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Newly purchased 931 - fuel relay
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Paul

Joined: 02 Nov 2002
Posts: 9491
Location: Southeast Wisconsin

Posted: Sat Feb 14, 2004 8:43 am Post subject:

Maybe the ROW cars did not have a catalytic converter, so they could use the rotor rev limiter and not worry about dumping raw fuel in a hot converter. BTW my rotor is definitely not the rev limiting type (cost a few bucks to replace), but my car still shuts down just above redline.

!tom

Joined: 28 Aug 2006
Posts: 1941
Location: Victoria, BC Canada

Posted: Tue Apr 20, 2010 2:13 pm Post subject:

Smoothie wrote:

Hey! I got a sudden burst of energy..

For '78 na:
31b - to airflow sensor switch
31? - to ground (no terminal number given - guessing 31)
50 - to 12v(+)
15 - to 12v(+)
30 - to fuse, then on to 12v(+)
87 - to single fuel pump

The other day while driving home, as I pulled into the driveway the engine stopped.

I quickly diagnosed that the fuel pump had stopped. Jumping 30 to 87 gets the car to run, but with the fuel pump constantly on.

I noticed that the jumper wire has very little resistance going into the 30 position. However, a tweak of the relay pin #30 still doesn't get the car to run, so some troubleshooting is in order. And yes, the fuse is fine. As Smoothie says, the problem could be any number of things, so I wanted to ensure the relay was functioning properly.

I pulled apart the relay (again; some years ago I put a MOV in the relay to protect the points), and with the help of some co-workers, we reverse engineered how the relay works so it is easier to troubleshoot the system.

The number from my relay: 896444000

There is a little schematic on my relay. I'll order pins here as they appear left-to-right on my schematic:

30: +12 always; power for fuel pump through fuse
87: +12 switched power for fuel pump
15: +12 power feed used to energize the relay coil and its circuitry
50: Signal, from TTS on CSV
31: Ground for relay coil and circuitry
31b: Signal, from AFM

Basically, the way it works in words:

Pin 15 supplies the power the relay uses to function. The relay is off if 15 has no power. The relay circuitry (including coil) is grounded through pin 31.

Here is a truth table of function:

Code:

15 31b 50 Coil
H H H ON
H H L ON
H L H ON
H L L OFF
L * * OFF

where H is +12 and L is ground. Note that if 31b is left floating, it is internally pulled up. This means the relay coil can be energized simply by connecting 15 to +12 and 31 to ground.

Basically, for those who have no clue what a truth table is, the relay is energized if 31b or 50 are energized. This is "or" in the logical sense, meaning the relay is energized if 31b is +12, 50 is +12, or 31b and 50 are +12. The relay is not energized if 31b is connected to ground and 50 is connected to ground. Also, nothing gets energized if pin 15 is not +12.

I have done this to eliminate the relay as a possible cause of the failure, and so that I know how to troubleshoot.

Here is my proposed troubleshooting procedure.

1) Plug the relay back in, and remove the fuse cover. A test light should indicate the fuse has power, as the fuse is connected directly to pin 30, which is always +12. If the test light fails to light, the issue is a poor contact between the car wiring and pin 30 on the relay. Since I can get the fuel pump to run using a jumper between pin 30 and pin 87, I know the wiring is okay, but I suspect a poor contact in the relay socket.

2) A test light between 31b and 31 should illuminate when the AFM is deflected.

3) A test light between 50 and 31 should illuminate when the CSV is energized.

4) A test light between 15 and 31 should illuminate when the key is turned to "on."

5) If the fuse is removed and a test light is put in its place, the test light will indicate if the fuel pump is receiving power. A test light won't pass enough current to run the pump though, but I already know the pump isn't running.

6) The relay should be energized when the test light in step 4 is on AND either step 2 or step 3 is on.
_________________
78 924 NA
5-lug

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