ECU or TFI? or something else?

[Dunderberg]

1990 B2 4x4 5speed with stock MAF system:
It died on the road- a couple of hiccups and ten seconds later engine died. Engine cranks, no start.
There was one code (don't remember the #) it said something about secondary fuel pump relay problem. Both Fuel pump and ECU relays operate when triggered manually. Key-on, fuel pumps do not prime. When the fuel pump relay wire at the ECU harness is grounded, the pumps work fine. Perhaps there is a failure in the ECU grounding the relay? A general grounding issue? Or the TFI is bad (but wouldn't the pumps prime as the TFI wouldn't be sending a cranking signal at that point?) or am I off base with this assumption? Or something else? When ECUs fail do they give a self diagnosis failure code or warning? Any ideas? Thanks for reading.

 

[rubydist]

On obd1 Ford vehicles, a code for secondary fuel pump issue is typically set every time the engine stalls, and can be disregarded.

In your case, you have started down the path you need in order to find the problem. You will need to determine if the pcm is commanding the fuel pump to run or not. If it is, there is an open in the wiring harness somewhere that need to be found and fixed. If it is not, then the pcm almost certainly needs replaced.

 

[Dunderberg]

Rubydist, thank you for your reply!
The wire from the PCM to the relay has no resistance, so the PCM has failed then.
Hopefully there isn't another harness issue that caused that particular chip to fail.

Curious as to whether the auto parts stores actually "remanufacture" the PCMs they sell,
or if they are units pulled from junked cars that still happen to function & cleaned up to sell......

Cheers, Jeff

 

[RonD]

Pull out the PCM(ECU) and have a look inside at the circuit board, the 2 or 3 blue capacitors specifically

These Caps often fail and leak after 20+ years, and no fuel pump is one of the symptoms
Under $5 to replace these and some soldering, replace all 3 if one looks bad

Look at the CEL(check engine light) it should come ON with key on and then go OFF as soon as you are cranking engine with starter motor, OFF means computer is seeing TFI timing pulse
If CEL doesn't go OFF then its not seeing timing pulse

But PCM doesn't start fuel pump again until RPMs are above 400
So just the Prime and then nothing until engine is started, cranking speed is 200rpms
Its a safety thing, if fuel line should break in an accident engine would run out of gas and stall, under 400rpms and PCM would shut off fuel pump power so wouldn't "feed a fire" if key was still on

 

[Dunderberg]

Ron this is great info- will recheck the main board.
Also will do the CEL idea- one of the easiest things to do!
Cheers, Jeff

 

[rubydist]

OP, I am not quite sure what you mean by "The wire from the PCM to the relay has no resistance, so the PCM has failed then. " The correct answer depends on what the pcm is doing when you are making that check.

The wire from connector to connector should have zero (or almost zero) resistance. While I don't have the wiring diagram for your truck, the way the pcm typically would turn on the relay for the fuel pump is to ground the coil on the relay, so if you are checking resistance to ground of the wire from the pcm to the ccrm, at the ccrm when the pcm is commanding the fuel pump to run, you should get zero (or nearly zero) resistance.

 

[Dunderberg]

Hi Ruby-
Yes it was oddly worded. I knew the relay operated, so I checked the wire from the relay plug to the
ECM harness connector. It check out fine (no resistance). So it's the ECM unable to turn on/off the relay by grounding it.
Cheers, Jeff

 

[wildbill23c]

Fuel pump shut off switch? My 88 Bronco 2 did the same thing, died, wouldn't start, no fuel pump operation. The fuel pump shut off switch was shot, would trip itself going over railroad tracks...replaced, never an issue sense.

 

[Dunderberg]

Follow-up / conclusion:

Replaced the two capacitors inside the PCM/ECU and reassembled.

The first five 'key on" tries, the fuel pumps were silent. On the sixth, all subsequent faux-starts, the pumps primed to 34psi.
I was wondering if the oversized caps I installed might have needed more "electrical priming" to operate?
They were 47 and 3.3 uF caps, but rated at 450v instead of the stock 16v-63v...........

So for about $10 in parts, the vehicle seems good to go. And I have enough spare caps to fix 9 more CPUs!

Cheers, Jeff