I was driving my truck home from work last thursday when it felt like i was running out of gas or the truck was misfiring, so i coasted for about a 1/4 mile then got back into the gas, it felt the same, and then just died. I put the truck into neutral and coasted to a safe place to pull off the road. I started by looking for blown fuses in the engine bay then the interior and finally found a 30a fuse blown in the #30 location in the interior box. after i changed it i tried to start up the truck and it started and ran for about 15-20 sec. then popped the fuse again. so i had the truck towed home. I pulled the coil's and plug's and noticed on the #5 coil it had a small crack and some red ,plastic like stuff coming from the crack. i tested the resistance (Primary/secondary) both fail. so i change the coil with #6 and put a new coil in #6, changed the fuse and started the truck. same thing. popped fuse and cracked the coil open. So I disconnect the bad coil and removed it from the truck. I put the cop cover back on and start the truck, It starts and runs on 7 fine. I let it run for 5-10 min than i took it for a few laps around the block and it is fine (if you don't count the misfire). well to get to the point, The coil's plug has batt + with key on but no negative firing signal from the PCM (running). i checked at the coil plug and at the pcm plug (while running) still no signal to fire from the pcm.

Is my pcm bad? What years can i take the pcm from?

My Truck is a 1999 Navigator 5.4L Dohc 4x4

Thank you, Robert

 

The PCM doesn't supply a "signal" to fire the coil. What the PCM does when it wants the coil to fire is it first "pulls" the coil primary to ground (which causes current to flow through the coil, energizing the coil) and then releases the ground. When the ground is released it stops current flow through the coil and causes the coil to fire.

This is just a guess on my part, but if the ground side of the coil is somehow "shorted" to ground, then the coil will have current flowing continually through it (whcih would cuase it to overheat) and the PCM wouldn't be able to pull the coil to ground or to release the ground to fire the coil (since the coil is continually shorted to ground).

You could try unplugging the PCM and putting your ohm meter on the ground side of the coil connector and a good ground. There should be no continuity to ground.

 

I did that . And checked with a test light from batt + to coil - neg runing ang got nothing on 5 but did get it to flash on 6.

 

So did you ohm out the coil neg wire on 5 by . . . . unplugging the PCM and putting your ohm meter on the ground side of the coil connector and a good ground. There should be no continuity to ground? <!-- / message --><!-- sig -->

 

Yes i did and their was none. So i made sure their was not a brake in the wire, so i checked both ends for continuity (pcm side and coil plug side) and it was fine. It has to be the coil driver in my pcm?

 

I know i will have to take the truck in to have some things programed to the new pcm once i install it so can i use one from a fue years newer. i.e. 2001? it is hard to find the 99 for some reason everyone has 2001.

 

If you need a new PCM the software in your current PCM will have to be 'unloaded' first, then install the new PCM, and finally, the unloaded software will have to be reloaded to the new PCM. (I've never done this, but maybe someone will come along and correct me if I have not got this correct.)
I don't know, but the dealer that is going to unload/reload your software should be able to answer that question.

 

I don't know the reason you are having this problem but the modular engines fire two plugs each time on the 2 valve and 4 valve engines. You would have the same problem with two cylinders if the PCM was bad. There is something else causing your problem, I believe. As you have a SOHC engine the PCM from a 5.4 Expedition, an Eddie Bauer as they are the most similar, might work if you can get a loaner to be able to try.

 

I do not have the sohc engine. its a dohc 4v.

 

If it is not the coil driver in the PCM or a short in the coils wiring what could it be?

 

do you know the fireing order and witch cylender firers with 5?

 

its 3 and 5. i think it would be runing much worse than it is. if it were 2 down cylenders. so their should be continuity in 3-5's neg. side? and the same with 1-6,4-7, 2-8?

 

If i checked the #5's Negitave side for continuity with a good ground and PCM unpluged and got Nothing, then checked for a short in the same wire by; Unpluging the Pcm, locating the 2 ends of the #5's negitave wire (coil plug side-PCM Plug side) and geting continuity in-between the two ends, and still get no signal (PULL) to fire #5. Than what do i do. I guess i will take off the COP covers back off and start the engine and see if #3 is fireing. And if it is, than what, Mcgiver 3 and 5 together? I think it has to be the pcm! anyone???

 

I also never said that it was not giving any check engine light, and is saying "Fail" when i try to pull code's

 

I know i will have to take the truck in to have some things programed to the new pcm once i install it so can i use one from a fue years newer. i.e. 2001? it is hard to find the 99 for some reason everyone has 2001.

I have changed the PCM in several vehicles without having the PCM reprogrammed. In the Mark VIIIs there are several differnt states of tune in the PCMs but they all seem to work just fine. Some people find that a certain PCM makes slightly more power on a dyno than others. I have found no difference. I don't believe you will need to have a reprogramming done.


I do not have the sohc engine. its a dohc 4v.

I looked at several '99s and they all had the SOHC engines. I assumed yours was the same. My bad.


If it is not the coil driver in the PCM or a short in the coils wiring what could it be?

It may be a broken wire, a bad coil or boot, you may have to search deeper.

do you know the fireing order and witch cylender firers with 5?
its 3 and 5. i think it would be runing much worse than it is. if it were 2 down cylenders. so their should be continuity in 3-5's neg. side? and the same with 1-6,4-7, 2-8?

The firing order is; 13726548 so 3 and 5 should fire together. You might try a jumper from 3 to 5 and if the problem clears up you wilol have a better idea of the cure as that would eliminate the PCM.

Make sure you check the injector on #3 to be certain you are getting fuel through it. Just a thought.