bad ecm?

mineman_bruce · Sep 7, 2014

Hey all need a second opinion. I need to drve my truck from San Diego to Seattle before I deploy. How ever since I bought this truck it's been nothing short of frustrating. 94 4.0 manual. I've been chasing a intermittent stall issue. It usually does it when it's the most inconvenient like a bridge or i5 in morning cali traffic. Sometimes it'll go a whole week no problem. Or it will do it many times a day. No error codes. So I cleaned the maf checked the 02 sensors the I actually replaced the thermostat and ect 2 wire sensor for the ecm. Etc etc. Nothing. And 2 very reputable shops can't figure it out either. So today I did a cold koeo test for shuts and giggles. Sensors clicked etc came back 111. Cool. So I started it and warmed the truck up, turned it off plugged in the scanner and it wouldn't start. Cranks fine etc. Thought ok this is good maybe my scanner will find something. Hooked it up did the procedures turned the key on pump primed etc. Turned on the scanner to test and the truck would do a self diagnosis. Let it cool for about 15 minutes tried the koeo test again and it did the diagnosis just fine. Warmed it up same issue. Scanner had the connecting symbol goi BFF but the truck wouldn't self test. Pcm is giving out power to the pump aND starter etc. This leads me to belive the ecu is going bad? Sometimes if I park at a store then come out it won't start and I wait 15 minutes cooling off and it starts and runs like there was never an issue. Please help me before I roll this thing off a cliff and deploy

RonD · Sep 7, 2014

Could be PCM(ecu) but it would be a long shot.
1994 Ranger PCM location, read here: http://www.therangerstation.com/forums/showthread.php?t=87246

The 1994 model is an EEC-IV pcm/ecu, which only runs the fuel system.
The spark is run by an EDIS module, on my '94 it is on the radiator support by battery(drivers side) but on the FRONT SIDE of the rad support.
Google: Ford EDIS module

To see what it looks like, 4.0l will have an EDIS-6(6 cylinder)

There are very few things that can cause an engine to just stall while driving.
Ecu may be cutting the fuel off, but more likely would be a bad wire or Fuel pump relay.

There is only 1 sensor that can stop the engine, the CKP(crank position) sensor, but these either work or they don't, never heard or read about a CKP sensor that was intermittent, hot or cold.
The CKP sensor is hooked directly to the EDIS module and the EDIS module is hooked directly to the Coil Pack, this system doesn't need the computer, it sends a pulse to the computer for injector timing and will get data back from computer for spark timing advance.

If you can get it to stall again I would have a can of starting fluid(ether) handy, try to restart, if it doesn't start then pull off the power brake vacuum line on the intake, spray the ether into the intake and replace the vacuum line.
Crank engine, if it starts and dies then fuel is the problem.
If it doesn't fire at all then spark is the problem.

There are 2 relays that pretty much run the whole shebang.
EEC relay, when you turn on the key the EEC Relay closes and sends power to ECU, Fuel injectors, EDIS module and Fuel Pump Relay.
Fuel injectors and the Fuel Pump relay have power when key is on but they are not active.
The Ecu grounds these to activate them.
Ecu will Ground the Fuel Pump Relay for 2 seconds when key is first turned on, it won't ground it again until RPMs are above 500, then it grounds it until key is turned off.

The fuel pump power comes from the Fuel Pump fuse, the fuel pump relay just connects the fuel pump fuse and fuel pump together.
The fuel pump power passes thru an inertia switch, passenger side kick panel or under glove box.
An inertia switch can become intermittent, it is there to cut fuel pump power in the event of a sudden stop or roll over, so accident.

mineman_bruce · Sep 7, 2014

Well I know the inertia switch is good as I used it to do the fuel rail cleaning at the shop. They tested for spark and it was hot and bright. I've swapped relays and that didn't fix the issue at a parking lot last weekend. It gets fuel I had 38 psi I want to say while cranking. This is rather frustrating. I'll check into the things you mentioned

mineman_bruce · Sep 7, 2014

I know it's heat related. I'll check the ignition module again to be sure.

RonD · Sep 7, 2014

"It had spark at the shop"

What does that mean?
Did it not start but had spark?

If you are pretty sure it is heat related, then it is most likely a bad wire connection.
Pull the fuel pump relay and the EEC relay, test for 12volts with key off, those sockets are from the Fuel pump fuse and EEC fuse.

Now turn key on and test the other sockets for 12volts, those are the "activate relay" sockets, make sure these show battery voltage, if battery is 12.5v then these should be 12.5v, not 11.9v, for example.

When a wire gets warm it's resistance goes up so the voltage passing thru it goes down, corroded connection or wires already have lower voltage, so heat lowers it even further.
The relays need XX volts to hold them closed, if voltage drops below the XX point relay will pop open.

black_demon69 · Sep 7, 2014

check fuel pump mine did the same thing and drove me crazy until i changed the pump... check the fuel pressure when you experience the problem..

Spott · Sep 8, 2014

Could be a faulty ignition switch, as well.

bad ecm?

mineman_bruce

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