Mass Air Flow Sensor and the Haynes Manual

[Postal_Dave]

I have a Crank, No Start problem with my 1996 Ford Explorer XLT and I'm testing my Mass Air Flow Sensor. The Haynes Manual says disconnect the harness connector to the MAF to test the DC voltage. It says that with the key on, the voltage going from the B+ terminal in the connector to the ground in the connector should be over 10 volts or close to the battery voltage.

From the B+ terminal in the connector to the Negative battery Terminal, I get 12.5 volts. However, I'm only getting 5.4 volts from the B+ terminal in the connector to the ground in the connector.

The Haynes Manual says that this should indicate a bad ground wire connection, so I checked the wire. I disconnected the PCM connector and did a continuity test from the MAF connector to the PCM connector pin and everything was fine there. I also checked the ground wire, to see if it could be losing voltage because of a break in the wire by running the battery's current though the wire. It came back at 12.5 volts that way. So the ground wire seems to be fine.

I'm guessing that something inside PCM is causing the voltage to drop by restricting the flow, or bleeding off the current. Do you think that's possible? Do you think this would cause a Crank, No Start issue?

Thanks for your help,
Dave

 

[crunchie_frog]

The Hayne's manual shows two grounds in the MAF connector. One will give battery voltage, the other 5 or so volts.

 

[Pete Deering]

The Hayne's manual shows two grounds in the MAF connector. One will give battery voltage, the other 5 or so volts.

The MAF uses reference voltage 5.0v.

 

[Pete Deering]

The MAF uses reference voltage 5.0v.

It looks like you have a 4 pin connector. Power in(12v) red, Ground G101 Black, pin 50, MAF signal wire light blue / Red ( 5v) and Pin 9 MAF signal return Tan / light Blue. The MAF sensor, measures air flowing into the engine for the PCM which uses the input to aid in calculating fuel delivery.

 

[410Fortune]

the MAS usually will not cause a no start, engine should start and run even with it disconnected
I would suspect the crank sensor (crankshaft sensor down on the crank balancer)
Easy to rule out though...

Air / fuel / spark
Do you have spark? If so then the crank sensor is good
Do you have fuel pressure 36-42 psi for a 96

 

[Postal_Dave]

The MAF sensor connector does have 4 pins. The "B+" red wire is the hot with 12.5 volts. Next to it is the PCM Power Ground black wire, I do get 12.5 volts when I connect those two. Next is the GND (ground) wire, Tan/Blue wire and it reads 5.4 volts. And lastly is the signal wire which is Lt Blue/Red wire and I also get 5.4 volts there.

So when the Haynes Manual says to connect the negative lead of the multimeter to the ground wire and it should read the same as the battery, it doesn't mean the one it marked as "GND", it's the one marked PCM Power Ground.

Thank you all. I guess that means that everything is working for the MAF.

 

[J_C]

Did this happen suddenly, or had it been getting worse running, or had it sat without running a long time?

It's easy enough to check for spark, but you could also hook up a scan tool capable of live data to look at the crank sensor signal (or pull codes if any are set), or spray starter fluid into the intake (after the MAF sensor).

 

[Postal_Dave]

I'm having a no spark problem that came on suddenly after changing my spark plugs and wires. As soon as I finished changing the plugs and wires, it started up and sounded perfect. As soon as the engine got warm, (5 minutes of running) it died like someone turned off the key. No sputter or anything. It would crank but no spark after that. The weird thing is, after 24 hours of sitting with the negative battery terminal unhooked, I hooked up the battery and it started back up and sounded perfect again. 5 minutes later it died. Once again, I unhooked the battery, waited overnight and it started again and died again. Now it won't start at all.

I've replaced the coil pack, the camshaft position sensor, the ignition switch, and the crankshaft position sensor. I've tested every fuse and tested every other sensor I can think of that could cause the computer to not tell the coil to fire.

The only thing I can think of is that one of the new spark plug wires somehow fried the computer.
Tomorrow, I'm going to the junk yard and getting a PCM and trying that. I'll let you know if that works.

 

[J_C]

I am wondering if the parts swapping caused a vac line break and now it's leaking, so runs until it tries to go closed loop, then after disconnecting battery, ran richer again on that attempt.

Wait, you did write "no spark problem" so it's definitely the case that your spark plugs aren't firing? If so then I wonder about some wire disturbed in the wiring harness.

 

[Postal_Dave]

There are a lot of strange things going on here. One of them is that the spark plug won't fire while cranking, but sometimes, when I quit cranking, it will fire.


I put one of my old spark plug wires and plugs on each input on the coil pack. I replicated this result on 4 of the 6 coil pack inputs. The middle coil for cylinders 2 and 6 didn't fire at the end. I don't know why it does that, but I did check the coil pack with the multimeter and it seemed fine.

Here is what happened the last time it turned off.


It ran perfect until it got to operating temp and then just turned off. I'd be glad to check all the vacuum lines but I'm wondering if there are any vacuum lines that go to something that would cause the no spark problem and keep the engine from starting or turn it off that way.

 

[Pete Deering]

There are a lot of strange things going on here. One of them is that the spark plug won't fire while cranking, but sometimes, when I quit cranking, it will fire.


I put one of my old spark plug wires and plugs on each input on the coil pack. I replicated this result on 4 of the 6 coil pack inputs. The middle coil for cylinders 2 and 6 didn't fire at the end. I don't know why it does that, but I did check the coil pack with the multimeter and it seemed fine.

Here is what happened the last time it turned off.


It ran perfect until it got to operating temp and then just turned off. I'd be glad to check all the vacuum lines but I'm wondering if there are any vacuum lines that go to something that would cause the no spark problem and keep the engine from starting or turn it off that way.

I'm having a no spark problem that came on suddenly after changing my spark plugs and wires. As soon as I finished changing the plugs and wires, it started up and sounded perfect. As soon as the engine got warm, (5 minutes of running) it died like someone turned off the key. No sputter or anything. It would crank but no spark after that. The weird thing is, after 24 hours of sitting with the negative battery terminal unhooked, I hooked up the battery and it started back up and sounded perfect again. 5 minutes later it died. Once again, I unhooked the battery, waited overnight and it started again and died again. Now it won't start at all.

I've replaced the coil pack, the camshaft position sensor, the ignition switch, and the crankshaft position sensor. I've tested every fuse and tested every other sensor I can think of that could cause the computer to not tell the coil to fire.
?
The only thing I can think of is that one of the new spark plug wires somehow fried the computer.
Tomorrow, I'm going to the junk yard and getting a PCM and trying that. I'll let you know if that works.

Its sure sound like a crankshaft position sensor failing. My neighbor told me, he had a similar problem. In the morning, he started his car, drove to highway (5 min drive) car up to operating temperature and once getting on the highway, its stop running. Where did you get the crankshaft position sensor?

 

[Josh P]

In another thread, the crank shaft position sensor and wiring from the ecm to the sensor has been tested, confirmed good. The issue is no signal from the ecm to the ignition coil pack. The ecm diode blew, that might be part of the problem, possibly a short in another sensor.

 

[410Fortune]

inspect the CRANK sensor again, check the wiring to it, the plug, the pins, etc
Maybe install your old one and see if it changes?
You may try doing the wiggle test, wiggle all wiring, it really sounds like a loose connection, like a loose ground or something is not tight

PCM is suspect at this point too.... and I hope you have ruled out fuel pressure. I know its a long shot, since it fires up and runs smoothly, then just suddenly dies, but as a mechanic we have to rule things out, otherwise we would just be throwing parts at things..........

 

[Postal_Dave]

I went to LKQ junk yard and got another PCM and installed it. Same problems. Crank, no start. Spark plugs fire after I quit cranking.

I checked the wires from the Crankshaft positions sensor to the computer completely. I get continuity and I get 1.4 volts AC at the PCM. I'll put my old CKP on and see what happens later. This is getting depressing.

The Question is: What sensor input would tell the computer not to spark, besides the CKP?

 

[410Fortune]

Camshaft synchronizer is used to time ignition spark

 

[Josh P]

Does the check engine light illuminate while cranking?

 

[Postal_Dave]

I haven't checked the signal from the camshaft sensor yet. I'll look at that next.

As for the check engine light: When I turn the ignition to "On", the light comes on. It immediately turns off when I start to crank the engine but it comes back on about 2 seconds into cranking and stays on. Then, after about 4 seconds of cranking, the 4WD and 4WD Low lights show up on the dash then they go away a couple of seconds later. The oil pressure gauge goes to operating pressure after cranking for about 4 seconds also.

 

[Josh P]

All of my stuff is older than 96, so bear with me. The check engine light coming on indicates the ecm is getting power and signal from the crankshaft position sensor, we can quit beating that dead horse. Anyone know if there is an external ignition control module? On the older fords I've had those go bad causing a no start condition

 

[Postal_Dave]

On the 1996 Explorer the EDIS, (which is separate on some older fords), is part of the ECM. That is the only other ignition control module that I know of.

 

[Josh P]

I guess we need to make an updated listed of what has been checked and ruled out. There are a few fuses related to the ecm, have you checked them? I'm wondering if this is a bad ground problem.