1996 Bronco stalls and intermittently won't start - no spark

tommarci

New member
Joined
Dec 31, 2020
Messages
4
Location
Iowa
My 96 started to exhibit this dreaded issue at the end of 2020. After reading through the many similar posts, I stated troubleshooting and here is what I found:
The truck stalls and sets code P0320. It won't start again unless I let it sit for several minutes. No spark. After a short period, it starts and runs fine until the next stalling event.
Reading through a bunch of 10-year-old posts, I changed out the ICM, Distributor and Crank Position Sensor ( yes, my 96 has this sensor mounted externally behind the balancer). Still had the same issue.

Then I started going deep. I used an LED test probe to test the PIP and checked voltages on the coil. Everything checked out. Plugs, wires, cap and rotor, all new. I even found a post about replacing the PCM power relay and swapped this with the horn relay. Same problem.

Recently, I noticed while the truck is sitting and will not start, if I leave the key in RUN, the PCM will reset all by itself. I hear the relay click, the fuel pump prime (1-2 sec) and then it will start. I started looking for loose or shorted wiring and could not find anything serious until I pulled the PCM relay diode. This little bugger tested good with the diode check mode on my DVM but it appears the contacts were oxidized and not making good connection. Just by pulling and reinserting the diode, I was able to scratch the contacts clean enough where the symptom appeared to diminish.

The strange thing about this, every time I turned the key on, I would hear the fuel pump prime, indicating the PCM powered up ok. But I am suspecting that when cranking the starter, the voltage dropped too low through the diode (and or contacts) and the PCM relay was opening up.

We just had 7 inches of snow and I ran the truck all morning plowing snow. Ran just fine but I am skeptical that the diode contacts were the only problem. I am now positioned with several signal and voltage probes temporarily wired into the cab to try and catch what the truck is losing when it stalls but of course the truck now starts and runs without issue, at least for the time being but I'm watching and waiting.

1996 Eddie Bauer, 5.8L 110k miles.
Meyer 7' Lot Pro
 

Tiha

Member
Joined
Jul 22, 2020
Messages
120
Location
Midwest
I was going to say the PIP or Coil, I have had both of those do hot start/stalling problems.

One thing you can do is clip a test light onto the positive terminal of the coil. See if it has voltage while cranking. Might help you narrow it down.

If it has voltage, light stays on. Then you are looking at a coil or pip issues. If not it is probably an ecm/wiring issue.
 

tommarci

New member
Joined
Dec 31, 2020
Messages
4
Location
Iowa
Thanks Al. I saw that laundry list of tests from a previous post and was hoping I didn't need to get into the majority of it. Since the truck runs fine most of the time, I would guess most of these tests would pass until the problem occurs.

I have test equipment setup in the truck to probe and narrow the problem down once it occurs again, although it hasn't since I "cleaned" the diode contacts. After chasing this for several weeks, I'm focusing on the intermittent loss of power to the PCM as this seems to be the symptom.
 

tommarci

New member
Joined
Dec 31, 2020
Messages
4
Location
Iowa
Tiha, I appreciate the input. I am checking the PIP constantly with an LED test light. It's a new distributor so I did suspect there was a problem with the pickup but it appears to be reliable. The coil voltage has been more challenging. Every time the problem occurs, by the time I put a voltmeter or probe on the coil, the problem has vanished. I started to think the low side of the coil wiring or connections were intermittent but could not confirm.
Waiting for the next episode so I can provide more info.
 

larry105

New member
Joined
Nov 24, 2008
Messages
2
My 96 started to exhibit this dreaded issue at the end of 2020. After reading through the many similar posts, I stated troubleshooting and here is what I found:
The truck stalls and sets code P0320. It won't start again unless I let it sit for several minutes. No spark. After a short period, it starts and runs fine until the next stalling event.
Reading through a bunch of 10-year-old posts, I changed out the ICM, Distributor and Crank Position Sensor ( yes, my 96 has this sensor mounted externally behind the balancer). Still had the same issue.

Then I started going deep. I used an LED test probe to test the PIP and checked voltages on the coil. Everything checked out. Plugs, wires, cap and rotor, all new. I even found a post about replacing the PCM power relay and swapped this with the horn relay. Same problem.

Recently, I noticed while the truck is sitting and will not start, if I leave the key in RUN, the PCM will reset all by itself. I hear the relay click, the fuel pump prime (1-2 sec) and then it will start. I started looking for loose or shorted wiring and could not find anything serious until I pulled the PCM relay diode. This little bugger tested good with the diode check mode on my DVM but it appears the contacts were oxidized and not making good connection. Just by pulling and reinserting the diode, I was able to scratch the contacts clean enough where the symptom appeared to diminish.

The strange thing about this, every time I turned the key on, I would hear the fuel pump prime, indicating the PCM powered up ok. But I am suspecting that when cranking the starter, the voltage dropped too low through the diode (and or contacts) and the PCM relay was opening up.

We just had 7 inches of snow and I ran the truck all morning plowing snow. Ran just fine but I am skeptical that the diode contacts were the only problem. I am now positioned with several signal and voltage probes temporarily wired into the cab to try and catch what the truck is losing when it stalls but of course the truck now starts and runs without issue, at least for the time being but I'm watching and waiting.

1996 Eddie Bauer, 5.8L 110k miles.
Meyer 7' Lot Pro
Check and see if drain between fender and firewall is clogged, water can pool between fender and firewall and seep into PCM. I removed PCM and dried it out (lucky no damage) . removed wipers and slotted vent and sealed area, so far no problem.
Also check wiring above distributor area, Had a wire short out intermittently and theat caused it to die suddenly
 

tommarci

New member
Joined
Dec 31, 2020
Messages
4
Location
Iowa
So far since I wrote this thread, the truck has been starting and running reliably.

I checked wiring around dist and coil again. Last thing I did was clean the connections on the PCM relay coil diode. I have an LED test light connected to the PIP and voltmeter ready to connect to coil if it happens again. Problem is so intermittent I have to wait until it occurs again.
 

Members online

No members online now.

Forum statistics

Threads
21,422
Messages
124,657
Members
23,202
Latest member
builtjrtough
Top