88 Cherokee - Won't Start - Brain Teaser

Ok my technical peers, a brain teaser for the first of the week

The stats:

*1988 Jeep Cherokee, has been starting without problems this morning won't start. Engine turns at normal speed just doesn't seem to crank up.

The Parts:
--New
*Jasper motor (3 mos old)
*Injectors (3 weeks old)
*MSD Ingition and coil (3 Mos old)
*Throttle Position Sensor (2 mos old)
*Vaccumm Harness (2 wks old)

--Old
*TDC Sensor (12 years)
* Starter ( 4years)
* Alternator (1.5 years)
* Battery ( 1 year)

The Brain teasor - Jeep has been running fine. Took her up 4 wheeling two weekends ago - no problems - drove her up in the mountains on Saturday - No problems - Didn't drive on Sunday. Today(Monday) Tried cranking, motor turns at normal speed, plenty of battery power, just doesn't seem to fire up.

As I understand it if the TDC Sensor died the jeep just would not do anything, right? I am thinking that, since I am blindly troubleshooting that I would check the fuel Pump, Starter, Battery, TDC sensor, and Alternator. The fact that it just went from starting fine to not starting has me irked though.

The X factor here is that it is Miller Moth season in Colorado and I don't know what those cursed things might have gotten into under the hood - Alternator Brushes?

Any guesses?

Mark

Mark,

From what you describe your XJ has a lack of fuel or lack of spark.

The Crankshaft Position Sensor (CPS) also called the engine speed sensor of the TDC sensor. It can just fail like you described. The battle damage repair method is to unplug and replug the CPS, that works sometimes to get you home.

If you have access to a timing light I would hook it up and see if the timing light is flashing when you crank the engine. If there are no flashes then replace the CPS.

You have an option on CPS, a normal and a high altitude. Make sure you get a High alt CPS at the dealer. The pick up mounted off center on the bracket on the high alt model. The sensor is centered on the bracket on the normal CPS.

The CPS controls when you have spark.

The Camshaft position sensor also known as the sync sensor is in the distributor. It tells the computer when to fire the injectors. If it is not working then you will not have fuel delivered to the cylinders. If this sensor is bad you have to remove the distributor to change it but your money ahead to install a rebuilt distributor than to try to change the sensor in your current distributor. Also with a rebuild you get new bushings which should improve your performance, worn bushings can retard your timing.

It a spark or a fuel problem, just have to figure out which one. You would know if a mechanic hooked up the test set to the engine. We are unlucky only the dealer or a garage with the snap on unit can examine the computer, no you or I.

The test set would tell you what wrong and confirm what working. If you have towning insurance/AAA then tow it to the dealer or a garage you trust, get a mechanic familiar with the Renix ignition system to check it out.

Sometimes you just have to pay for professional help but my guess is replace the CPS.

Martin

Thanks for the reply martin - to reiterate though when you say CPS and I say TDC we are talking about the same thing right? This is the sensor on the bell housing, passenger side right?

Concur that you should check the CPS as mentioned -- on the bellhousing. But, you may also want to check your ignition relating to spark -- coil, cap, rotor, and plugs. These do not show as error codes, but the high voltage coils can erode your ignition components more quickly than the stock ignition. I don't have the specs on the MSD coil, but you should look for a few ohms across the primary and in the thousands of ohms range across the secondary.

Isn't there a low-tech way to check spark?

Mark,

The low tech way to check spark is with a volt/ohm meter as Pec suggested. Do you own a volt/ohm meter or a Digital Multimeter? If not it time to go buy one, very handy for finding electrical problems.

I noticed I mistyped the sensor on the bellhousing at the 10 o'clock position is known as:
* Crankshaft Position Sensor
* Engine speed sensor
* Top Dead Center sensor

Three names for the same item.

Yes, as Pec said you need to check out the ignition system to make sure everything is correct.

Have you tried to use a timing light to see if you're getting spark to the plugs?

Also have you checked to see if you have fuel pressure. There is a schrader valve on the fuel rail, put some rags under it with the engine off and see if you have fuel come out under pressure. You could have a failed fuel pump and that why it does not run... no fuel.

It is still a question of fuel or spark, you are not getting one of them... just what causing it is the question.

Martin

Running

Got multimeter - checked spark and cool

Before changing CPS tried turning ignition on until the noise from the fuel pump stopped and viola