Why is red fuel pump relay special?

[simsalabim]

Hello all,

I have been sorting out a newly purchased, and newly rebuilt 81sc. Almost there, but in an attempt to figure out a difficult starting issue (5 seconds of cranking and then a couple more of sputtering until the engine starts), I decided to begin with some simple electrical stuff. Let me first say that the WUR is a new digital unit from unwired tools, and that once the engine is started, it seems to work well as it cycles through its cold start enrichment phase (idle is steady at 13-14 hundred for a few minutes until a gradual decrease to around 950).

Anyway, my suspicion was a fuel starvation issue and I thought I would check out the fuel pump fuse and relay before digging into other possibilities. Fuse was fine, but I noticed that the P.O had swapped a standard black relay for the specified red one (and used the red one where a standard one should be ...?).

I remember reading somewhere that the red ones are a bit more robust than the standards, and that they have an additional contact that allows the fuel pump to be activated when the ignition is switched to the "on" position (unlike the standard relays). If this is true, then this might explain a starvation issue as the fuel pressure/flow may be somewhat compromised upon starting. When I switched (and replaced with a new one) the relays back to specified, all is now as it should be and the engine starts with just a few cranks and no sputtering.

Does this make any sense? Are the red relays indeed wired differently as I had been led to believe?

Thanks for any input, I hate guessing!

[Ronnie's.930]

Here you go -

Relays... whats the difference?

[porwolf]

This is the difference as I understand: The red fuel pump relay, besides having the integral diode, also has a terminal 87A that is the "normal closed" contact of the relay. The other round black relays do not have that terminal hooked up internally. Here are the relay schematics from my 1978-1983 SC Bentley manual:

Fuel Pump Relay (red):





Horn and other relays (black):

[T77911S]

th eblack ones have 87a, dont go buy the drawings. they just leave it off if it is not used

[psalt]

A stumble or stall after firing is a common problem on Lambda CIS 911SC's. The engine should fire immediately on the first crank, then settle to a strong cold idle. If the engine does not fire immediately, check the CSV circuit, then the output. The CSV is irrelevent once the key is off cranking. If it fires then stalls, disconnect the vacuum retard , reset the idle to 950 hot and try again tomorrow. Most engines have vacuum leaks at the injector sleeves and new boots, O rings and seals cures this. Two minutes with an unlit propane torch reveals this problem. If you don't like guessing, hook up an analog dwell meter to the Lambda CIS test port and it will tell you quite a bit.

Paul

[T77911S]

i did not even read the post, just saw the relay title.

as paul said
air leaks, CSV/circuit and with the WUR. it may just be fine tuning the mixture and cold pressure.
make sure plugs and ignition are good

[simsalabim]

Thanks for all the great responses.

Like I said, once I replaced the black relay with the specified red one, the motor does indeed start right up and go through the proper WUR cycle. No more sputtering first and then starting.

Looking at the Bentley diagram (thanks Porwolf), it does look like there is circuitry associated with the red relay (87A) that activates the fuel pump during the start/run ignition cycle (unlike the black relays). Seems to me that this would explain the sputter first condition as the fuel distributor struggles to find fuel during the cranking phase. Unless I am completely off base with this assessment, I am tempted to conclude that the mystery is solved.... anyone have a better theory?

[psalt]

Not my experience, swapping new red vs black FP relays has no effect.

The FP circuit is complex, the difference in the start and run power supply is the air flow sensor switch. Add the rev limiter and the alarm and you have a lot of actual testing to do to find an intermittent problem.

[wwest]

The relay has what is referred to as a "damping"/"swamping" diode used to suppress the relay armature coil inductance's voltage "kick" that otherwise might well damage the solid state components in the rev-limiter circuit.

[T77911S]

are the amp ratings different?
along with the diode i have thought that maybe the red one handles the current of the FP better.

the relay you had in the there could have just had dirty contacts.

[porwolf]

Originally Posted by T77911S

th eblack ones have 87a, dont go buy the drawings. they just leave it off if it is not used

That is what I understand. Yes, the black relays have the 87A pin but it is not internaly connected inside the relay.

[boyt911sc]

Originally Posted by porwolf

That is what I understand. Yes, the black relays have the 87A pin but it is not internaly connected inside the relay.

porwolf,

This not true. You could switch the red and black FP relays anytime with no problem. The difference between these two relays is the 'diode' in RED relay and none in Black relay.

If the 87A is not internally connected how do you explain the FP getting energized? When the air flow sensor's switch breaks the ground (#85) the power supply is switched back from 87-30 (normally open) to 87A-30 (normally closed).

Terminal #30 (FP terminal) switches between 87A (run) & 87 (start). HTH.

Tony

[porwolf]

Originally Posted by simsalabim

Thanks for all the great responses.

Like I said, once I replaced the black relay with the specified red one, the motor does indeed start right up and go through the proper WUR cycle. No more sputtering first and then starting.

Looking at the Bentley diagram (thanks Porwolf), it does look like there is circuitry associated with the red relay (87A) that activates the fuel pump during the start/run ignition cycle (unlike the black relays). Seems to me that this would explain the sputter first condition as the fuel distributor struggles to find fuel during the cranking phase. Unless I am completely off base with this assessment, I am tempted to conclude that the mystery is solved.... anyone have a better theory?

Actually the way I understand the fuel pump relay function on the SC's is this: In order to prevent the fuel pump from running with the ignition switch in the middle, on-position, the air flow sensor switch, by being grounded in the rest position, powers the red relay by grounding it at pin 85. This disconnects the power to the fuel pump from pin 30 via pin 87A, before the engine is running normal. The power to the fuel pump is then provided by the ignition switch in the 3rd position, the engine crank position, via pin 87. When the engine fires and runs on it's own the ignition switch is returned to the 2nd position, the engine-on position, and the fuel pump then gets it's power from pin 30, via pin 87A, with the relay de-powered because the air flow sensor switch is not grounded anymore. The other connection from pin 85, the relay coil ground connection, goes the the rev-limiter switch whitch also grounds pin 85, and therefore cuts off the power to the fuel pump at excessive revs.

[wwest]

Originally Posted by boyt911sc

porwolf,

This not true. You could switch the red and black FP relays anytime with no problem. The difference between these two relays is the 'diode' in RED relay and none in Black relay.

If the 87A is not internally connected how do you explain the FP getting energized? When the air flow sensor's switch breaks the ground (#85) the power supply is switched back from 87-30 (normally open) to 87A-30 (normally closed).

Terminal #30 (FP terminal) switches between 87A (run) & 87 (start). HTH.

Tony

"..switch....anytime..."


Yes, but you are chancing the failure of the solid state rev-limiter circuit absent the protective diode in the red relay.

[boyt911sc]

Originally Posted by wwest

"..switch....anytime..."


Yes, but you are chancing the failure of the solid state rev-limiter circuit absent the protective diode in the red relay.

WWest,

The red FP relay should be used for the FP and I would not use a black relay unless in an emergency or a temporary fix. And would not recommend using a black relay for a long period of time. And people had used this black relay innocently for many years.

Tony

[boyt911sc]

Originally Posted by wwest

"..switch....anytime..."


Yes, but you are chancing the failure of the solid state rev-limiter circuit absent the protective diode in the red relay.

WWest,

What has the FP relay to do with the rev-limiter? The rev-limiter is connected directly to the FP relay socket @ 86 and powered directly from the ignition switch. Even without a FP relay installed, power goes to the rev limiter. Please refer to wiring diagram.

Tony

[T77911S]

Originally Posted by porwolf

That is what I understand. Yes, the black relays have the 87A pin but it is not internaly connected inside the relay.

dont go by the drawing. the PO actually poved it for you when he had the black one in there and it worked.

[T77911S]

Originally Posted by porwolf

Actually the way I understand the fuel pump relay function on the SC's is this: In order to prevent the fuel pump from running with the ignition switch in the middle, on-position, the air flow sensor switch, by being grounded in the rest position, powers the red relay by grounding it at pin 85. This disconnects the power to the fuel pump from pin 30 via pin 87A, before the engine is running normal. The power to the fuel pump is then provided by the ignition switch in the 3rd position, the engine crank position, via pin 87. When the engine fires and runs on it's own the ignition switch is returned to the 2nd position, the engine-on position, and the fuel pump then gets it's power from pin 30, via pin 87A, with the relay de-powered because the air flow sensor switch is not grounded anymore. The other connection from pin 85, the relay coil ground connection, goes the the rev-limiter switch whitch also grounds pin 85, and therefore cuts off the power to the fuel pump at excessive revs.

correct

[T77911S]

tony,
if pin 85 is connected to the rev limiter circuit to provide a ground to kill the FP, west would be correct in that the red relay COULD be in there to protect the solid state rev limiter circuit.

i dont know what the actual rev limiter circuit looks like. it could just use a relay to ground p85 or could use a transister to ground it.