'00 4.3ltr blazer won't start

Thanks for getting me here, I'm so far into my daughter's truck, I'm lost and need help. No spark from Coil, had DTC P0336 replacd Crank Pos sensor No Spark. Cleared code, now have pending P0336.

I should also say up front that I've had the PCM flashed and changed timing chain and gears.

With the ignition in the RUN position, crankshaft position sensor unplugged, does the pink wire show battery voltage in the harness connector?

Yes, though slightly discharged showig 10.7 volts at both, connector pink wire and at battery posts. Hooked up charger...

Remember you also asked..., this truck's an automatic with 3 button 4WD.

With the ignition in the RUN position, Does the pink wire at the ignition coil and at the ignition module show battery voltage?

If so, connect a test light from battery positive to the white with black stripe wire at the ignition coil, then crank the engine.... does the test light blink while the starter is engaged?

Old Master, we've got battery voltage on the pink wires at the coil and the control module, but nothing whatsoever on the white and black striped wire while cranking (hooked up on Pos).

I've got a ground on the whte/Blk striped wire # 451 socket C on the control module.

Think I should do an ohms check on wires 423, 573 & 574? Prety sure I can fnd the sockets on the PCM connector...

The CKP is where the PCM gets its data for determining spark. The PCM sends a pulsed ground to the ignition module and the ignition module sends a pulsed ground to the ignition coil to fire it. With the P0336 DTC in memory, it indicates a problem in circuits 573 or 574, (purple and yellow wires). They are the signal wires from the CKP sensor to the PCM. It would be a good idea to check resistance on them with the CKP and PCM unplugged. They should both show less than 5 ohms resistance from end to end. They also should show infinite resistance to ground. With the CKP harness connector unplugged, and the PCM connected, the purple wire, (sensor ground) should show less than 5 ohms resistance from the CKP harness connector to chassis ground. You can check resistance on circuit 423 from the PCM to the ignition module, should be less than 5 ohms, but I think you'll find a problem in the yellow or possibly purple wire from the CKP sensor.

Lord, I hope you're right. I've been through what I thought was every inch of this system. Beleive I'd checked the referenced circuits but I'll pull the connectors and check them tomorrow and we'll see what we've got.

While you're down by the CKP sensor, check the mounting of the sensor: make sure its seated all the way in, and tight. Also, make sure the reluctor wheel is not damaged or loose.

Did you perform the crankshaft position sensor relearn? Whenever the CKP sensor is disturbed, moved, removed, replaced, or the timing cover is removed, the relearn MUST be done. If it's not done, the engine will still run, but ignition timing will be incorrect.

Didn't know of the re-learn requirement. How is that done?

The reluctor ring looks right but while checking with depth guage, saw it's about 2.5mm off the crank sensor. I measured the length of the sensor from mounting flange to end, then the distance from fnt cover mounting seat flat to reluctor ring, and subtracted the difference. Book says sensor for the 4cyl anyway protudes to within 0.050 of the reluctor ring.

Two questions;
What type signal do you think I should see on the white wire#423 at the coil control module at socket B while cranking?

What type signals shoud I see on te yellow and purple wires 573 & 574 at PCM connector sockets 31 & 28. I seem to remember these two sockets being in seperate connectors?

PS. Merry Christmas! This is the first whte Christmas hre in KY/TN for 19 years!

Old Master, I hope we're on to something. All wires are showing good conitinuity end to end, (423, 573 & 574) from sensor sockets to PCM Blue connector "C1" socket #s 28 &31 .

Hooking up the white wire 423 at control module to a test light and battery POS, I get a solid illumination with no pulse while cranking.

Only problem I see contrary to your comment, "They also should show infinite resistance to ground." The yellow off the sensor showed no continuity to ground, however purple did. Would key being off while checking cause this?

There is no published air gap between the CKP and the reluctor ring, but past experience is about .010". The signal to the PCM is AC voltage. If the air gap is too large, no signal will be sent to the PCM and...no spark! Relearn is done with a bidirectional scan tool that can access the procedure.

With the CKP sensor and the PCM unplugged, resistance end to end on the yellow and purple wires should both be less than 5 ohms on each wire. Also measure resistance at each connector to ground on the yellow and purple wires, should show infinite resistance, (open circuit). Basically you're making sure both wires are good and that neither one is shortd to ground.

With the PCM connector plugged in, the purple wire should show less than 5 ohms to ground when measured at the CKP harness connector. The PCM grounds the purple wire internally, this test makes sure that ground is good.

The white wire, (423) is normally grounded in the PCM. When the starter is cranking the engine, or the engine is running, the PCM sends a pulsed ground to the ignition module through this circuit. After the PCM determines what ignition timing should be, it sends a pulsed ground to the ignition module. The ignition module sends a pulsed ground to the ignition coil and it fires. This circuit is referred to as Electronic Spark Timing, (EST).

Old Master, I've gne through all your questions and verfied / Re-checked all the conditions. I'm thankfull for your patience while I continue to fumble through diagnosing this job, and hope we can isolate the problem. Trying to stay on "track" I'll list my findings in the order you asked (more or less) I'll list the ohms readings got using the 200 scale on my meter...

Battery voltage is present on all pink wires at CKP, Coil & Module.

Test light gets no illumination on wht/blk striped wire at coil any key position on, off or while cranking. End to end this wire shows 00.4ohms from module connector to coil connector and infinite resistance (Open) to ground.

The CKP wiring, and white #423, all show from 00.4 - 00.9ohms end to end, and infinite resistance to ground. (open)

With PCM hooked back up, White wire #423 shows constant ground by solid illumination of test light all key positions, but no pulse while cranking.

With PCM hooked back up, PPL #574 shows 00.2 ohms to ground.

One question here, with key off and coil connctor plugged in I get test light illumination and 00.4 ohms resistance at module connector wht/blk socket from coil pink wire. Is this right?

I pulled the CKP, wrenched the crank slightly to ensure poper position of reluctor wheel for another measurement and it still shows a .1" or 2.5mm air gap. wheel looks right, centered on well, and tight with no aparent damage. Re-seated sensor, plugged everything back n and still no spark. I might check to ensure I got the correct CKP upon replacement? And start shopping around for a garage that can help me with CKP re-learn and new passkey re-learn?

Suggestions?

I measured old sensor and find they are the same length so appears I have correct new replacemnt, and provided depth of new timing chain cover well is same, should have same air gap as original set up.

I put a probe wire into back of CKP connector at yellow wire and cannot see any AC signal while cranking. Lowest setting on my multimeter is 200VAC. Would an analog meter be better to use? I confirmed Battery voltage on the pink wire and ground on the PPL with key on.

I measured old sensor and find they are the same length so appears I have correct new replacemnt, and provided depth of new timing chain cover well is same, should have same air gap as original set up.

I put a probe wire into back of CKP connector at yellow wire and cannot see any AC signal while cranking. Lowest setting on my multimeter is 200VAC. Would an analog meter be better to use? I confirmed Battery voltage on the pink wire and ground on the PPL with key on.

All of your test results in post #13 are correct. Battery voltage on the pink wire and ground on purple, (at the CKP) means both of those circuits at the CKP are good. Your resistance reading on the yellow wire is good, which means the PCM can receive the pulses... if the CKP is producing them. When the starter is cranking, the CKP creates a magnetic field. When the crankshaft rotates, the reluctor wheel changes the magnetic field. The CKP sensor converts the changes into pulses and are measured as a duty cycle with a Digital Volt Ohm Meter, (DVOM) capable of measuring Hz, (cycles per second). When the engine is cranking, the duty cycle should be 40% to 60%. So, with what you have checked, and the results you got, we can be pretty darn sure the CKP is not producing the pulses. The ONLY way to know for sure is to check the duty cycle. As I mentioned in an earlier post, if the air gap is too wide, the reluctor will not change the magnetic field and no pulses will be produced.

Something to check: Rotate the crankshaft manually until one of the teeth on the reluctor lines up with the CKP sensor, then measure the air gap with a feeler gauge and post your measurement. In the meantime, I'll go measure a "known good" air gap on a 4.3L. If your air gap is out of spec, the problem might be the new timing cover is not orienting the sensor correctly.

EDIT: AC voltage across the yellow and purple wires while cranking should be at least 20mv, and the air gap should be no more than .050" ...from a pretty reliable source ;)