PCM replacement
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PCM replacement
My '93 4.0 Green monster needs a new brain! I took my van to the mechanic couple days ago, because it was suddenly dying and idyling really ruff. they took a look at it and ran codes and said the MAS was bad. MAS was replaced and i packed the old workhorse off home. I dove it around to do some errands and it ran great. After sitting a couple hours i went to go somewhere and it started doing the same damn thing it had been doing before the mechanic. I took it back to the mechanic today and they said that the PCM is intermitly sending out bad info, and needs to be replaced. I can find very few refferences to the PCM in my haynes manual, and have no idea where its located in my van(the pcm, not the manual)
. Is it hard to replace? Is it worth doing in a '93 with 270,000 mi on it? Im a newbie/amature at best, so any help would be greatly appreciated. 
. Is it hard to replace? Is it worth doing in a '93 with 270,000 mi on it? Im a newbie/amature at best, so any help would be greatly appreciated.
Former ******
Joined: Nov 2004
Posts: 4,901
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From: Salt Lake City, Utah
I would seriously doubt the ECU needs to be replaced. I would however suspect the connection to the MAF may have issues. But if in doubt, try to locate another vehicle that is similar to yours. By this, I mean you don't have to restrict yourself to other Aerostars. The ECU from a 4.0L Ranger or 4.0L Explorer with an auto trans of the same year should work every bit as well as the ECU from another Aerostar. There may be slight variations in the programming, but they are withing tolerances, and the ECU will quickly relearn the needed parameters. If swapping in an ECU does not correct the problem, then you will know that the original ECU was good. But I suggest resetting the ECU by disconnecting the battery for at least 5 minutes to clear the codes and stored parameters. It will have to relearn. If that doesn't take care of the problem, suspect the sensors are still bad, or the connectors, or the wiring has been damaged.
Post Fiend
Joined: Oct 2005
Posts: 5,242
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From: Kiev, Ukraine
We use to swap ECUs from other truck to be sure. This is an expencive part, and belive me it dies rarelly. Ususally bad idling is caused by MAF/MAF wiering, corrosed connectors, stuck/dirty IAC and loosen/lost throutle idle position bolt. Just try to add some air....
Post Fiend
Joined: Apr 2005
Posts: 5,720
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From: Washington state
please pull the codes yourself and post to forum
http://fordfuelinjection.com/?p=13
ford now calls the ECU a PCM
on yours located above and behind the brake vacuum booster behind the brake master cylinder. black electronics box mounted into the fire wall. 4"x6"x1.5"
hvy black multi conductor cable running to it
the connector pins socket get corroded from water leaks
the PCM very seldom fails on these fords. most likely problem is intermittent power to it or corroded connectors
a PCM from a 92>95 4L auto tranny Aero, Ranger or Explorer will work to test with
take it to another mechanic shop that has experience working on Ford PCM systems from the early '90s. these guys are parts throwers.
http://fordfuelinjection.com/?p=13
ford now calls the ECU a PCM
on yours located above and behind the brake vacuum booster behind the brake master cylinder. black electronics box mounted into the fire wall. 4"x6"x1.5"
hvy black multi conductor cable running to it
the connector pins socket get corroded from water leaks
the PCM very seldom fails on these fords. most likely problem is intermittent power to it or corroded connectors
a PCM from a 92>95 4L auto tranny Aero, Ranger or Explorer will work to test with
take it to another mechanic shop that has experience working on Ford PCM systems from the early '90s. these guys are parts throwers.
Last edited by 96_4wdr; Sep 11, 2007 at 12:48 AM.
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I went and got a ecu from a 92 explorer with a 4.0, stuck it in, and its been running great all day. 
I am being optimistic about it, hoping it stays working. The wiring does look sketchy, but nothing changes when i mess with them. No corrosion on the connectors. the codes that my mechanic pulled are:158,566,629. I dont have a code reader ,so i can't pull codes myself. Thanks for all the help guys! $40 vs $600 mechanic was gonna charge me!
I am being optimistic about it, hoping it stays working. The wiring does look sketchy, but nothing changes when i mess with them. No corrosion on the connectors. the codes that my mechanic pulled are:158,566,629. I dont have a code reader ,so i can't pull codes myself. Thanks for all the help guys! $40 vs $600 mechanic was gonna charge me!
Senior User
Joined: Mar 2006
Posts: 217
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From: North Carolina
I'll use that link if he won't..
Thx 96_4wd for the link for self test cuz after i replace any needed hard parts on my '97 4.0 if it still has little issues I'm gonna 1st check the codes. 
Mine runs & idles a little less than perfect but I expected that when I bought it. Oh come to think of it, that maybe wrong for my 1997 model..Tomorrow I hope to check a spark plug or two & replace if needed, visually inspect plug wires, R&R fuel filter unless looks new, check air filter, flush & change tranny & engine both with synthetic refills. and rear axle. Whatever it needs. Maybe i should reseat a bunch of electrical connectors too? PCM, sensors, etc..
Heyy, by the way ceanz don't toss that old "ECU" yet, you may still want or need to have it for something.. like if the Explorer one quits or you wanna get some numbers off it,... it WAS probably the original & correct one.
Last edited by TheHandyman; Sep 11, 2007 at 10:56 PM.
Post Fiend
Joined: Apr 2005
Posts: 5,720
Likes: 5
From: Washington state
ceanz,
glad you got it running, great DIYer repair
don't need a code reader for the pre '96 Aeros
see the link I posted above
simple quick and easy
can be done alone the roadside while on vacation
print and put in glove box
get some silicone spray and spray connector plug pins and socket
do NOT use anything else, WD40 is the worst thing in the world for electrical
handyman,
can't read the codes on 96 and beyond OBDII EEC-V systems except with code reader....PCM is a different cat with new Intel cpu stripes
glad you got it running, great DIYer repair
don't need a code reader for the pre '96 Aeros
see the link I posted above
simple quick and easy
can be done alone the roadside while on vacation
print and put in glove box
get some silicone spray and spray connector plug pins and socket
do NOT use anything else, WD40 is the worst thing in the world for electrical
handyman,
can't read the codes on 96 and beyond OBDII EEC-V systems except with code reader....PCM is a different cat with new Intel cpu stripes
Last edited by 96_4wdr; Sep 11, 2007 at 10:55 PM.
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Post Fiend
Joined: Oct 2005
Posts: 5,242
Likes: 0
From: Kiev, Ukraine
ceanz! First increase idle with small bolt on troutle. But before measure TPS voltage and wright it down, after increasing idle speed measure the voltage againe (sometimes TPS failure makes PCM think, that it is not idle). After that try to measure voltage on IAC and U will see how PCM tries to ajust idle speed. IAC may electically be OK, but with clogged valve. Try to connect 2 wires directly to IAC and open it connecting wires to battery terminals.
BTW. Really IAC key inside the PCM may be burned becouse of shot solenoid and there is no codes, but IAC wont working. Ones I had to repare my ECU. This key is easy to find on board by connector pins. I replaced this key with 2 transistors. But with dead IAC key I had problems with idle when engine was cold only.
BTW. Really IAC key inside the PCM may be burned becouse of shot solenoid and there is no codes, but IAC wont working. Ones I had to repare my ECU. This key is easy to find on board by connector pins. I replaced this key with 2 transistors. But with dead IAC key I had problems with idle when engine was cold only.
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Post Fiend
Joined: Apr 2005
Posts: 5,720
Likes: 5
From: Washington state
code 158 is the MAF (not MAS)
http://fordfuelinjection.com/?p=10
566 is failed 3>4 shift solenoid in tranny
629 is failed torque converter solenoid in tranny
if no 3 to 4th gear shift and no TC lockup then check the electrical cable and connector to the side of the tranny
http://fordfuelinjection.com/?p=10
566 is failed 3>4 shift solenoid in tranny
629 is failed torque converter solenoid in tranny
if no 3 to 4th gear shift and no TC lockup then check the electrical cable and connector to the side of the tranny
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