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Jeremy airjer W

Seeing the whole picture, diagnosing a no start!

11 posts in this topic

The other night a Nissan Maxima came in on the hook. It would crank over fine but would not start. Most techs would start with the gut feeling, as I did. I couldn't hear the fuel pump running while I cranked it over and I never smelled the odor of gas. This led me to believe that there was a fuel issue. The first step was to see if there was fuel pressure and there wasn't. Next step is there power to the pump? The pump is accessible under the rear seat which made it easy to check. The black with yellow wire supplies power to the pump and much to my surprise there was no power!

mazdapump.jpg

So the next step was to check the fuse which had power to it. I decided that I needed to check the fuel pump relay which was located behind the ABS module that was behind the left front kick panel. The PCM grounds the relay which energizes the solenoid that closes the circuit that allows power to go to the pump. The PCM was not doing its job?

I decided to hook up the scan tool to see if there was any data that might be helpful. The scantool would not communicate with the PCM? What is going on?

Then it dawns on me, theres no check engine light when I turn on the key or crank the vehicle over.

mazdanolight.jpg

Now I start to see the whole picture. The problem isn't just the fuel system its all the systems! So now I need to find out if its the PCM that has failed.

I run through the diagrams and find all the power and grounds that I need to check. They all check out except for a white with blue wire that feeds the PCM and ECS relay. It starts in the underhood fuse block, then goes to the underhood #1 relay box where it splices into three wires, it then runs back to the underhood #2 relay box and the PCM after it splices off into three more wires somewhere in the harness that runs between the two relay boxes.

I start by checking the fuse block to make sure theres no corrosion or open wiring. It was fine.

mazdafb1.jpg

I then double check the ECS relay for power on the white with blue wire. No Power!

mazdarb2.jpg

Then I tear apart the #1 relay box to find the first splice. I had power on all three wires coming out of the splice so it was good. the middle wire (red arrow) was the power feed from the fuse the one to the right (yellow arrow) was the feed to the splice in the harness. I'll get to the yellow arrow in a second.

mazdasplice.jpg

I decided I needed to find the splice in the harness. The yellow arrow is the power supply wire to the relay box splice. The red arrow is the feed to the PCM and ECS relay. I had continuity from the relay to the spot where the arrow is pointing but no continuity from the arrow to the relay box splice so I knew the problem was in the hardest to get at last foot of harness.

mazdaharness.jpg

Sure enough just outside of the relay box was the white with blue wire that had completely corroded through (yellow arrow).

mazdasplice.jpg

Close up of the culprit

mazdaproblem.jpg

And the repair

mazdarepair.jpg

The first thing I did was turn the key on and there was the check engine light.

Mazdalight.jpg

A couple of cranks and the engine started!

Even though my gut instinct was theoretically correct, taking a second to see the whole picture made this diagnoses a little easier!

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Very cool Jer!

Pretty nice of you to take the time to do this. Time is money wink

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Ah the wonderfull life of wire chasing! Thanks Airjer, some of these posts you spend time to create for us shows others how hard our jobs can be, and its not just hook up the computer and the computer tells us what is wrong!

Thanks for taking the time to post these steps for everyone!

By the way, while you are working on these cars and taking pictures, does you co-workers or boss wonder what you are doing?

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Amazing! What a pain to diagnose. Definately a smart man you are! Cool post!

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Im glad I was a service advisor and manager and never a tech!

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By the way, while you are working on these cars and taking pictures, does you co-workers or boss wonder what you are doing?

Everybody I work with is really supportive of the things I do! Mike (one of our service managers) reminds all the time to take pictures!!

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"Even though my gut instinct was theoretically correct, taking a second to see the whole picture made this diagnoses a little easier!"

Taking a second??? C'mon, you were on your thirds, fourths and fifths here.

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Taking a second??? C'mon, you were on your thirds, fourths and fifths here.

I had about 4 hours into this one from start to finish, So you are correct!! grin

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You lost me pretty quick on this one, some fixes are worth every penny. smile It usually is the piece that is hard to get at, makes you wonder what mechanics think of engineers.

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We think alot of engineers. They have made me laugh, swear, cry, scream, and just plain shake my head with confusion on why! I am sure their are more, but those are the most comon things!

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I was told a long time ago that the cost engineers are mostly to blame for the laughing, swearing, crying, screaming, and head shaking.

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