No Voltage to Fuel Pump Relay

[AFB24911S]

Please help me plot a course for diagnosis!

2 days ago, the car drove great. Now, it won't start.

1977 Porsche 911S Targa, 211k mi, original motor, original injection system. Fuel pump replaced in 2019, lots of recent work on vacuum leaks related to the injection system.

Cranks great, but I pulled the #4 injector and put it in a jar to see if any fuel came out during cranking; none. No pressure when I lifted the air flow plate; it hung in midair.

Tested for voltage between positions 86 and 87a on the fuel relay port; no voltage, in "run" or "crank" positions.

Tried to jump fuel pump using wire with switch between position 30 and 87a; no dice.

No broken fuses.

What do I check next? My first thought is to just do continuity between the battery and the fuel pump relay port, but what doesn't make sense is that the car ran two days ago, drove straight into the garage. What could have changed in 2 days of sitting in the garage?



Thanks in advance for your help!

[boyt911sc]

Search and read DKLever48 thread about fuel pump relay terminals tests. Everything you need to know about the 5 terminals:

• #30
• #86
• #87A
• #87
• #85

Use a 12-volt test light for this type of troubleshooting work. And make a sketch or drawing of the fuel pump relay socket to visualize and understand how they work. If you have problem following the results of the tests, just ask. Someone will surely help you.

Tony

[Schulisco]

Check out the very informative web site from our user AndrewCologne:
https://nineelevenheaven.wordpress.com/the-protection-circuit-of-the-fuel-pump/
Note: You have to click on the circuit image to step through the whole sequence to see how it works in detail and to see which lines are when powered.
This security circuit is afaik also in your car, it was introduced earlier, supposingly in '75 or 76...?

Thomas

[AFB24911S]

Thank you @boyt911sc and @Schulisco, super informative.

Here are my results from the tests prescribed:

Ignition OFF:
87 - no power - confirmed
87a - no power - confirmed
86 - no power - confirmed; continuity with 87a - confirmed
85 - no power - confirmed; grounded? NO, UNGROUNDED!
30 - no power - confirmed; not grounded? NO, GROUNDED!

Ignition RUN:
87 - no power - confirmed
87a - power - confirmed
86 - power - confirmed
85 - no power - confirmed; ground continuity? - NO
30 - no power - confirmed; ground continuity? - confirmed

I cleaned up all the terminals with the relay out and the battery off, that did not change any of the test results.

I thought the circuit to the fuel pump might be grounding out on the warm-up regulator somehow, disconnected it and tested again, no change.

Am I correct in thinking that circuit 30 to the fuel pump is the next place to look?

[AFB24911S]

Decided to investigate circuit 85 first. It's not grounded when the plate is down, but it is continuous with ground when the plate is up.

How could that possibly be? The car ran when I drove it into the garage, seems unlikely the wires could have switched themselves. Plus, those CIS connectors aren't reversible; they only go on the one way, I think.

If I lift the car up and examine the fuel pump wiring, I have a 12V source that I can use to test the fuel pump's functionality without the car's circuits or battery.

[PopsRacer]

You can hotwire the pump at the fuse panel. Super easy and it is even in the factory shop manual as a technique. Just grab an alligator clip.
When your car is in "run" you should see (assuming this is about the same as an 88?)
30 - power (always hot)
87b runs to fuel pump (power)
85 should be grounded. On my 3.2 this is on the Cyl 2 intake runner. (the main ground for the engine electronics - worth taking apart & cleaning up if you have any doubts about it)
85b is fuel pump control on the DME cars. Not sure about earlier cars?
87 should have power (from ignition sw) - this is power for the DME (if you have one)

I would check that ground to pin 85 first, it may be the simplest explanation. (?)

Apologies in advance if the 77's are completely different! I don't have a schem handy.

[AFB24911S]

OK, update: I lifted the car up and supplied power to the fuel pump with my 12V source; the fuel pump works.

Diagnosing circuitry now. I think that circuit 85 is grounding everything out or that there's a crossed wire just behind the relay, it looks like my mechanic moved the fuel pump relay when he did some work on the car recently.

@PopsRacer, I tried jumping it the way you described between 87a and 30 as the Bentley manual suggests and no dice. Thanks though!

[Schulisco]

Originally Posted by AFB24911S

Decided to investigate circuit 85 first. It's not grounded when the plate is down, but it is continuous with ground when the plate is up.

Make a bottom view photo of your sensor plate when it rests on the mechanical stop (engine off, no ignition, no nothing) and show it here, so that we can see where is the sensor plate located when resting. Thanks.

Did you stepped through Andrew's image (5 images) series?
https://nineelevenheaven.wordpress.com/the-protection-circuit-of-the-fuel-pump/

Originally Posted by AFB24911S

Diagnosing circuitry now. I think that circuit 85 is grounding everything out or that there's a crossed wire just behind the relay, it looks like my mechanic moved the fuel pump relay when he did some work on the car recently.

Aha, the plugs of the sensor plate switch and the cold start valve are interchangeable and not coded, both 2pins....probably is this your culprit...?!

Refer to:
https://forums.pelicanparts.com/porsche-911-technical-forum/897745-cold-start-csv-location.html#post8948317



Thomas

[AFB24911S]

[IMG][/IMG]

[AFB24911S]

[IMG][/IMG]

[AFB24911S]

OK, I sprayed the contact point shown in the blue circle with a contact cleaner and it now does what the other posts say it should: circuit 85 grounds out when the airflow plate is down and it is an open circuit when the plate is up.

[AFB24911S]

Woohoo! That was it! I also took the fuel pump relay out and pulled the prongs apart a little, it felt loose in the sockets. Probably a combination of both.

Old car stuff! Would rather be out in my garage banging rocks together like this than paying someone to fix my car with a laptop.

THANK YOU so much to everyone who replied to this post!

[Schulisco]

Congrats!! Glad to hear :-)

I hope your contact cleaner spray was not the common WD40...It may work for a short period but then it evaporates. I recommend to use a real contact cleaner and protectant spray to keep the contact clean.
Happy X-Mas!

Thomas

[AFB24911S]

@Schulisco thanks! Yeah, I use an electric-specific spray cleaner. Good tip!

I was surprised by how dirty that intake area was. Very grimy.

Also: I think the previous posters on this topic have claimed that circuit 30 (from the relay to the fuel pump) should be ungrounded, and I think that's wrong; looking at the diagram, there's no reason that circuit should ever be anything but grounded, I should think. But opinions are like elbows, everyone's got at least a couple, I would be interested to hear other thoughts.

[wazzz]

Circuit 30 cannot be grounded, since it is power (+12V) to the fuel pump.
But depending on the tool used to test for ground, you may be tricked by the fact that the fuel pump is an electric motor and as such, a coil that has a relatively small resistance that may be confused with continuity to ground, since it has one leg that is permanently grounded.
Are you using an ohmmeter or a test light? If using an ohmmeter, what ohm value do you read to conclude for continuity?