Blazer crank/no start

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The patient: 99 Chevy Blazer 4 door, 4.3 (VIN W) automatic, four wheel drive.
The problem: Crank/no start due to no spark.

I just bought this truck so the history is very limited. What I can tell you is that the truck went to a shop with a no start problem and they diagnosed it as a bad fuel pump. Their customer declined repair and that customer gave the truck to my Dad. When I first looked at it, I was able to fire it right up and it ran great. I could hear the fuel pump prime when cycling the key on. I didn’t have a fuel pressure gauge then, so I settled for tapping the schraeder valve and spraying the firewall down with gas as my sign the fuel pump was operating for the time being.

I decided to take a blind shot at spark problems in spite of a running engine. With KOEO, I poked at the ignition module a little and found battery voltage where there needed to be battery voltage and the ignition module ground was a clean zero volts. The timing signal wire showed 2.5 volts during cranking. I know that doesn’t mean much with a voltmeter, but to me it says that the PCM is talking to the ignition module about timing. I did get to stab that signal wire once with engine running and it showed me .6 volts. I was talking with my Dad trying to make a point about something so I pulled the fuel pump relay and the truck died instantly. I reinstalled the relay and the truck hasn’t ran since that moment.

At that point, the truck sat for about a year and a half with a little dabbling on it when I had a moment in between. During that moment in between, I just swapped the coil and ICM out of my S10 with no change in spark status. IIRC, the Blazer parts started the S10. I returned all parts to original locations. Somewhere along the way, I did confirm the pump was somewhere between dying and already dead. My Dad never did anything more with it until his landlord told him to have it gone before winter. So, I bought it and I now have to have it gone before winter.

I eliminated a couple variables already. My first order of business was to replace the fuel pump. The pump works and delivers 57 psi to the rail. Rest pressure is 50 psi. I did some reading and investigating on a possible Passkey module failure. The security light shuts off within five seconds of turning the key on and it will shut the security light off immediately when cranking. These are both indicators that the Passkey system is functioning normally. The service information also says that it will only disable the fuel injection if it thinks a theft is being attempted. It says nothing about disabling spark.

My new diagnostic weapon of choice is a Snap On Modis…which is kinda broken. (No more eBay for me, I don’t care how good the seller’s feedback is.) It usually skews the voltage by .2 to .5 volts to the negative of what it should be reporting, but otherwise looks to be reporting the correct hi to lo voltage range. I’ve had to risk other diagnostics with this problem and I’ve actually been able to make consistently accurate repairs so far. I started my day at 6AM and I’m just as disfunctional as any broken Modis I own at that time of day. Here’s how some scope testing went:

I was looking for cam and crank signals at the PCM and I usually got a normal cam sensor signal, but my crank signal was all over the place. I found a lot of confusing moments where I found my ground scope lead was disconnected, but I might have got a wave form anyway. Moments where I knew I had everything connected right and got erratic signals anyway. Other moments where I couldn’t quite get a damn back probe to get touchy feely in the right way with the PCM pins. The one that got my attention though showed me a crank signal that flatlined high at battery cranking voltage. I know for sure I had everything connected right for that round. The factory wiring diagram shows that the crank sensor gets battery voltage. I can’t find anything in factory information that says how the crank signal is reduced to five volts. Assuming my Modis hasn’t totally shit itself inside out on me, could it be possible that the crank sensor has shorted itself internally bypassing a built in resistor?

There are a couple more things I have to address. I know I forgot to reconnect a vacuum line at the front of the fuel tank so it’s hard to tell if the fuel I smelled this morning was from flooding or tank fumes. Come Wednesday, I will have time to work on the Blazer again and I’ll keep you posted on what I learn as I learn it. For now, I’d be happy to hear your thoughts on the crank sensor in spite of any problems with the scope or its user.

Topic

[Its me]

Great Diagnostic work so far…

How fresh is the fuel?

Also, get some back probe pins for those connectors

MODIS is an awesome labscope and powerful tool…

Keep at it and you will find it…

-Karl

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The fuel is a blend of 1.5 year old gas and 1.5 day old gas…heavy on the day old stuff. The old gas still smelled like gas and most of it got pulled out of tank and poured into another car which is drinking it up just fine so far. Maybe I’ll have a new problem to report on a Buick next week.

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What I eventually figured out about that other scope trace for the Blazer is that in spite of trying to push the back probes in straight, you can find the wrong terminals through a weather pack connector. My channel one cycling voltage was because I was hitting the same terminal as channel two. Channels three and four were likewise both back probing the coil driver terminal. Things like this is why this is my personal motto: “If it can be done wrong, I can do it.” Once I got my shit together, I was able to quickly diagnose a bad ignition module. I borrowed the module out of the S10, promptly zapped myself with the Blazer coil wire, and then ordered a new module off Amazon. As luck would have it, I’ve lost fuel pressure again. At least there’s time to address that while waiting for the module to get here.

[Author]

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