igende

I came across this article while researching how the ECM is notified to prime the fuel pump. Thought I would share to help others searching for the same. Written by someone named Cliff Harris, about vette's but, it's all GM, so pretty much the same for our birds. I highlighted in bold the pieces that show how the ECM works with the Fuel Pump relay OPSU and ICM. The focus of his article is to debunk myths around how the Relay and how it works, but the sections I highlighted give good info on how things interact to make the system actually work. Enjoy!

http://www.misterpeachy.com/VetteStu...elay_Myth.html

Fuel Pump Relay Myth Debunked

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I have recently become aware of a "myth" that has been spreading around the Corvette world. I have heard two versions of basically the same myth.

First version of the myth:

The first version says that the fuel pump relay is used ONLY to prime the fuel injector rails for starting. It runs for two seconds when the ignition key is turned on and then turns off and never turns on again.

Second version of the myth:

The second version says that the ECM turns on the fuel pump relay for two seconds at key on and the engine must be cranked within that two second window or the fuel pump relay will turn off and never turn on again. This version says that the fuel pump relay will stay on if cranking begins during that 2 second time frame.

In both versions it is alleged that the oil pressure switch "takes over" and turns on the fuel pump when it sees at least 4 PSI of oil pressure. This is also touted as a safety feature, as supposedly the fuel pump will turn off if the engine loses oil pressure.

References

This quote supposedly comes from the 1982 Corvette FSM (Factory Service Manual). The 1984 crossfire injection setup is identical:

"The electric fuel pump draws high current, so it is not controlled directly by the ECM. Instead, the ECM controls a fuel pump relay which in turn controls system voltage to the fuel pump circuit. When the ignition is turned ON, before the starter is engaged, the ECM turns on the fuel pump relay for 2 seconds by providing ground. If the ECM does not receive ignition reference pulses from the ignition module within the 2 seconds, it shuts off the fuel pump relay until the engine is cranked. The ECM grounds the relay circuit as long as it receives ignition reference pulses.

The 1986 FSM (page 6E3-A-26) says: "When the ignition is turned "ON", the Electronic Control Module (ECM) will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving HEI distributor reference pulses".

Page 6E3-C2-4. In the "fuel pump electrical circuit" section, it says: "... If the engine is not started within two seconds, the ECM will shut the fuel pump off and wait until the engine starts. As soon as the engine is cranked, the ECM will turn the relay on and run the fuel pump".

Page 6E3-A-82: "There should be about 12 volts on CKT 120 for 2 seconds after the ignition is turned on or any time reference pulses are being received by the ECM.

1989 Corvette FSM (Page 6E3-C2-3):

"When the ignition switch it turned to the "ON" position (engine not running) the ECM turns the fuel pump relay "ON" for two seconds, causing the fuel pump to pressurize the fuel system. If the ECM does not receive ignition reference pulses (engine cranking or running) within two seconds, it will shut "OFF" the fuel pump relay, causing the fuel pump to stop."

"As a backup system to the fuel pump relay, the fuel pump can also be turned on by the oil pressure switch. The oil pressure sender has two internal circuits.

(On thirdgens we usually have two separate oil pressure sensors, the one closest to the oil filter is the OPSU which will run your pump as a backup to the fuel pressure relay, the one above the OPSU just controls info to the oil pressure gauge on your dash. Back to his article now.)

One operates the oil pressure indicator or gauge in the instrument cluster, and the other is a normally open switch which closes when the oil pressure reaches about 28kPa (4psi). If the fuel pump relay fails, a Code 54 will be set in the ECM, and the oil pressure switch will run the pump.

"An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn "ON" the fuel pump as soon as oil pressure reaches about 28 kPa (4psi)."

Comments

The myths seem to be faulty interpretations of the information in the FSM. The facts are that when the ignition is first turned on the FMD (Fuel Management Device) integrated circuit inside the ECM (Electronic Control Module -- the "computer") turns on the fuel pump via the fuel pump relay for 2 seconds. The purpose of this is to prime the fuel rails in preparation for starting the engine. The ECM shuts off the fuel pump after 2 seconds so that it will not run continuously if the ignition is turned on but the engine is not started. When the ignition switch is turned to the crank position the engine starts rotating and the ignition module inside the distributor starts sending out DRPs (Distributor Reference Pulses). When the FMD IC detects DRPs it turns on the fuel pump relay. The FMD will keep the fuel pump relay turned on as long as it receives DRPs, which means whenever the engine is rotating, whether cranking or running.

When the oil pressure reaches about 4 PSI the oil pressure switch (which is in parallel with the fuel pump relay) will also turn on. This is a backup for the fuel pump relay in case the fuel pump relay fails. Since the fuel pump relay is the main source of power for the fuel pump, loss of oil pressure and therefore the opening of the oil pressure switch will NOT "save the engine" by turning off the fuel pump (it can't, since the fuel pump relay is keeping it running).

This diagram from page 6E3-A-68 of the 1986 FSM shows the fuel pump connections. I modified this diagram because it did not show the "FP" fuel pump fuse, added the fusible link, corrected the circuit 458 connections, added connector numbers and corrected several other minor problems. The diagram also indicates that the quote from the 1989 FSM regarding error code 54 is incorrect. If the fuel pump relay fails and the oil pressure switch supplies power to the fuel pump, the ECM will still see the fuel pump voltage and will NOT set an error code 54.



Busting the myth part 1: the empirical test

According to Google, empirical means "based on, concerned with, or verifiable by observation or experience rather than theory or pure logic". I call it the "try it and see what happens" or "it is what it is" test. The method:

1. Unplug the oil pressure switch.
2. Turn on the ignition but don't start the engine.
3. Wait 5 to 10 seconds.
4. Start the engine.

Unplugging the oil pressure switch means that it can't "take over". According to myth version two, you will still be able to start the engine if you start cranking within two seconds of turning on the ignition. That's why we wait at least 5 seconds -- to make sure that 2 second time period has passed. If the engine starts then the myth is busted. I did this test and the engine DID start, so MYTH BUSTED.

Busting the myth part 2: the technical test

The oscilloscope screen shot below shows the signal I measured on pin C of my fuel pump relay. This is the signal that turns on the relay and it comes from pin A1 of the ECM. The wire color is dark green with a white stripe.

The scan rate is 1 second per each vertical division. The vertical scale is 2 volts per division. The arrow on the left with the 1 (for channel 1 on my 'scope) is at zero volts.

The ignition was turned on at 1 second and the voltage jumps up to 12 volts for 2 seconds. This turns on the fuel pump and pressurizes the fuel rail in preparation for starting the engine. After 2 seconds the fuel pump is turned off so that it doesn't run continuously if the ignition is turned on but the engine is not started. Note the inductive kickback as the magnetic field collapses in the relay when it turns off at the 3 second mark (the voltage goes negative briefly).

At approximately the 5 second mark the ignition was turned to the crank posisition. The engine started cranking, the ECM detected DRPs (Distributor Reference Pulses) and turned on the fuel pump. The voltage is low (about 10 volts) because the starter is cranking the engine and loading down the battery. As the engine comes up to speed the voltage rises because the starter motor isn't working as hard. When the engine starts the voltage jumps up to about 14 volts because the alternator is now supplying the voltage. I thought it was interesting to see the battery voltage varying as the starter motor cranked the engine. The oscilloscope trace clearly shows MYTH BUSTED.



Made with Mac Send Email to Cliff Harris Updated 1-10-2015

RBob

iTrader: (1)

I stopped reading after this false statement. The ECM provides +12 volts to the fuel pump relay to activate it.

RBob.

igende

The ECM sends a signal via the green/white wire right? How much would you expect to see there?

RBob

iTrader: (1)

Most of the time, there are always exceptions.

Vehicle voltage, at key-on, engine-off, this will be battery voltage. With the engine running it will be whatever the alternator is providing.

RBob.