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Re: ECU = Electronic Control Unit Posted by Justin VanAbrahams [Email] ![]() ![]() In Reply to: Re: ECU = Electronic Control Unit, Sunset Park, Wed, 24 Jul 2013 09:20:03 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I think you have it exactly backwards - the bad unit isn't ignoring something, it's failing to act on something.
1. Ignition switch on
2. "System" relay coil closes
2. ECU powers up
3. Turn key to start
4. Hall-Effect or crank position sensor generates signal
5. ECU "sees" signal, grounds "fuel" relay coil
6. "Fuel" relay coil powers fuel pump and injectors
7. ECU pulses injectors by grounding them
A. If the "System" relay doesn't get power from the ignition system or it otherwise failed, the ECU never powers up
B. If the ECU doesn't get a tach signal, it doesn't power up the "Fuel" relay and the pump & injectors don'twork
C. If the ECU fails internally it can't power up the "Fuel" relay and the car can't run
Your "bad" ECU is bad because it's failed in the same way most '89 ECUs fail - the circuit that grounds the "Fuel" relay is fried. Jumping the "Fuel" relay effectively bypasses this safety mechanism so the car runs.
If you're *certain* the reason the car died on the road is because the fuel pump shut down, either:
I. The pump is bad
II. The "Fuel" relay is bad
III. The ECU is bad or not getting power
IV. The tach signal to the ECU is bad
V. Ignition power to either/both the "System" and "Fuel" relays are bad
posted by 12.195.130...
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