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Unread 04-08-2011, 02:47 AM   #1
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1995 ZJ 
Join Date: Apr 2011
Location: Bangor, Maine
Posts: 41
No Spark after CTS replacement

Does the '95 Laredo (6 cylinder, 2 wheel drive) have a separate coolant temperature sender (not sensor) to the dashboard gauge?

Long story: After tune up in October at national chain auto center, tune up included coolant exchange, but after, temp gauge on dash stopped responding, needle just flopped over to the cold side and lay there. No problems otherwise. Jeep ran great, smoother for the tune up, no problems at start up in spite of bitterly cold winter. Finally in March, brought jeep back to have the coolant temp gauge re-engaged. Auto center replaced coolant temperature sensor, (not sender) after which car would crank but not spark - so no ignition - no start up (impossible to test temp gauge) or drive the vehicle. Had jeep towed to a mechanic's shop. So far we've replaced the Crank Position Sensor - still no spark. Wondering if the replacement of the Coolant Temperature Sensor at auto center may be cause for no-spark.

What do you suggest?

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Unread 04-08-2011, 04:44 AM   #2
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2003 WJ 
Join Date: Mar 2011
Location: akron, ohio
Posts: 130
Cts would not cause a no spark condition, however if the crank position sensor is not oem, that could. Does a 95 have a distributor or coil?
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Unread 04-08-2011, 06:36 AM   #3
It's the crank sensor!
tjwalker's Avatar
1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,704
I agree, that coolant temp sensor will NOT affect spark.

There are quite a few reasons for a no-spark condition. The #1 suspect is the crankshaft position sensor, which you have already replaced.

Don't replace any more parts until testing verifies the failed part. I hope your mechanic has the knowledge of how to test to the point of failure. If not, find a technician who does.

Start with testing of the ignition coil. They can be tested for primary/secondary resistances very easily.

Other possibilities include a bad ASD relay or a problem with the ignition switch. The ASD relay is located in the power distribution center (black box) under the hood on the passenger side. That relay can be swapped with another relay in the troubleshooting process, as these relays are of the same part number.

Here is the route of how spark is delivered......

Spark: 12 volts from ignition switch to the ASD relay; when the key is first turned on the PCM provides a ground to the ASD relay to energize it, then the ASD relay passes 12 volts to the coil primary side. If no crank sensor signal is generated after a couple of seconds the PCM cuts the ground to the ASD relay (and fuel pump relay too) and that cuts power to the coil (and fuel pump). Key to START and once again the PCM energizes the ASD relay. Now, a critical element is the crank sensor--no signal from the crank sensor and the PCM won't trigger the coil to provide spark
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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crank , cts , sender , sensor , spark

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