Non-Start Diagnostics

Last year one of my friends rebuilt a motor for Michael, a new 260Z owner. Michael got the engine back in, and my friend tuned the engine.

Fast forward to January of this year. Tim, Michael’s father, posted on Facebook that the car was running fine for a while, but after warming it up, it wouldn’t run. In answering a question to his post, Tim said there wasn’t power at the coil.

I sent a message to Tim to arrange to come over and do diagnostics. I suggested a couple of likely culprits and how to resolve issues with them. Tim talked with my friend who built the engine. My friend told Tim that it was unlikely to be mechanical, and it would be better to have me look at it. With the long weekend coming up and unseasonably warm temperatures, it seemed like a great opportunity to figure out what was wrong.

I packed plenty of testing and tuning tools into the car in an effort to leave no stone unturned and drove over to meet up with Michael. We dragged the tools and factory service manual (FSM) to Michael’s 260Z and set to work. The first thing was to check the battery voltage. It was 12.46VDC. That is on the low side. It worried me that it could be a factor. We put the key in the ON position, and Michael tried to check the voltage at positive terminal of the coil, but he couldn’t get a good reading. Unsure of what the voltage was at the coil, we tried to start the car. The engine turned over just fine, but there was no spark.

When I looked at the ballast resistor, I could tell at a glance that it was not the resistor for a 260Z. The stock ballast resistor for the 260Z and 280Z has three terminals while the 240Z ballast resistor has only two terminals. Unfortunately I didn’t do much besides glance at it. (Keep that in mind.) I showed Michael the wiring diagram in the FSM and gave him a quick tutorial on how to read it. I was focusing on the wiring between the ballast resistor and coil. That was when I noticed the second issue.

Ignition coils have polarity. He had the ballast wired to the negative terminal on the coil, and the positive terminal was wired to the condenser and distributor negative. This in of itself won’t cause a non-start, but it does weaken the spark. I showed Michael how to check the coil by using the resistance of the wires in the coil. The ratio of the resistance between the secondary (high voltage) side of the coil and the primary (12VDC) side of the coil should be about 1000:1 or more. The coil in the car was less than that. Michael got another coil, and we check it. The ratio was better, so we installed that one in the car. We also inspected the distributor cap and rotor looking for damage or excessive wear. Both were in good condition. One thing that did concern me was that the wire for the #1 cylinder seemed to be rotated too far counterclockwise, especially compared to my car. I sent a photo of the distributor to my friend who rebuilt and set the timing of the motor. He confirmed that it was where he set it, and I confirmed the distributor was still locked down tight.

We got the wiring back in place and tried again…well we couldn’t REALLY try again because the battery was dying. With no battery charger, we couldn’t continue diagnostics. Michael did say that the turn signals weren’t working, so I showed him how to use the drawings in the Body Electrical section of the FSM to diagnose that circuit. We traced the problem down to the turn signal switch itself. We then packed up my tools and loaded them back in the car. Michael told me Tim was coming home from work, so I decided to wait until he got home before I left. When we told Tim the battery was dead, Tim directed Michael to take the battery out of another car and put it in the Z. Michael went to do that while Tim and I got the tools back out.

With the stronger battery in place, we resumed testing. I was checking voltage at the coil while trying to start the car. It was only 6VDC. I made a jumper wire to go between the battery and coil, and we tried again. Still the car wouldn’t fire. I placed an inline spark checker on the first plug wire, but Tim said there was no glow. At this point I was thinking that it might be a grounding issue. I looked over the wiring diagram for a grounding point, but nothing jumped out at me. So, maybe it was a grounding issue or other problem with the transistor ignition unit (TIU).

I told Michael to look in the passenger side footwell for the TIU. He couldn’t find it. Funny, it isn’t that small, and it is a natural metal finish against the black background of the footwell. I didn’t think it would be difficult to find. I looked myself. It wasn’t there. I showed Tim and Michael where it was mounted originally in my car, pointing out the connector that it plugged into. Looking at Michael’s car, I found it in a slightly different place. I was perplexed. How could the car have run without an ignition unit? The wiring at the distributor did not show any signs of another ignition unit. I just couldn’t figure it out.

That was when Michael went looking around in the shed. He came back out with the aftermarket AC evaporator. It had the TIU attached to the backside of the bracket that held the evaporator in place. He removed the TIU from that bracket and installed it back in the car. On the next try, the car still didn’t start. Okay, I still thought the voltage at the coil was low. We put the jumper wire back on to the coil and battery. The car fired up.

Why was the voltage low? I looked at the wiring on the ballast resistor again. The wire from the start circuit and wire from the run circuit were wired to the same point on the ballast resistor and on the opposite side from the wire to the coil. I moved the wire from the start circuit over to the other side of the resistor. The car would start, but it would die as soon as the key was released. Yes, it made sense. I measured the resistance across the ballast resistor. It was over 10 Megaohms. That was way too high. The diagnosis was confirmed. The ballast resistor was the problem.

Since it is difficult (and expensive) to get a replacement 260Z ballast resistor, I suggested that they try a 3 Ohm coil and have all three ignition wires on the same terminal at the ballast resistor. It’s a solution I’ve been running for a while with no issues.

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