'78 SC What makes the fuel pump run with the ignition in START position?

[cpcooper]

I've been straightening out my fuel pump circuit wiring and have everything working, EXCEPT fuel pump is not running when the starter is cranking. I confirmed this with a jumper lead from terminal 30 on the fuel pump relay to a test light to ground. Hit the starter, engine cranks, light stays off until engine fires, then lights up. Remove the relay and jump from 87 (yellow from starter switch) to 30 (black to fuel pump) motor starts immediately when cranked. (The relay clicks when the ignition is turned on and pump runs when the engine starts and/or airflow sensor arm is lifted)

I know the fuel pump is supposed to run when the starter is cranked (and motor is cold?) but I can't seem to figure out how to make that happen. What am I missing?

[Flat6pac]

The air flow sensor connection on the front of the injection gets mixed up with the cold start valve. The airflow sensor is controlled bu the relay
The cold start injector will ohm with the wire from the left chain cover probe connection making sure you're connected properly.
The AFS connection is controlled through the red relay in the front
Key in the ON position lifting AFS under the air cleaner will turn the fuel pump on.
Bruce

[cpcooper]

That part works... Per my post, lift the AFS with the ignition on and the pump runs. The pump doesn't run when the starter engages.

Somewhere in these pages I saw an explanation of the various circuits involved with the fuel pump and fuel pump relay, but I can't seem to find it now.

[Bob Kontak]

Terminal 85, feed (ground) from the air sensor relay is tied into the RPM limiter and the alarm system if so equipped per the 82 diagrams.

Once you crank engine, air sensor lifts, even when cranking, right? Relay should be energized.

Using the wire in the 87/30 sockets bypasses the air sensor control.

Humor me and plug one of the black relays into the fuel pump relay and see what happens. Just for grins. Suspect no change.

Here are the 82 diagrams. I have the 78's at home in hard copy.

http://www.pelicanparts.com/911/911_parts/911_electrical_diagrams.htm

Subscribed.

[Bob Kontak]

Originally Posted by Flat6pac

The airflow sensor is controlled bu the relay

The air flow sensor is controlled by the air sensor. Lift it and the relay's electromagnet can energize.

Starter controls the CSV firing, not the air flow sensor. Starter disengages, CSV stops pissing fuel but fuel pump keeps running because air sensor is lifted.

However, yes, fuel pump should be running once key is turned on AND air flow sensor lifted.

[ossiblue]

Originally Posted by cpcooper

I've been straightening out my fuel pump circuit wiring and have everything working, EXCEPT fuel pump is not running when the starter is cranking. I confirmed this with a jumper lead from terminal 30 on the fuel pump relay to a test light to ground. Hit the starter, engine cranks, light stays off until engine fires, then lights up. Remove the relay and jump from 87 (yellow from starter switch) to 30 (black to fuel pump) motor starts immediately when cranked. (The relay clicks when the ignition is turned on and pump runs when the engine starts and/or airflow sensor arm is lifted)

I know the fuel pump is supposed to run when the starter is cranked (and motor is cold?) but I can't seem to figure out how to make that happen. What am I missing?

Everything you post is normal, except your jumper test of the relay at terminal 30:

Relay clicks when ignition is turned on.

Power to the relay from starting circuit at terminal 87.

Pump comes on when sensor plate is lifted.

Engine starts and runs.

You didn't state you were having a starting problem, so, unless you are, I can't see an issue other than the odd behavior of the test light.

As Bob suggested, try another relay. The only thing that could be wrong is the internal connection between 87 and 30 of the relay is, somehow, faulty.

[Bob Kontak]

Originally Posted by ossiblue

You didn't state you were having a starting problem

OP did say that when wires are placed in the 87 30 sockets the car starts immediately.

Assuming there may be some issue, but it's not articulated.

Originally Posted by ossiblue

The only thing that could be wrong is the internal connection between 87 and 30 of the relay is, somehow, faulty.

+1
Relay clicking when key is turned on is not the magnet energizing when air sensor is lifted. Hoping it's a simple as a tired relay.

[fred cook]

Check to make certain that the plug for the safety valve is plugged in. If it is plugged in and everything else is working normally, the fuel pump will only run when the key is in the start position and the air plate is slightly lifted either by hand or by incoming air. If the safety valve plug has come loose or been removed, the pump will run with the key in the run position. Hope this helps.

[cpcooper]

Bob: Thanks for directing me to the wiring diagrams! I printed them out and what I need to know is there, I just need to noodle it out I guess.

To back up a little, my car had an extra relay scabbed in outside the fuse panel and it simply turned the fuel pump on when the ignition was turned on. No starting problems, but some idle issues so I decided to get the fuel pump right then proceed with pressure tests. I traced wires and got a new red relay in the right place and everything seems to work as it should, with the exception of the fuel pump not running when the the starter is engaged. Not a serious starting problem, but doesn't catch as soon as it should and since it works so much better when I jump across 87 and 30 on the relay socket, I thought it was worth pursuing. Also, I may be just the tiniest bit compulsive.

[ossiblue]

Originally Posted by cpcooper

Bob: Thanks for directing me to the wiring diagrams! I printed them out and what I need to know is there, I just need to noodle it out I guess.

To back up a little, my car had an extra relay scabbed in outside the fuse panel and it simply turned the fuel pump on when the ignition was turned on. No starting problems, but some idle issues so I decided to get the fuel pump right then proceed with pressure tests. I traced wires and got a new red relay in the right place and everything seems to work as it should, with the exception of the fuel pump not running when the the starter is engaged. Not a serious starting problem, but doesn't catch as soon as it should and since it works so much better when I jump across 87 and 30 on the relay socket, I thought it was worth pursuing. Also, I may be just the tiniest bit compulsive.

BenchTest the continuity of your relay.

Apply 12v to pin 86 and ground pin 85--relay should click. Then, test for continuity between pin 87 and 30. Given what you've told us, this is the most logical culprit as all else works normally in the relay circuit.

Just a question for clarity. When you jumped 87 and 30 in the relay socket and got an easier start, how did you do it? If you removed the relay to jump the socket, how did the pump get power once the engine started? Did you just pull the relay out a bit and jump the pins of 87 and 30?

[cpcooper]

Interesting... Bench tested relays. My new red one acted like 85 and 86 were connected internally; no click, just big spark. Old black one clicked and showed continuity between 87 and 30 when energized. Might have screwed the pooch on the red one. Now when I plug it in the fuel pump comes on with the ignition. Black one acts as it did before, clicks when ignition is turned on, fuel pump runs once motor starts but not with the starter. Maybe sensor does have to be lifted for pump to run, but CIS Primer says "when switch is turned to "start" - starter voltage goes to fuel pump through the NO (Normally Open/87) contact. Fuel pump runs while engine is cranked"

[ossiblue]

Originally Posted by cpcooper

Interesting... Bench tested relays. My new red one acted like 85 and 86 were connected internally; no click, just big spark. Old black one clicked and showed continuity between 87 and 30 when energized. Might have screwed the pooch on the red one. Now when I plug it in the fuel pump comes on with the ignition. Black one acts as it did before, clicks when ignition is turned on, fuel pump runs once motor starts but not with the starter. Maybe sensor does have to be lifted for pump to run, but CIS Primer says "when switch is turned to "start" - starter voltage goes to fuel pump through the NO (Normally Open/87) contact. Fuel pump runs while engine is cranked"

85 and 86 are connected internally as they are the two ends of the electromagnetic coil wire that makes the relay switch. They should not have shorted out unless you misconnected power to 85 instead of 86 as there is a diode between the two terminals that could have blown and shorted the whole coil. Yes, the red relay is now broken as the pump comes on with the ignition. If the black one showed continuity between 87 and 30, it's working correctly.

You've already checked that 87 gets power from the ignition. Now you know the black relay has continuity between 87 and 30 when energized. The relay clicks when the ignition is turned on. I can see no reason why the pump isn't working during start. Again, are you certain the pump isn't working?

[boyt911sc]

Cpcooper,

Search for DKlever48's post about fuel pump relay test. Take your time to read that post. Everything you need to know about how to test the FP relay and testing the relay socket were discussed in detail for everyone to follow and understand. HTH.

Tony

[Bob Kontak]

Originally Posted by boyt911sc

Everything you need to know about how to test the FP relay and testing the relay socket were discussed in detail for everyone to follow and understand.

Thanks Tony. Here is the link.

http://forums.pelicanparts.com/porsche-911-technical-forum/673014-fuel-pump-relay.html

Interested to see how ossiblue's thoughts pan out.

[cpcooper]

Thanks guys, gives me a way to go. I'll print out the test and dig into it in the morning.

I got curious about what I did to the red relay and I think I got unlucky with polarity and the diode. Two possibilities, and I got the wrong one. Figuring I had nothing to lose, I cut the cover off and snipped out the diode. Now it works normally when power is applied to 85 and 86. Lesson learned, and now I have a relay I can look inside to help me visualize how they work.

[cpcooper]

Here's another thought though; For the fuel pump to run with the ignition turned on, motor running, it gets grounded by the air flow sensor. If the circuit to run the fuel pump when the starter is independent of the AFS, how is it grounded? If it's not independent of the AFS, why is it necessary?

[ossiblue]

Originally Posted by cpcooper

Here's another thought though; For the fuel pump to run with the ignition turned on, motor running, it gets grounded by the air flow sensor. If the circuit to run the fuel pump when the starter is independent of the AFS, how is it grounded? If it's not independent of the AFS, why is it necessary?

You've got it reversed. Fuel pumps with the engine running because the relay is not grounded. Grounding comes from the AFS and causes the relay to energize with ignition on--that's the click you hear--switching terminal 30 (fp) to 87 (start circuit). As soon as vacuum in the intake moves the sensor plate, the AFS breaks the ground to the relay, coil de-energizes, and it switches terminal 30 to 87a, the ignition power. A dead or non-powered relay connects 30 and 87a.

[boyt911sc]

Originally Posted by cpcooper

Here's another thought though; For the fuel pump to run with the ignition turned on, motor running, it gets grounded by the air flow sensor. If the circuit to run the fuel pump when the starter is independent of the AFS, how is it grounded? If it's not independent of the AFS, why is it necessary?

cpcooper,

Since you are interested and curious to know the principles behind the FP relay and AFS switch operation, you need to familiarize yourself with the wiring diagram first. Then make sketches or drawings of the FP relay, AFS switch, etc. and read DKlever48's thread about FP relay test. This subject has been discussed in this forum too many times already. Ask questions after you made an effort to read the available information. And you would be well informed to share your knowledge with others.

Tony

[Bob Kontak]

Originally Posted by ossiblue

You've got it reversed. Fuel pumps with the engine running because the relay is not grounded. Grounding comes from the AFS and causes the relay to energize with ignition on--that's the click you hear--switching terminal 30 (fp) to 87 (start circuit). As soon as vacuum in the intake moves the sensor plate, the AFS breaks the ground to the relay, coil de-energizes, and it switches terminal 30 to 87a, the ignition power. A dead or non-powered relay connects 30 and 87a.

In fairness to cpcooper, I am the one who had it reversed as I told him lifting the AFS creates the ground when lifted. That is incorrect. Thanks ossiblue for the explanation.

That said, the relay with key on is just dying to provide juice to the fuel pump. Break the ground, please, it's hoping. Wouldn't cranking the engine, assuming a slight delay in instantaneous starting, break the ground with vacuum created and resultant lifting of the sensor plate before the engine starts?

[ossiblue]

Originally Posted by Bob Kontak

In fairness to cpcooper, I am the one who had it reversed as I told him lifting the AFS creates the ground when lifted. That is incorrect. Thanks ossiblue for the explanation.

That said, the relay with key on is just dying to provide juice to the fuel pump. Break the ground, please, it's hoping. Wouldn't cranking the engine, assuming a slight delay in instantaneous starting, break the ground with vacuum created and resultant lifting of the sensor plate before the engine starts?

I've thought the same thing, but I have no way to know how long the start circuit powers the pump before the ground is broken. My thought is, it isn't very long, and that's why I suspect the OP's start circuit is working after all.

Look at it this way, with the relay, the pump starts the instant the key is turned to start, and continues to run the instant enough vacuum is sensed in the intake, but on a different circuit. If the fuel system is sound and the residual pressure and/or CSV are in spec, fuel will reach the cylinders almost instantly. What is the duration of that "instantly?" Don't know, but a CIS engine in proper spec has no trouble starting when the pump is engaged at the outset of the start cycle, as dictated by the relay.

Now, go to your room and say twelve "Hail Ferrys" as you count the links in your timing chain, as penance for misleading the OP on the relay!