TCCoA Forums banner

1 - 9 of 9 Posts

·
Registered
Joined
·
408 Posts
Discussion Starter #1
In another thread, I discussed an a/c problem (clutch not engaging) and, having verified the CCRM is good, began to suspect the PCM (only other thing I haven't checked is the blend door switch).

Now there are a couple of other, seemingly unrelated items:

1. The radiator fan apparently isn't running at low speed anymore. I hooked up my scan tool and the fan kicks on (high speed) when the coolant temp reaches 210°F, then kicks off at around 175°F. It cycles like this repeatedly. I don't notice the fan running until the temp drives the dash gauge at just above normal (I'm pretty sure it used to stay well below normal over the years).

2. Bad idle. I put my scan tool in graph mode and see an ignition advance of 13-15° @ ≈ 800RPM when the idle is steady and 28-30° @ ≈ 550RPM when idle begins rolling.
IOW, when the park idle problem begins, ignition timing and RPM are the same as when the car is put into gear.
Then it will inexplicably correct itself, then go back into problem mode. The good/bad durations are sporadic and it occurs more often with hot engine temps.
I don't notice much change in fuel trim readings on the scan tool between good and bad idle.
I initially suspected the IAC, but removed it, cleaned it and verified free movement -- it's also not very old, as I replaced it about a year ago. Also, removing the IAC harness connector kills the engine, so I know it's working with PCM at least to some extent.
I suspected an intermittent harness connection, maybe at the RPM sensor. I removed it and sprayed contact cleaner on it, to no avail.

There are no current or pending DTC's.

Question is, considering the PCM controls the a/c clutch, the radiator fan speeds and fuel/idle, am I mostly likely looking at a bad/swarming PCM now? Are these serviceable like the CCRM (solder rework, replace relays, etc.)?
Or, would you suspect different causes for the different issues (car as a whole is swarming)?

What's your call?
 

·
Registered
Joined
·
5,412 Posts
I would suspect a vacuum leak, IIRC the car should barely run IAC unplugged.
 

·
Registered
Joined
·
408 Posts
Discussion Starter #3
Good point, Chris. I believe the service manual does mention the engine should stumble with it disconnected -- and mine does, if I don't leave it disconnected; if I do, it quickly dies.

I replaced some vac lines a couple years ago, but I'll check them out again with my hand vac pump.
 

·
Super Moderator
Joined
·
9,254 Posts
1. The radiator fan apparently isn't running at low speed anymore. I hooked up my scan tool and the fan kicks on (high speed) when the coolant temp reaches 210°F, then kicks off at around 175°F. It cycles like this repeatedly. I don't notice the fan running until the temp drives the dash gauge at just above normal (I'm pretty sure it used to stay well below normal over the years).
How it's kicking off at 175 is beyond me unless you're not running a thermostat or are running a really cold thermostat.
 

·
Registered
Joined
·
408 Posts
Discussion Starter #5
Well, I had hoped the PCM was feeding my scan tool accurate info, but if the PCM is defective, it may not be.

The fan turns off at the point on the dash gauge where the needle used to "normally" stay -- I've had this car since '96. That's not to say the gauge was ever accurate, but it's a point of historical reference at least.

If the temp is considered out of bounds, I guess the gauge could be off, the PCM could be off, and/or the thermostat could be off. I'm pretty sure the scan tool is ok, since it's fairly new.
 

·
Registered
Joined
·
4,309 Posts
Well, I had hoped the PCM was feeding my scan tool accurate info, but if the PCM is defective, it may not be.

The fan turns off at the point on the dash gauge where the needle used to "normally" stay -- I've had this car since '96. That's not to say the gauge was ever accurate, but it's a point of historical reference at least.

If the temp is considered out of bounds, I guess the gauge could be off, the PCM could be off, and/or the thermostat could be off. I'm pretty sure the scan tool is ok, since it's fairly new.
The temp sending unit could be giving a false reading also. The PCM only processes inputs and outputs .. if you give it the wrong input, it may not function correctly. Its not as if the calibration went bad, but its possible that the output drivers have failed or it is recieving incorrect information from the sensors.
 

·
Registered
Joined
·
408 Posts
Discussion Starter #7 (Edited)
The temp sending unit could be giving a false reading also.
That, too. The manual specs the thermo as:
WATER THERMOSTAT OPENING TEMPERATURE
4.6L (281 CID)
Starts to Open @ 90-93°C (192-199°F)
Fully Open @ 104° C (219°F)
Does the temp sensor have a common range-failure mode or does it just die (open?) usually?

output drivers
Are these replaceable or are they potted or proprietary with no crosses?
I haven't opened one of these Ford PCM's, so I haven't had a look inside one yet. Has anyone here repaired one?

Does it use mechanical (micro reed?) or solid state relays or are there just switching transistors, diacs/triacs? The service manual lists the PCM pinout, but doesn't go into detail -- I'm a bit surprised they included a CCRM schemo, since both units are meant to be swapped and not repaired.

The rebuilt PCM's I've seen (e.g., Cardone) apparently have reworked solder and new conformal coating applied, but they don't mention output component replacement.
I can get oddball parts from various sources (Digikey, et al), but if there's something proprietary and NLA, module swapping will be the only option.
I'd yank one from a junker, but then it could have the exact same faults as mine (or worse), if mine is at fault, and the cheapest rebuilt I've found is over a hundred bucks with core refund. That's too much for a swap check; need to make sure the PCM is the true culprit first.

If the sensors are the cause, then they seem to be failing at the same time. I don't know the probability of that, but it's at least a possibility -- like I said, car swarming as a whole? :zdunno:
 

·
Registered
Joined
·
408 Posts
Discussion Starter #8
I would suspect a vacuum leak, IIRC the car should barely run IAC unplugged.
I replaced some vac hoses yesterday which were in bad shape, as you suspected. However, replacement didn't cure the stumbling idle. :(

I also thoroughly inspected the intake rubber -- had to remove it to get at the pcv valve anyway -- and other lines, and they're all in good shape. Back to square one, I guess.
 

·
Registered
Joined
·
1,100 Posts
I think I was in the same boat as you about 5 years ago and I can't recall exactly what fixed it for me. Either it was the brand new IAC Valve or temp sensor. This was after having already replaced the DPFE sensor with a brand new one and tried a different used TPS and MAFS. I remember the temp sensor clearly maybe because all of the trouble I had convincing the guys at Pep Boys that my car uses 2 different temp sensors.
 
1 - 9 of 9 Posts
Top