How to wire fuel pump to shut down if engine stops?

[billjam]

I am planning to replace and rewire the fuel pump relay in my '88 Carrera shortly when I install a 3.6 with MegaSquirt MS3X.

I copied the note below from somewhere and I want to retain the original function where the fuel pump relay shuts off the pump when the engine is not running.


My friend "Search" didn't really yield anything specific, so I am looking for some info on how to wire up the relay so that it opens up if engine stops. Using tacho signal somehow maybe?

[steely]

Hi Bill
If you can maintain the stock wiring from the DME to the fuel pump relay, it should kill the pump like it normally does via the relay.
When the starter is running, a signal is fed to DME pin 4. The DME then enables the fuel pump relay (85b) ground path via DME pin 20. If the engine starts, the micro controller (uC) now takes over and latches/provides the control pin 20 (assuming the starter signal is no longer there on pin 4).

The DME uC needs to see that the engine started by looking at the speed and Ref signals. If they aren't there - no output on pin 20, and no ground for 85b.
I hope this is what you were looking for.

on edit: I am not familiar with the Megasquirt - I can only answer the fuel pump relay question.

[HawgRyder]

The old way of doing it was to use an oil pressure sender.
When starting...the sender is bypassed to allow ignition and fuel...and if the oil pressure drops too low...the ignition and fuel are stopped.
This used to take into consideration the car being upside down as well.
Bob

[billjam]

Dan,
Its handy to know the fuel pump circuit logic as that's what I'm trying to replicate, more or less.
As far as using any of the original wiring - it is all being replaced. Actually, that's a bit misleading ... it's all being deleted and a simpler version put in its place!
The DME control unit will be completely replaced by the MegaSquirt MS3X ECU.
The associated DME relay that normally powers the fuel pump is also going to be replaced by a simpler normally-open relay, although it might be possible to reconnect it to work with signals from the MS3X.
Ideas anyone?

Bob,
In the absence of any suitable outputs from the MS3X to trip the relay when the engine is stopped, I like the idea of using the oil pressure switch. I think the on/off idiot light switch might be simpler than using the pressure sender (maybe that's what you meant?).

Looking at wiring diagrams, I see that most relays (incl the DME relay) have diodes built-in.
Is there a simple answer to: how essential are these and what is their purpose?

[HawgRyder]

The diode in the relay is to prevent (or cut down) on the EMF (back) pulse when the relay releases.
The coil has a charge when activated...and when the power is cut off the coil suddenly sends a big spike down the line...yes just like a coil in the ignition circuit...how about that?...LOL
Bob

[steely]

Originally Posted by HawgRyder

The diode in the relay is to prevent (or cut down) on the EMF (back) pulse when the relay releases.
The coil has a charge when activated...and when the power is cut off the coil suddenly sends a big spike down the line...yes just like a coil in the ignition circuit...how about that?...LOL
Bob

Bill
+1 - sometimes referred to as snubbing diodes, they are only there to suppress chatter (rapidly being turned back on/off by the spike Bob mentions). We're talking milliseconds.

And thanks for the insight into megasquirt info.
Does it look for speed, reference signals like the DME?

[al lkosmal]

Bill, A quick search on the Megasquirt forums came up with this nugget.................

"as I think I understand it, MS provides a ground for the fuel pump relay control for normal operation, then UN-grounds it when MS gets no tach signal. "

Wiring the Fuel Pump

To activate the fuel pump, MegaSquirt® provides a ground for the fuel pump relay circuit -see the main wiring diagram.
Ordinarily, at power on, Megasquirt will run the fuel pump for 2 seconds, then when you start cranking the fuel pump is enabled again. If you stop cranking before the engine starts of you stall, the pump is turned off.

You might want to consider a safety switch in the fuel pump circuit when installing an electric fuel pump. Holley has one (12-810, ~$20) that will ensure the fuel pump will not run unless the engine has oil pressure. It stops the pump from running if the motor stalls with the ignition on. Wiring the switch through the starter solenoid circuit energizes the pump on engine start-up. Once the engine has started, the switch continues to provide power to the pump as long as there is oil pressure to keep the switch turned on.

Note: An inertial safety shut off switch should be installed and used to kill power to the pump upon significant impact to vehicle.

These switches are available in junk yards from EFI Fords. The switch is on the drivers side in the trunk, near the trunk hinge, mounted so that it is between the interior bracing and the rear quarter panel (protected from being knocked around if you stuff your trunk full of stuff). It is mounted with the reset switch straight up. Note switch mounting orientation probably matters.

It is Ford Part # F2AB-9341-AA. The wire going into it is about 14 gauge, so it should be capable of handling the full current of the fuel pump.

The markings on the switch show that it has NO/NC (normally open/normally closed) positions so that it should be able to accommodate any possible fuel pump configuration.

I'm not sure if this is correct, but it is worth further investigation.

[fanaudical]

Another idea - 70's and 80's VW and Audi used a "fuel pump relay" that referenced a tach signal and incorporated a time delay. Fuel pump would start with ignition and then die 2.5 seconds after loss of tach signal. Sounds similar to the Megasquirt. I tried searching here on Pelican for the part number of a replacement, but no luck so far...

[billjam]

Originally Posted by al lkosmal

Bill, A quick search on the Megasquirt forums came up with this nugget.................

"as I think I understand it, MS provides a ground for the fuel pump relay control for normal operation, then UN-grounds it when MS gets no tach signal. "

Wiring the Fuel Pump

To activate the fuel pump, MegaSquirt® provides a ground for the fuel pump relay circuit -see the main wiring diagram.
Ordinarily, at power on, Megasquirt will run the fuel pump for 2 seconds, then when you start cranking the fuel pump is enabled again. If you stop cranking before the engine starts of you stall, the pump is turned off.

Thanks Al, that sounds promising. I'll do a bit more research on the MS setup.
It will be really neat if MS comes with that functionality out of the box - it would seem logical really.
BTW, junkyards full of American Fords are non-existant down here, but I'll look into the inertia switch idea.

I am getting closer to actually configuring my MS3X. So far, all I have done with it is take it out of the box, pull it apart (of course!), put it back together again and thank God that I bought it fully assembled . I am going to have plenty of mental challenges just getting this thing running. The MS learning curve is going to get much steeper soon.
I haven't even looked at the tuning software options yet. I know there are several - I'd be interested in your recommendations.

fanaudical - that VW relay sounds like the perfect solution if the logic isn't already available in MS. Thanks for the tip.

[billjam]

Confirmed. Megasquirt manual says:
"Fuel pump - the fuel pump output on the Megasquirt is ground switching for a relay. When the Megasquirt is first turned on it will run for a couple of seconds. As soon as tach input pulses are seen the pump will turn on again. If the engine stalls, the pump is turned off."

That's the best news I've had all day, and it is only breakfast time!
I love you Al.

[al lkosmal]

Originally Posted by billjam

Confirmed. Megasquirt manual says:
"Fuel pump - the fuel pump output on the Megasquirt is ground switching for a relay. When the Megasquirt is first turned on it will run for a couple of seconds. As soon as tach input pulses are seen the pump will turn on again. If the engine stalls, the pump is turned off."

That's the best news I've had all day, and it is only breakfast time!
I love you Al.

More good news...........................I think that I have several of the Ford inertial switches out in my EDIS stash. I will check and get back to you.

It's a love fest.

regards,
al