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post #1 of (permalink) Old 05-14-2000, 12:56 PM
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88 Cherokee no spark

I have an 88 Jeep cherokee with 4.0 I6. I am getting no spark. So far I have changed the Ignition control module and crank position sensor. The manual, Chiltons, says I should be getting 5V AC to the Cam or Sync sensor in the distributor with the ignition in the on position. I am not. Next it tells me to use a ms 1700 tester or equivalent to test the computer. Does anyone have any other ideas?

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post #2 of (permalink) Old 05-15-2000, 11:49 PM
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Re: 88 Cherokee no spark

The early Renix type 4.0ls will run and start with the distributor sensor disconnected, but they will need a longer crank time than usuall and sometimes have a hesitation. Unplugging the sensor causes the EFI system to go to a simultanious injection pulse instead of the sequential pulse that is normal.
Sometimes, a shorted cam "sync signal generator", as it is called, will cause a no start, unplugging it will isolate it as the cause or not.
More likely, the PCM or wiring problem is the cause. The large bulkhead mounted connector, near the emissions label on the cowl of '87s and '88s, is usually full of corrosion, bypassing it by soldering together each wire on opposite sides of the connector or replacing it with the '89-'90 style harness
is the way to fix this problem. Soldering the wires wont cost much more than the time, the new replacement harness costs over $400.
The PCM , we usually test by replacing it with a "tester" we keep in the shop.
Rebuilt ones are available at the dealer and from the aftermarket. Double check the power and ground circuits to the PCM, the shop manual should have a schematic. You may have a bad ignition switch, blown fuse or fusable link.
When you replaced the crank sensor, you did install the jumper harness that goes directly to the PCM, didn't you? This is very important to install on the '87 and '88s, and recommended on the '89s and '90s too.
The MS 1700 tester went out with the '80s, replaced with the DRB2, only a Chrysler/Jeep dealer will likely have this unit. Using it would help pin down the problem, in most instances. Most dealers will want to charge an hours' labor for a diagnostic test. This may be money well spent before replacing expensive parts that you can't return.
Hope this helps, good luck!

post #3 of (permalink) Old 05-18-2000, 12:02 AM
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Re: 88 Cherokee no spark

Thanks for the help! Found my problem. The PCM was faulty. The large diode in the middle was fried along with some of the traces. I believe this happened because the nut on the distribution block or solenoid next to the battery was loose. I bypassed the diode with another one ( same type) and it started right up.

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post #4 of (permalink) Old 05-18-2000, 10:24 PM
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Re: 88 Cherokee no spark

How did you test the diode? Did you have to disassemble the PCM? Very interesting! It's probably all they do to "rebuild" them!

post #5 of (permalink) Old 05-18-2000, 11:46 PM
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Re: 88 Cherokee no spark

I disassembled the PCM out of curiousity. The diode didn't need tested; it was cracked open and the board traces on both sides were fried. In this case it probably would be all they would do to rebuild or remanufacture this one. I purchased the proper size diode from a local electronic supply warehouse, soldered across the board and everything is running fine. The only thing I don't have is a warranty as to how long it will work, but for the $130 difference I can take the chance.

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