98 PCM replacement question


Karcastic

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Hello everyone,
I have a question about replacing my 98 mazda b4000 PCM. I have a spare from another truck (exact same model) I can use but does anyone know if it will work without any modifications?

Back story:

My truck died abruptly while driving and just rebuilt top half, custom 97 converted to 98 exhaust system. Exhaust manifold to y needs more tightening (f ing with) and leaked enough for a constant code ever since the rebuild (1 year ago). I was hoping it just maxed out trim ratios for so long that it refused to start, but I tried to scan it and I could not establish communication with the PCM (F). It cranks great, batts good, CEL is on, disconnected batt for an 1hr and still no start. I did notice 1 sensor wire was shorted from some wicked lubricant from the 80’s I sprayed (in a pinch) to free up the throttle air intake spring assembly. I thought it was an injector plug but it had two leads, maybe it was the temp sensor.. I’ll have to check again. Anyways, maybe that wrecked the computer, it might still be shorted too.. I’ll definately clean that up before replacing the pcm. Any input is appreciated, thanks

Karcastic
 


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RonD

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PCMs need to match engine model(4.0l OHV), transmission type and model, and emissions, i.e. EGR or no EGR

Do 50/50 test
Spray gasoline or ether(quick start) into the engine
Try to start it
If it starts up and then dies you have no fuel getting into the engine
If it doesn't start then you have no spark
50/50 test

Yes, a failed PCM can cause a no start, although they do fail that often
Only sensor that can cause a no start is the Crank sensor(not cam sensor)

When an engine starts to run poorly and then gets a no start 9 times out of 10 is has no fuel, 1998 has 20 year old fuel pump
Do you hear the fuel pump when you turn on the key?
It only runs for 2 seconds but is loud enough to hear in the cab
Repeat key on and off as much as you need to confirm you hear or do not hear the fuel pump
 

Karcastic

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Thanks for the response RonD,
I have not yet tried the 50/50 test yet (no starter fluid handy) but I tried listening for fuel pump. It does have a slight constant hum when keyed on after the beeps stop beeping. I also notice some light relay clicking sounds. I just thought a pcm swap would be easiest as I have two of these trucks side by side and it fails to communicate. I guess I should try to stir up an engine compartment relay diagram and try swapping fuel pump relays maybe. And of coarse I should probably pickup some starter fluid just in case for the 50/50 test you have suggested
 

RonD

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Yes, PCM from same 1998 model would work
1998 Ranger did not have PATS, but it was an Option

PATS(passive anti-theft system) prevents swapping PCMs without reprogramming, 1999 and up Rangers with V6 engines all had this
 

Karcastic

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Nice! I thought that was the case but wasn’t certain. So I guess it’s as simple as making sure the sensor wires are not shorted anymore and swapping out the PCM. I’ll update this thread, I’m hoping to swap tomorrow. Thanks again RonD
 

RonD

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Unhook the battery FIRST :)
 

Karcastic

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Update,
I finally had time to check all PCM and fuel pump fuses, swapped both pcm and fuel pump relays and repaired shorted fuel injector wires and still no start. I swapped PCMs and she started up. Thanks for the assistance again RonD and a Good Friday to you sir
 


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