What sensor would tell ECM to not allow spark?

[Postal_Dave]

1996 4.0 Explorer. I changed the spark plugs and wires. It started up and ran great until it reached operating temp then it shut down. It did Not sputter, it just turned itself off like you would do with the key. I tried to restart it, it would crank, but it created no spark. It would not restart again til the next day, and again, it ran perfect for 5 minutes. Then engine reached operating temp, and it shut itself off again. The third was was a repeat of the first two.

Since then I've changed out the camshaft position sensor, the crankshaft position sensor, the ignition switch, the coil pack, and the computer. Nothing has changed. I've checked every fuse and relay, all are fine. I've checked the wiring completely over and I'm getting the right voltage and continuity.

I believe that some sensor over heated in those 5 minutes that it ran and told the computer to shut down. Then finally, on the third day, it blew. That overheated sensor is keeping the computer from allowing the engine to spark.

So the question is, what sensor would not allow the ECM to start my Explorer? Would an exhaust sensor do that?
Thanks for your help,
Dave

 

[J_C]

I'd hook up a scan tool capable of live data, looking for anything out of place but particularly the crankshaft position sensor signal. Maybe it's not the sensor itself but rather the wiring or connector is bad. You can note the wire colors, look at same color wires at the ECM (or see if you can find a wiring diagram for that ECM, don't think I have one for a 4L OHV), and measure for low AC voltage with a multimeter while cranking.

I don't follow what you're stating about the sensor overheating. Why would it have overheated if the rest of the engine didn't?

I'd also check these:

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Sometimes a gremlin can be caused by the smallest thing. There are 2 large wire bundle connectors on the firewall. Both are held tight with a 10mm bolt. Any bad connection in either plug can cause just about any issue. from bad transmission shifting, bad o2 sensor readings, no start, no...

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[Josh P]

This one has me stumped, you tested multiple ways to determine the ecm is getting the signal from the crankshaft sensor.

 

[Turdle]

How are you sure you don't have spark?

Fuel gauge level now? Parked level or downhill?

 

[Postal_Dave]

J_C: My scan tool isn't too sophisticated and it doesn't show anything happening using live data that would indicate a reason for it to not spark. I did check the Crankshaft position sensor. I check all voltages to and from the sensor. I disconnected the ECM harness and tried cranking it and I did get 5 volts AC back to the ECM.
I also checked that wiring connector thing, it was fine.

Turdle: This is what happens when I crank the explorer. No spark until the key is released.


Guys, I believe that the explorer was giving me a clue when it was dying. Because it ran perfect for 5 minutes, I don't believe it was the crankshaft position sensor or the camshaft position sensor because they wouldn't have really changed. It could have been the ECM but I changed that out and there was no change.

The only thing that could have gotten that hot in that short of amount of time and may have caused an over heated sensor is something in the exhaust system. Looking at the wiring schematics, we have 4 "Heated Oxygen Sensors".
If any of those blew, would the engine refuse to start?

 

[Turdle]

I don't think so.
I'm suspecting the ignition switch now.

 

[Postal_Dave]

I already changed that out too. lol

 

[Runnin'OnEmpty]

Not sure if a bad O2 sensor would cause a no-start condition, but they usually trigger a fault code. You might try unplugging them one by one to see if the engine then starts....

Ignition coils will short out with heat, but I think you've already swapped that out...?

 

[Runnin'OnEmpty]

I don't think so.
I'm suspecting the ignition switch now.

I ran upon this same situation, where the engine would fire at 'start' then die at 'run'. Ignition switch was the culprit, but this was on an old school system with a carb and coil with resistor.

Explorer doesn't have a ballast resistor, but I wonder if it has two ignition circuits, to give the coils more voltage when starter is turning???

 

[Postal_Dave]

I was thinking that the coils weren't getting enough voltage but I checked the voltage going to the coils while cranking and it was over 10.5 volts. I even charged the battery in the explorer then put a jumper battery on it to give it extra push, nothing changed.

Google shows some articles that say the ECM will not allow a vehicle to start without an O2 plugged in, some say it will just ride rough. I'll pull the harness for the ECM and check the wires for the O2 sensors there and see if I get current back from the O2 when the switch is on. I'll let you know what readings I get.

 

[C420sailor]

99% sure these things will run I open loop with the oxygen sensors disconnected. Could be wrong though, often am

After she shuts down after five minutes, will she start right back up? If not, how long until she will?

 

[Josh P]

Something I've found, there is a 5V reference to the tps, dpfe and fuel tank pressure sensor, if one of the sensors or the reference circuit is shorted it can create a no spark no start condition.

 

[Pete Deering]

I think its the coolant sensor. When the coolant sensor goes bad the engine will not stay running when warm. Cold start, the engine (ECM) uses the default setting. The warm starts the (ECM) engine, uses the sensors input. I know there is a starter relay (95) that provides power too the ECM

 

[Postal_Dave]

C420: It will crank, but won't start anymore. When it would start, it took a whole day of rest before it would start. After the first time it died, I waited 8 hours and it still wouldn't start. It only started 3 times.

Pete: I thought that at first, but it won't even cold start now. Also, the temp gauge still works fine in the car. The gauge would get to operating temp and the explorer would shut down.

Josh: I've been going over the wiring schematics and there is a grey wire with a red stripe that seems to be the one that carries the 5 volt reference charge to the sensors. I printed out the schematics and used an orange highlighter to follow where all those wires go. It looks like a road map to a major city. I've back probed a bunch of those sensors and they all seem to be fine. I've also tried unplugging those sensors to see if that affected anything and it didn't help at all.

Guys, I want you to know how much I appreciate all your advise. Please keep sending me your thoughts on this. I've tried a lot of things on this Explorer to get it running again but I'm really hoping that someone will find the secret that is escaping me here. If nothing else, someone coming behind us might find this thread useful.

Thanks again guys,
Dave

 

[Josh P]

What is the voltage you are getting on the 5V reference wire?

 

[Turdle]

Also. Engine coolant sensor is not the gauge sender. 2 different sensors. Gauge will work fine with engine coolant sensor disconnected.

 

[Pete Deering]

C420: It will crank, but won't start anymore. When it would start, it took a whole day of rest before it would start. After the first time it died, I waited 8 hours and it still wouldn't start. It only started 3 times.

Pete: I thought that at first, but it won't even cold start now. Also, the temp gauge still works fine in the car. The gauge would get to operating temp and the explorer would shut down.

Josh: I've been going over the wiring schematics and there is a grey wire with a red stripe that seems to be the one that carries the 5 volt reference charge to the sensors. I printed out the schematics and used an orange highlighter to follow where all those wires go. It looks like a road map to a major city. I've back probed a bunch of those sensors and they all seem to be fine. I've also tried unplugging those sensors to see if that affected anything and it didn't help at all.

Guys, I want you to know how much I appreciate all your advise. Please keep sending me your thoughts on this. I've tried a lot of things on this Explorer to get it running again but I'm really hoping that someone will find the secret that is escaping me here. If nothing else, someone coming behind us might find this thread useful.

Thanks again guys,
Dave

The temp gauge sensor(for gauge) is different than the coolant temperature sensor (for ECM)

 

[Postal_Dave]

Josh: I'm getting 4.8 volts through the reference line. So that should be fine.

Turdle and Pete: I didn't know that there are two coolant temp sensors. Thanks for the info. I had to pull off the throttle body off to get to it and test it. The sensor tested fine. 25k ohms, which is the correct amount of resistance when cool. Also checked for continuity for the wires back to the ECM and it was fine. It was also getting 4.8 volts sent to it.

BTW: I checked the heaters on the O2 sensor and they were fine. I still want to check the resistance for the O2 sensors to see if they fall in the correct tolerances.

Please keep sending in the ideas. I'm learning a lot here. lol

 

[J_C]

I've extracted what looks like the relevant section from a '96 Workshop Manual, the Integrated Ignition - No Spark, section and a few sections it linked to. If there are any referenced links in it that are missing from what I've attached, let me know and I'll try to extract those.

 

[Pete Deering]

Josh: I'm getting 4.8 volts through the reference line. So that should be fine.

Turdle and Pete: I didn't know that there are two coolant temp sensors. Thanks for the info. I had to pull off the throttle body off to get to it and test it. The sensor tested fine. 25k ohms, which is the correct amount of resistance when cool. Also checked for continuity for the wires back to the ECM and it was fine. It was also getting 4.8 volts sent to it.

BTW: I checked the heaters on the O2 sensor and they were fine. I still want to check the resistance for the O2 sensors to see if they fall in the correct tolerances.

Please keep sending in the ideas. I'm learning a lot here. lol

From my experience, I measure the resistant of coolant sensor( 60 degrees F, 24k ohms on 200k scale, 0.61 volts . when at 190 degree F, 2.8 ohm, 3.0 volts. My sensor did not shown up As bad, till I replaced it. It came apart. The 02 sensor you need to measure the voltage, when the engine is running you will see the switching. up stream sensors. O2 sensor just adjust the fuel mixer.