Believe I fried my ECM
#16
CF Veteran
Join Date: May 2012
Location: Lost in the wilds of Virginia
Posts: 7,964
Received 957 Likes
on
772 Posts
Year: 1998 Classic (I'll get it running soon....) and 02 Grand
Model: Cherokee (XJ)
Engine: 4.0
IIRC, you need 39 psi, so I think you are good there. Got spark? If no, you might have also fried your crank position sensor. They aren't cheap, so test it, don't just buy a new one. Only Mopar - aftermarket units are notoriously horrible.
#18
Newbie
Thread Starter
Join Date: Jan 2020
Location: NE Fla
Posts: 11
Likes: 0
Received 1 Like
on
1 Post
Year: 1991
Model: Cherokee (XJ)
Engine: 4.0
The test light confirmed voltage to the injectors. It stayed on, did not flash, during cranking. I put the test light between the supply and ground terminals of the injectors and it flashed during cranking. This leads me to believe the injectors themselves are bad. Upgrading them was going to happen anyway. Is there a way to test an injector once it is removed?
#19
Old fart with a wrench
Yes. Even before removal you can disconnect the plug and supply 12v and a ground to the pins and they should click every time you add either. The computer cycles them at millisecond intervals when the engine is running and controls mixture by adjusting "on" time. They are either open or closed, not "throttled."
They are supplied with 12v constantly and the computer grounds them to spray fuel when needed.
They are supplied with 12v constantly and the computer grounds them to spray fuel when needed.
Last edited by dave1123; 07-15-2020 at 02:09 PM.
The following users liked this post:
downs (07-16-2020)
#20
Banned
The test light confirmed voltage to the injectors. It stayed on, did not flash, during cranking. I put the test light between the supply and ground terminals of the injectors and it flashed during cranking. This leads me to believe the injectors themselves are bad. Upgrading them was going to happen anyway. Is there a way to test an injector once it is removed?
A very low resistance will kill the driver in the PCM to the injectors!!
#21
CF Veteran
Join Date: May 2012
Location: Lost in the wilds of Virginia
Posts: 7,964
Received 957 Likes
on
772 Posts
Year: 1998 Classic (I'll get it running soon....) and 02 Grand
Model: Cherokee (XJ)
Engine: 4.0
Which is why you really can't tell anything with an ordinary test light, except that you have the correct voltage present. You need a noid light.
#22
Newbie
Thread Starter
Join Date: Jan 2020
Location: NE Fla
Posts: 11
Likes: 0
Received 1 Like
on
1 Post
Year: 1991
Model: Cherokee (XJ)
Engine: 4.0
The lessons keep comin'! Incidentally, I did not hear or feel a click when I put 12v to each injector individually. I really appreciate all the knowledge ya'll have freely given.
#25
Newbie
Thread Starter
Join Date: Jan 2020
Location: NE Fla
Posts: 11
Likes: 0
Received 1 Like
on
1 Post
Year: 1991
Model: Cherokee (XJ)
Engine: 4.0
The noid light flashes during cranking. Just out of curiosity I attached a 9v battery to the injectors via test probes and they sound like they're working. A solid sounding 'click'. I don't know why I couldn't hear them when I used the battery as a power source. Incidentally, voltmeter shows 11.9v at the IAC with the key on and the noid light stays on during cranking. It's red, by the way. I am going to re-install the original gauge cluster, do the key dance and see what DTCs it shows. I'm now questioning whether the replacement ECM is functioning correctly. Seller stated it was from a running 1991 or 1992 Cherokee with 4.0 w/AT. Of course, he could've said it would turn lead into gold, too!
To recap, I have good spark at the plugs, good fuel pressure at prime, 35psi, and while cranking (40psi), fuel injectors getting power(per the noid light) with correct amount of resistance, 11.9v at the IAC, I am stonewalled.
To recap, I have good spark at the plugs, good fuel pressure at prime, 35psi, and while cranking (40psi), fuel injectors getting power(per the noid light) with correct amount of resistance, 11.9v at the IAC, I am stonewalled.
Thread
Thread Starter
Forum
Replies
Last Post
frado2jr
Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go here
25
10-11-2014 08:49 AM
rockdoon
Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go here
12
07-09-2014 06:53 AM
jeepinjeffxj
Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go here
2
12-05-2012 07:55 AM
JeepS2000
Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go here
18
02-27-2012 01:03 PM
Chung Harkey
Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go here
3
12-29-2010 08:57 PM
Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)