Sunday, October 6, 2013

2004 Lexus RX330

I get a call from a shop concerning a 2004 Lexus RX330 with 143,707 miles. Apparently the vehicle was jump started backwards and was towed in to this shop. The shop found numerous fuses blown. They corrected all the blown fuses and got the vehicle to run. The vehicle did not run well and at this point the shop is thinking the backwards jump start hurt the vehicles PCM. They pulled some codes and then wanted me to come down and either confirm or rebuke their assessment. A little background here. Toyota/Lexus PCM's I have found to be pretty tolerant to backward jump starting. The industry term is "robust". The other item of note is Toyota/Lexus PCM's are very expensive for the most part.

I arrive at the shop and start the vehicle. The vehicle starts and idles fair. Hitting the gas the vehicle stumbles to a stall. I retrieve some codes. Hmmm, all 6 Ignition Coil circuits being bad. We have a common issue. I can't see all 6 Ignition Coils going bad. But, it was jump started backwards. Or, maybe it does have a bad PCM. Let's roll up the sleeves and get some information. My mind right now is thinking we have a power issue feeding the coils. Maybe, instead of the coil primary circuit getting battery voltage it is getting a reduced voltage and that is why we have the codes and the lack of power. I got this vehicle figured out already. Yeah right.

I break out my scope. I know it will give me the most information in the least amount of time. Next is to get the "lay of the land" of what is involved. Knowledge is power so they say.

Here is a partial wiring diagram of the coil circuits. Each coil has four wires. They have a shared battery power, ground, and IGF. Then there is IGT. Lets go over Toyota ignition systems. IGT is the signal from the PCM to fire the ignition coil. I always refer to IGT as ignition "trigger". IGF is the confirmation signal sent back from the coil to the PCM that the coil has fired and to allow proper fuel injection operation. Now we can setup a battle plan.

My next piece is to know the coil arrangement and the firing order.

Ok, I typically like to trigger off of #1 cylinder. But, #1 on this vehicle is buried under the intake and is a pain to get to. So I will use coil #2, it is easy. I am using Pomona test leads. I absolutely love these for testing. Secure connections, minimal damage to insulation, and well built. I get mine from These guys are awesome! So the first thing I do is make sure the ground at #2 coil is good using voltage drop. It was. Then I am going to hook my scope up to Battery power, IGF, and IGT signal wires at coil #2 and start the vehicle.


Channel 1 in yellow is our battery voltage at coil #2, Channel 2 is coil #2 IGT signal (it is also what I am triggering off of), and Channel 3 is IGF at coil #2. Anything jump out at you? Well my super genius idea of low battery voltage is off the table. Battery voltage is at charging system voltage. I do see something right away.

It looks like we have a Lexus 5 cylinder. I see no deviation in battery power, no IGF signal, and furthermore the IGF signal seems low to me. Typical IGF signal is about 5 volts in amplitude. Lets add some notes. Remember, we are triggering off of coil #2. Remember the firing order.

Adding some notes it fills in the blanks. It is obvious we have issues with coil #1. But, what about that poor IGF signal? I had a theory. My theory was if coil #1 was bad enough it could pull down the IGF signal since this signal is shared amongst all six coils. How could I test my theory? I had an idea.

I reached around and disconnected #1 coil connector and restarted the vehicle. Bingo! Look at the IGF signal. More importantly the vehicle revved up well. It did have a misfire obviously but accelerated well. Let's check those codes now. If my theory was correct I should only have a #1 coil code due to it being disconnected.

Nice! Now I test battery power, ground, and IGT trigger at coil #1 connector. I explain my findings to the shop owner and recommend an OE coil unit for #1 cylinder. The unit was installed and the vehicle delivered.