mystery sensor at bell housing

[brian]

I'm stumped.... again :hmm: I just put a used tranny in my 89 mj last night. all went pretty well . But this morn. when I went to move the beast out of the garage. It started but runs as if the wires had been switched on the rotar cap. Well, almost that bad. It barely runs.. missing and coughing and small backfires! Won't rev at all ...just idles.

Knowing it must be something that changed either in the spark or fuel, I unplugged that 2 wire connector that runs from some sensor mounted just inside the top of the bell housing and couples up to wires under the hood. This kept the truck from starting at all. So I plugged it in again and the engine starts but runs as I described before.

 

What the bleep is that thing? I have the old one from my busted tranny but before I exchange it for the one causing trouble ...fill me in will ya!? :grrrr:

thanks

[Automan2164]

Crank position sensor.

 

Rob L.

[brian]

thanks Rob. as in crankshaft position?why would that cause the engine to barely run??

ok I'm editing my post here. I looked up crank sensor and found yes it is sensing ht relation of the crankshaft to the camshaft. that explains why the misfire but how in the H does this little baby which is mounted to an open space at the top of the bell housing, detect crank position? I guess it's magnetic or magic.

since it is nearly impossible to get at the sensor end of the thing now that the tranny is in.. I'm wondering if the old one from the bell housing I just took out will solve the problem and can I swap the two w/out pulling the tranny again?

momma told me there'd be days like this :grrrr:

[Geonovast]

That sensor is what tells the computer when there's a piston in position to fire. If the sensor's not working, it may be late, early, or skipping telling the ECU to fire. Another good chance is the sensor was damaged when you put the transmission in. It's very easy to do.

 

This happened when I did a motor swap in my 88 XJ and neglected to remove the CPS before reinstalling the motor: Good thing I had a few Renix CPSs laying around.

 

Take a multimeter, and set it to read resistance(ohms). Connect the two terminals on the sensor's plug, it should read between 125 and 275 ohms.

[Geonovast]

It's magnetic. It reads indentations in the flywheel.

 

And, it doesn't read anything to do with crank position in relation to the cam position. It just reads when there's a piston in the fire position. There's three pickup indents on the flywheel.

 

There's a sensor inside the distributor that determines that, in conjunction with the CPS going off, as the CPS and CMPS together determine injector control.

[brian]

thanks again

Yeah.... it probably got damaged in installation or removal.

I'm hoping the old one that was original to this rig will work if I swap them. I'm not looking forward to trying to do a swap since the thing is in such a hard location.

:doh: don't make me pull the tranny again!!!

[Geonovast]

It can be done without pulling the trans.

 

Make sure your original sensor is still good before you swap, and make sure the old one is bad.

 

Yours is 4wd, yes? It's not that bad if you remove the front driveshaft first. Those bolts are 11mm heads. You may have to break them loose from the back with a socket and extensions, then work them out from underneath in front of the trans. Be very careful not to drop those bolts into the bellhousing. It's not fun to fish them out.

[brian]

yep, mines a 4x4

 

I agree that I need to check both before I swap but assumng I need to change it out, I just tried to reach those 2 bolts (holding the the sensor in place) from the top with an extension and also from the hole in the tunnel since my shifter is still out.I tried a flexsocket but it looks impossible!

The blasted bolt heads are sooo close to the sheet metal tunnel there doesnt seem to be room for a socket or a hand from underneath. Incredible!! :wall:

[Geonovast]

Yeah, they didn't make it very fun to change.

 

Easy as pie on a 2.5. I changed the one on my 2.5, takes about 5 minutes, going in from the top.

 

Pull out the front driveshaft, and go in from the bottom. There's still not a lot of room, but it's damn near impossible to do anything from the top, unless you have ninja grip fingers, and three elbows in each arm.

 

Another suggestion:

 

Since you just had your trans out, you obviously don't need to worry about crossmember bolts breaking. Get a jack under the back of the transfer case, take the frameside crossmember bolts out, and let it down a few inches.

[brian]

Hey man thanksfor all the good info and support. I just crawled out from under my little yellow mule. I got the bad one off and the good one on but it took me and my son to do it. 4 hands seemed to be the ticket... plus ( i hate to admit this) :oops: I cut a hole in the firewall to get at one of those devils.

Anyhow it runs like a top ! :thumbsup: smoooooth

here'to ya :cheers:

[Geonovast]

Glad it was something simple!

 

As for cutting the hole in the firewall.. as far as I'm concerned, there should be a hole there from factory.

 

But now you know that whenever you pull a motor and trans apart, always, always remove the CPS before pulling them apart, and put the CPS back on after they're together.

[Oizarod115]

DEFINITELY symptoms of a CPS. mine did the same thing about a week ago.