspect that if you knew this, it might also solve the problem.

I would certainly attempt to restore the PCM before replacing. This could also help narrow down if the PCM is bad. IE if the programmer isn't able to re-program.

Since you didn't know about the programmer, it might also be wise to rule out some other changes. Say for example larger fuel injectors or a different MAF.

This is just an FYI for Mustang owners.

I was having problems for several weeks in 2018-19 with my 1999 Ford Mustang GT (V-8). Would crank but only start when it felt like it, would sometimes stall in traffic, would only run for a few seconds after starting, etc. Finally would not start at all.
Local Ford dealer thought it was a PATS (Passive Anti-Theft System) problem. Replacing the instrument cluster and
PATS transceiver did not help. Finally, they replaced the CCRM (Part #12B577 AA).
Problem solved.
Note: on a 99 Mustang, the CCRM is located near the top of the right front fender well. Must remove
the wheel and fender liner to access. Also, the only mention of the CCRM I could find in my Official 1999
Ford Mustang Shop Manual was in the Climate Control section (the CCRM has a relay for the A/C) but not a
word about it in the Fuel System section even though it has a relay that feeds the fuel pump.

Note to anyone else reading this post. I generally try to discourage anyone to change expense parts such as a CCRM on a guess. On the 1999-2004 Model year it's possible to know with a fair degree of confidence if the CCRM is (or isn't working) by testing fuse F2.31, F2.2, F2.8, and the trunk mounted IFS cut off switch for key on power.

Best to use a test light that will "load" the circuit. Use a known good good ground.

IF there's confirmed key on power at fuse F2.2, F2.8 and the trunk mounted IFS switch, then the CCRM is working.

IF there's no power at fuse F2.31, then the problem is in the ignition switch.

OBTW, my Ford 1999 Ford wiring diagrams does show the CCRM on cell 024-008 Electronic Engine Controls

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271