99 Chevy Tahoe 5.7l. No start.

[Jsdirt]

Just verify your 5v reference is still good - check it at the MAF sensor. If you have 12v on it there, you might have a reference circuit shorted to battery voltage.

[Carla Jean simmons]

On my 99 Chevy Tahoe 5.7l vortex.. No codes, no security light. Truck just died like it ran out of gas. Now I have a no 

Start issue for a month. On the ICM mounted beside the coil with a 4 pin connector, it has a

Pink wire tests 12volts, and a white wire tests 1 to 4 volts also the LED light flashes, and a 

Black with white stripe wire which is the ground it tests good, the fourth wire is white with a black stripe which 

Is the switching signal and I can not get a signal out of  the circuit. 

I have had the ICM tested and they say its good. I have ohm it myself and all prongs read the same any way I put my leads

But the switching signal doesnt . it reads

With red on ground and black on the switching signal it reads 2.671 mega ohms

With black on ground and red on switching signal it reads absolutely nothing.

Why is the switching signal not reading?

Does that mean my ICM is bad? Please help.. Thank you

[Carla Jean simmons]

U say check my 5v reference. What do u mean if I have 12v at my maf I have a bad  ground to battery

[Jsdirt]

Most 3-wire sensors on any vehicle have a 5v reference wire straight out of the computer (I've heard Chrysler uses 7 or 9v on some, but that's not important ...). What the computer receives back out of that 5v determines what value it sees. A shorted reference will be pulled to ground, while an open circuit will be at or near 5v. If there's no 5v reference present, a sensor could be shorted, pulling the entire reference right to ground - the vehicle will not run in this case, and in some cases will not even crank.

 

There is very little amperage on the 5v reference. Just touching a 250mV test light to it, will pull the whole thing to down ground. (.2v or less)

[Doug_Scott]

1 hour ago, Carla Jean simmons said:

U say check my 5v reference. What do u mean if I have 12v at my maf I have a bad  ground to battery

You also must check those values with the device connected to them.  If you disconnect the wire to check voltage you stand a good chance of causing damage if you test with a 12 test light.  You also will get incorrect volt readings on an open circuit.  5v connections are very sensitive to voltage drop caused by bad or loose connection, especially the ground side.  What used to be acceptable in the old 12 volt world can cause all kinds of strange issues. 

[Snowcamo]

Wouldnt it be crazy if it was the neutral saftey switch this whole time...

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[Jsdirt]

She said it just died like it ran out of gas in her first post. It was already running, so doubt the neutral safety switch is bad.  A bad one would cause a no-crank.

[Snowcamo]

Its worth testing. Especially since there is a no start condition now.

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[Jsdirt]

It's a "no START", not a "no CRANK" .

[Snowcamo]

Well at least I tried. What do you think the problem is?

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[Jsdirt]

It could literally be one of 1,000 or more things. Without having the truck right here in front of me, everything is just a guess.

[Carla Jean simmons]

Jsdirt can you try naming off some of the 1000 or mpre things because i have replaced and tested my little heart out and im still sitting here with a no start issue. Its about to drive me insaine. I mean one day i have fire and no fuel the next fay i hace fuel and no fire. I mean ugh....  why is the computer sending the trigger signal to the icm, but the icm wont send the switching signal the coil. Icm has tested to be good. 

[Carla Jean simmons]

Is there anything anyone can think of a sensor or anything that would cause the icm not send signal to the coil

[Jsdirt]

Usually when the computer has everything it needs to send a signal, but isn't sending a signal, the computer is bad. It's a tough call to make, but when everything else has been exhausted, it has to be made at some point.

 

As much as I'm against firing the parts cannon and winging parts at a problem, the only thing I can suggest is grabbing a junkyard or eBay computer, and trying it out ...  IF you're 100% sure EVERYTHING - including the wiring & computer inputs - is good.

 

 

I had a '96 Tahoe mop the floor with me 6 years ago. Took me almost 2 years to figure out the junkyard lied to me, and sold me an engine computer that was out of a S10 blazer - no wonder the engine would only run on 6 cylinders! The original crank, no-start problem on that was fixed by the computer - it now ran, but only on 6!  Now, that computer would have worked had it been reflashed to the Tahoe 5.7 mapping, but I didn't know that at the time. It was easier to pay $100 on eBay for a computer, which the seller would program the VIN into before shipping, so it was a plug and play deal. That truck has been running over 20k miles since, and just turned over 200k.

[Doug_Scott]

14 hours ago, Jsdirt said:

Usually when the computer has everything it needs to send a signal, but isn't sending a signal, the computer is bad. It's a tough call to make, but when everything else has been exhausted, it has to be made at some point.

 

As much as I'm against firing the parts cannon and winging parts at a problem, the only thing I can suggest is grabbing a junkyard or eBay computer, and trying it out ...  IF you're 100% sure EVERYTHING - including the wiring & computer inputs - is good.

 

 

I had a '96 Tahoe mop the floor with me 6 years ago. Took me almost 2 years to figure out the junkyard lied to me, and sold me an engine computer that was out of a S10 blazer - no wonder the engine would only run on 6 cylinders! The original crank, no-start problem on that was fixed by the computer - it now ran, but only on 6!  Now, that computer would have worked had it been reflashed to the Tahoe 5.7 mapping, but I didn't know that at the time. It was easier to pay $100 on eBay for a computer, which the seller would program the VIN into before shipping, so it was a plug and play deal. That truck has been running over 20k miles since, and just turned over 200k.

I had one of those.  Car came in with rough idle.  Hooked the scope up to it and it had 14 spark lines.  It back in the days of the Allen Smart Scope.  I kept looking at that screen showing 14 cylinders.  It even stumped the smart scope.  Customer finally showed up so I could talk to him.  He kept saying it ran fine until it quit running.  I asked him what did they do to get it running again.  He got his receipt out and it just showed an Echlin part number that at the time I knew it was a pick up coil for a Chevy.  this car was a small block Chevy in a Buick(in Canada everything pretty much got those)  I pulled the cap off and checked to make sure the wire wasn't trapped or something.  It just looked odd to me.  The little teeth were not so little.  It only had 6 teeth.  The guy that changed the pickup coil put an even fire V6 pickup coil and the reluctor wheel was the correct one (they never failed unless you broke it taking it off),  Put the correct pickup coil in, and that fixed up the issue right away, no more 14 cylinders on the scope.  This customer actually complained to head office for having to pay for the scope time because the scope could not tell me what was wrong.  If it wasn't for it showing 14 cylinders I don't know how long it would take anyone to figure it out, because there is no way it can run with a v6 pickup coils in a v8.  Head office never let us respond to complaints, they would just refund the customer and apologize, then ask me to explain why I would ever tell a customer that I had no idea that would be the issue.  They were lucky I wasn't allowed to explain my actions to the customer.  After a few months the guy at customer service was in the store and I asked whatever came of that case.  He said they had just finished off with the guy, it turned out that it was his brother the barber who changed the pickup coil, and that very same barber told his brother and customer service that there was no way in hell it was the pickup coil, the car would never run if he put the wrong one in.   It was a packaging error.  I can see it happening, I would never expect it to run either.  But it would be a bitch to fix in your backyard.  It was a rolling misfire, never the same cylinder twice in a row.  Never saw it again.