Using Pro Output to switch a fuel pump relay-Useful for safe

-Ad- · Post by **-Ad-** » Wed Jan 09, 2008 9:35 pm

Is this possible?
Just been having a think, and realised that you can set MJ to earth the output wire at a specific rpm.
So if you set the prog output settings to switch at a very low rpm(below cranking engine speed), it would in theory ground the relay when cranking(and keep it grounded whilst rpm are above ***rpm), switching it, thus supplying 12v to the fuel pump?
Would be helpful in terms of safety, as if the engine cutout, the pump wouldn't keep on running like it does on a manual switch. Also if the cars ever involved in a crash.

Any input?
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MartinM · Post by **MartinM** » Wed Jan 09, 2008 9:47 pm

Don't quite see what you mean - need a drawing like your other thread!

...but the lowest rpm you can set a programmable output to is 500rpm, which is above cranking speed but below idle speed.

britlude · Post by **britlude** » Wed Jan 09, 2008 10:05 pm

Rover V8 SD1 have this set up in a SPDT switch on the oil pump... if the pressure is low, the pump is fed via the started motor line, when the pressure is high, it's supplied by the ignition controlled supply. hense, if the engine stops, so does the pump! handy when i rolled the car!!!!

the only prob I see is if the mjlj goes down for whatever reason, you lose the limp home, as you have no fuel pump...

Post by **brentp** » Wed Jan 09, 2008 10:47 pm

The MJLJ firmware cannot measure below 457 RPM, so the programmable outputs would not be suitable for this application.

However, it is technically possible - it would require a firmware modification to do something like deactivate an output if an ignition event isn't seen within X seconds.

-Ad- · Post by **-Ad-** » Thu Jan 10, 2008 3:56 pm

Ah right, ok then.
It wont work then, as the cranking speed is below what Mj first registers.
So it wont turn the pump relay on, until the engine is running, which wont happen without a fuel pump,lol.

Unless I rely on the fuel sitting in the carb float bowls to get the engine started.

I'll have to wire the pump to either a manual toggle switch, or oil pressure switch instead then.
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MartinM · Post by **MartinM** » Thu Jan 10, 2008 4:08 pm

So it wont turn the pump relay on, until the engine is running, which wont happen without a fuel pump,lol.

...yep, that's the bit I couldn't get my head round

britlude · Post by **britlude** » Thu Jan 10, 2008 6:28 pm

it can work if you wire the pump via a change over relay.

the common connector to the pump,

normally closed terminal.... T into the starter solenoid lv cable, so that the pump runs WHEN the engine is cranking.

the normally open terminal is connected to ignition live.

the MJLJ then controls the relay.

the pump runs via the starter until it fires, then the mjlj output fires the relay and powers the pump that way. if the engine stops for whatever reason with the ignition on, the mjlj o/p will drop below the idle/trigger level, and drop out the output to the relay and so stop the pump... in this state, the pump can ONLY be powered again by cranking the engine.

simple!

-Ad- · Post by **-Ad-** » Thu Jan 10, 2008 7:18 pm

Thats quite a good idea/\

But i'll think it'll be better to keep the pump wired seperately from MJLJ, just incase it stops working for some reason.
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Post by **brentp** » Thu Jan 10, 2008 8:20 pm

But why ever would it stop working?

I trust you will find the MJLJ a very reliable piece of equipment!

Anyway, fuel pump control really does require special behavior on top of a switched output. For example:

1. When first powered up, the controller would activate the fuel pump, priming the system.
2. If the engine is not started with within some time, say 2 seconds, then the fuel pump is disabled.
3. If an ignition event is detected (cranking), the fuel pump is re-activated and the 2-second timeout is extended.

-Ad- · Post by **-Ad-** » Fri Jan 11, 2008 3:58 pm

Sorted it now, im wiring the pump up to an inertia switch.
To trip off if the cars ever involved in a crash.
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