Renix no spark

Renix no spark

A few weeks ago I went to start up Scuffy my 1988 XJ and it would not start. The starter spun but the motor would not start. A quick check showed no spark.

I suspected a broken wire but I did not see anything obvious. I began with the usual suspects and checked the connection for the CPS and the ground wire by the dip stick. These both seemed fine.

I then went to check the distributor wiring and found a wire pulled out of the plug. I am not sure this was before or after I unplugged the connector to check it.

I decided to then drag the Jeep into the shop for better diagnosis.

I put a scope on the CPS and it read the correct 500mv. However the wire looked in really bad shape so I ordered a new one. I also ordered a new distributor since the wires were falling out of the connector.

When they arrived, I installed them and expected the engine to fire right up. No such luck. I still had no spark. I hooked up a test light and confirmed the injectors were firing so the computer had to be getting its input signals. Just to be sure, I cleaned the C101 connector at the firewall anyway.

I then started to focus on the coil. I removed it from the ignition control module and tested it. It made a spark out of the system. I pulled the ICM and took it to autozone to be tested. The tech seemed to be unfamiliar with the test procedure but he eventually figured it out and declared the unit bad. He happily sold me a replacement.

I installed that and again no spark. I began to suspect a faulty ECM output. I checked the trigger wire which is yellow and could not detect any pulses. I used a test light to simulate the pulse at the ICM and sure enough the coil would fire.

I was really dreading puling the ECM from under the dash because my racing seats and roll cage make access difficult. However, I needed to test continuity of the yellow wire from the ECU to the ICM.

I began on the engine side since it was easier to access. I began pulling off the wiring harness cover and running my hand along the yellow wire. I soon found where it had been chewed in two.

I quickly spliced the wire and the other one near it that was also cut. The Jeep started right up.

I did leaner several things about the Renix system that I did not know before. It has several differences from the Mopar controller I am more familiar with.

The CPS (crank sensor) on the Renix system has two wires while the Mopar has three wires. The Renix CPS generates a voltage due to eh starter teeth passing the sensor on the flywheel. There is a missing tooth where the coil fires.

The Cam position sensor in the distributor has nothing to do with firing the coil. I have trouble wrapping my head aroun dhti sone but it seesm to be true according toe h Renix control manual. The cam sensor signal helps the ECU determine which of the missing teeth is for Cylinder 1. If the compute can’t get a reference signal it just makes up something. If it guesses wrong, the engine will still run just not as well as if it guesses right. So maybe my broken distributor wire explains why sometime my engine would feel off power and others times run fine.

The Renix coil is fired by a transistor pack under the coil. This pack gets a 12V pulse from the computer when it is time to fire the coil. Timing, dwell etc. are all controlled by the ECU.

The Renix injectors get a 12V pulse to fire and use a common ground wire for all six. The opposite is true for the Mopar that feeds all six with 12V and grounds the one that it wants to fire.

One Reply to “Renix no spark”

  1. I have 89 jeep Cherokee Laredo 4.0 and sometimes cranks with no start. ?? Starts later in day. ?? Runs strong. Little low on idle. Just replaced idle air control throttle position sensor crank position sensor starter, battery , cables relays….ballast resistor, map sensor…starter relay box….hard to find answer

Comments are closed.