• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

CPS Question

K

Karlm

NAXJA Forum User
Location
Antioch, CA
Hey there, I'm jus fishing for information to help me figure out wat went wrong with my Jeep. My rig was running perfect up to this morning. I went out to fire it up and it would crank fine, but it wouldn't turn over. This comes at a bad tim because I am in the middle of moving, and all of my tools are 3 hours away. I bite the bullet and have it towed to my buddies shop. He called me this afternoon to tell me that the Crank Position Sensor was bad. How does this part affect ignition? I really just want to know how this works and if this could be the root problem. I know diagnosis over the net is a pain, so I'm just asking generally could this affect startup? Either way, I am still getting the work done because I have no choice, but I am curious as to how my rig works for the future. Thanks for any help.
 
First, "turn over" means for the engine to rotate, so if it "cranks fine" I suspect it's turning over. I believe what you meant is that it won't fire.

The CPS is the classic culprit for this. The CPS tells the ignition when to fire, as well as telling the injectors when to fire.
 
Cool, thanks. Yeah, I meant that it wouldn't fire. Looks like its gonna be a $200 hit, but there is nothing I can do. I have no tools and no time.
 
I think he got the part from the stealership. Its a buddy of mine that runs the shop, so he isn't ripping me off that bad. He said the part was a little over $60 and labor was about two hours. He said he would knock a few bucks off if he could (boss out of the shop). It looks like something that I could fix no problem if I had my junk and a day or so to fool around. I was just interested in how the CPS would effect operation.
 
stealership price... $43.00 just got one today... labor to install even if it takes twice as long should only be 1 hour... really... 1 plug and 2 bolts.. it is in a tight spot but you can do it get an 18" extention and a u-joint adapter.. i tried to teach my 6 yr old how to do it but she just wanted to go jump rope.. give the kids ear plugs when you do it..
i hate to say i think you're gettin screwed but i think they need to adjust the labor end of it at least..
good luck...
mike
 
Yeah, it seemed high to me I figured that it probably included diagnosis time and running it on the computers (at least that is what I am trying to convince myself so I don't feel too bad about it :D) Hey, it was just bad timing for me. I just ended up with a really expensive lesson in CPS problems. Thanks for the info. I know now for next time.
 
Way too high. The part at the dealer is about $40 and time to install is less than an hour. I installed one in the dark in 25 minutes on my back under the jeep w/o a lift and using a flash light. Call the dealer and ask them what their book time is on the install. Greg
 
I think that you would need a flashlight even if you were to do this at noon outside! the light should be shining down also. I don't think it's a rip-off if you need the car and have no way of doing it yourself. It's a b!tch to change even with the right tools (just did this last month).
 
CPS function

Karlim:

The CPS works as follows:
The flywheel has a series of precise "notches" cut into it at specific intervals. Some are larger than others.

The CPS is nothing more than a small wound magnetic pick up coil. The sensing end of the CPS sits just above the flywheel and notches. Thus, you basically have a tiny a/c generator, the flywheel creating a moving field, the CPS a coil.

When the flywheel turns, a small a/c voltage is created. Each time a notch in the flywheel moves under the CPS, the volatage is interrupted. This system provides a very precise timing signal to the computer. That signal feed the tach, and the Ignition Control Module which in turn signals the coil to fire.

Make sense?

Michael
 
Re: CPS function

michael-esq said:
Karlim:

The CPS works as follows:
The flywheel has a series of precise "notches" cut into it at specific intervals. Some are larger than others.

The CPS is nothing more than a small wound magnetic pick up coil. The sensing end of the CPS sits just above the flywheel and notches. Thus, you basically have a tiny a/c generator, the flywheel creating a moving field, the CPS a coil.

When the flywheel turns, a small a/c voltage is created. Each time a notch in the flywheel moves under the CPS, the volatage is interrupted. This system provides a very precise timing signal to the computer. That signal feed the tach, and the Ignition Control Module which in turn signals the coil to fire.

Make sense?

Michael
Click to expand...

Well sort of and on 87-90. The CPS on 87-90 models is a two wire sensor that generates a AC sine wave (.5-.8V AC) which the engine computer uses to fire the coil and for base injector timing.

The CPS on 91-01 is a three wire powered sensor that generates like a DC square wave which the power train control module uses for firing the coil and base injector timing.
 
Okay, according to their TSB's and such a replacement of a CPS "should" take 0.6 hours. But, the first time I did it it took me about 1.25 hours because I just couldn't come up with a good way to do it and didnt' have all the right tools.
 
Not out of line

$200 is about the right price to have a shop diagnose and change a cps. You're paying for an hour of diagnosis time and an there is almost always a minimum of an hour for labor. Guess you know why I do all my own work. Well that and I'm a mechanic by trade. :D
 
I've heard alot of disscussion about this part. At what point do these start going out? My 2001 has abut 48K on it. Is this a part I should be carrying now?:confused:
 
It's impossible to know for sure. The original on my '88 lasted about 180,000 miles, but they usually start failing around 100k. Variation among units obviously plays a role, and I assume that some operating conditions are harder on them than others but I don't know what would be worse ... heat, vibration, ???

I carry spares in each vehicle.
 
You must log in or register to reply here.

Similar threads

Root Moose
Restoring a Uniframe
Replies
7
Views
146
DirtySouthXJ
D
W
How to secure dome light
Replies
14
Views
2K
XJlimitedx99
X
D
1996 4.0 No Crank, Starter Works When Jumped
2
Replies
20
Views
300
DirtySouthXJ
D
O
0331 Strikes Again...?
Replies
11
Views
303
Ol Blue
O
Xtreme XJ
Crank, no start even with new CPS...
Replies
13
Views
2K
Xtreme XJ
Xtreme XJ
Back
Top