Can someone design a simple circuit that will send +12V to the fuel pump relay ONLY when there is a tach signal present?

Thanks!

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Why would you want to use a tach signal to drive your fuel pump? strange but you can use a relay to switch a 12 volt source the only problem is if there is enough current to drive the input leg of the relay and then the source will fluctuate with the rpm.What I would do is to connect the pump power wire to a switched 12 volt source that drives the pump with the key on where as if you have a problem with the ignition your pump will not run.The relay can have 4 or 5 terminals but you will only use the four outside terminals.
84- goes to trigger tach wire.(this should be a 12v source but then again it depends on the current flowing if it is too low it will not trigger the relay)
85- goes to ground.
87- goes to fuel pump.
30- goes to12v power source

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Tuners of the Quickest 4.9
12.51@118 mph

4.5 (RSR)Hi Perf. Caddi-V8
In place and running.

Why not use the oil pressure warning switch as your signal source? That'd give you the same net effect, with the bonus of it automatically cutting the fuel if you lost oil pressure...

quote Originally posted by Master Tuner Akimoto: Why would you want to use a tach signal to drive your fuel pump? strange but you can use a relay to switch a 12 volt source the only problem is if there is enough current to drive the input leg of the relay and then the source will fluctuate with the rpm.What I would do is to connect the pump power wire to a switched 12 volt source that drives the pump with the key on where as if you have a problem with the ignition your pump will not run.The relay can have 4 or 5 terminals but you will only use the four outside terminals. 84- goes to trigger tach wire.(this should be a 12v source but then again it depends on the current flowing if it is too low it will not trigger the relay) 85- goes to ground. 87- goes to fuel pump. 30- goes to12v power source

Tach wire is AC voltage so a DC relay will not work.

Honestly, IMHO, find another wire to trigger from...AC relays are difficult and expensive...

It's not that he wants the tach signal to operate the fuel pump, He's just trying to set-up the safety feature that turns the pump off in case of an accident or if the engine dies.

Why not use this?
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=724&prmenbr=361
or this:
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=807&prmenbr=361
It would be easy to tee off the oil pressure pipe.

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Happiness isn't around the corner...
Happiness IS the corner.

OK, those would work, but a relay still would not...AC will not throw the pole on a relay.

quote Originally posted by FieroAudio: OK, those would work, but a relay still would not...AC will not throw the pole on a relay.

I think by the title of the thread he wants a logic circuit to detect the tach signal then engage a relay. Not so much as just having the signal operate the relay.

quote Originally posted by FieroAudio: OK, those would work, but a relay still would not...AC will not throw the pole on a relay.

Never heard of alternating current in a car unless it is an ac/dc inverter, the 12v goes into the coil where it is stepped up to high current not voltage then into the distributor and is transfered to the spark plugs.

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Tuners of the Quickest 4.9
12.51@118 mph

4.5 (RSR)Hi Perf. Caddi-V8
In place and running.

Uh, this function already exists in the Fiero. The ECM commands fuel pump relay on for 1.8 seconds upon key being turned to "RUN", then will shut off the fuel pump until EST pulses are detected from the distributor module. If the engine stops running then the EST pulses stop also, so the ECM kills the fuel pump. Try it and see, with the engine running, pull the coil trigger wire to kill the engine. The pump will stop immediately.

JazzMan

quote Originally posted by JazzMan: Uh, this function already exists in the Fiero. The ECM commands fuel pump relay on for 1.8 seconds upon key being turned to "RUN", then will shut off the fuel pump until EST pulses are detected from the distributor module. If the engine stops running then the EST pulses stop also, so the ECM kills the fuel pump. Try it and see, with the engine running, pull the coil trigger wire to kill the engine. The pump will stop immediately. JazzMan

He's using a carb'd V-8 and he pulled the ECM out of the car.

Actually there is a very good reason for this. A big one is the car will start faster than waiting for the oil to pump up, especially in cold weather.

I don't have time to draw or build it... This should get someone that can build it going in the right direction.

That can be done with a 555 IC timer and some conditioning of the tach signal. Most of the info you would need is in a little book about the 555 that Radio Shack sells. It is also on the internet.

You want the 555 to get power when the key is on.
If I remember right... you want to set up the 555 as retrigerable monostable.

In that mode... The 555 will enable the output as soon as it gets even one pulse from the ignition. As long as it keeps getting pulses, it will be continuously resetting the timer and keep the output on.

I would set the timer value for 10-15 seconds. It doesn't have to be exact, just long enough that even when cranking it will get enough pulses to hold the output on. A couple seconds would do but 10-15 seconds will hold the pump on a bit if you have to try starting more than once. You can set the timer longer or shorter as you see fit.

Since the timer would be wired to get power only when the key is on, it won't hold the pump on when you stop the car. (The Oil Pressure switch will hold the pump until oil pressure drops, which can take up to a minute depending on oil temperature.)

You will need a relay. The 555 won't handle big loads. Depending on the relay you may need a driver transistor as well. (the 555 turns on the transitor what then turns on the relay.) The relay will also have to be a diode supressed type like the rest of the relays that are run by an ECM.

I would wire the relay the same way GM wired the OE one... in parrallel with the oil pump.

There may be other ways to make it work... one thing is that you don't want a big load on the tach line... Anything that loads up the tach line can cause the ignition to not work right. The 555 circuit would not place any more load than the tach already does.

It should be posible to build the 555 into the tach itself but I don't have enough details to tell any one how to do that. By putting it up with the tach you could pick off the signal from the tach board. That would save you conditioning the signal if you pick from the right spot on the tach board.

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Be alert. The world needs more lerts...

The Ogre's Fiero Cave (It's also at the top of every forum page...)

Thanks guys.

Ogre and Oreif are with me on this one. I don't want to drive the fuel pump with the tach signal, I merely want the fuel pump to receive +12V ONLY when there is a tach signal present (only when the engine is running). I am using the ECM in another car and don't want to get another one just to perform this function. I also have the fuel pump wired into the oil pressure sender, but at operating temp the oil pressure drops to the point that the pump starts to oscillate on/off. If I let it idle long enough, the pump will stop and the float bowl will drain, killing the engine. I have great pressure above idle and it's not dangerously low, so I am not worried about that. I have tried a nre oil pressure switch and got the same result. I can also use this in another (non-Fiero related) project I have.

So, as I said before, the circuit only has to sense that the engine is running, and then supply +12V to the fuel pump relay if it is. I am running a carb, so timing on the circuit is not critical. The car will run on whats in the float bowl for a bit, so the pump doesn't even need to run while cranking.

I am not an electronics guy, so I don't know much about this stuff. I thought this would be simple...

Orge, why would you set the timer value so long? Setting it to 10-15 seconds means that his fuel pump will be running for that long after he shuts the engine off. Even at 100 rpm, the tach wire will put out 5 pulses per second.

No need to re-invent the wheel. Find yourself a fuel pump relay out of an earlier VW Fuel Injected vehicle. You can find these relays in many 1980s vintage VWs. They MAY still be using the things, but I haven't touched a VW electrical system in a few years. Wiring one of these in would be simple & painless.

How does this relay work? Much like what the Ogre has suggested for building one - it's most complex piece is the 555 timer. Wish I still had my old relays - threw them out a number of years ago. I couldn't give them away!

That's just what I was looking for! Tell me more. All I need to do is get one from a VW and hook it up? Where can I find a schematic with the information I need?

Maybe I'm missing something, but the ECM runs the pump when it receives injector pulses which are really just tach pulses. Therefore the ECM is already doing what you want.

TK

As stated above, I have no ECM. This is a carbed V8.

As I stated... 10-15 seconds is so the pump will stay running if the thing doesn't start first crank.

Running the pump 10-15 seconds after shut down isn't a problem. The OP switch can hold it far longer than that when the oil is cold. Like when you start it in cold weather and shut it off or have it stall before it warms up.

The exact run time of the timer is not critical. Yes, 1-2 sec is enough if that's what one wants.

The circiut you are looking for is available from Toyota, they call it a rev switch. I'll see if I can get you a used one, as they are probably quite expensive new. Simple four wire hook-up; +12v, gnd, tach, and relay driver out. Sensitive enough to trigger during cranking. Just a small brown module less than the size of a pack of cigarettes.

quote Originally posted by Master Tuner Akimoto: Never heard of alternating current in a car unless it is an ac/dc inverter, the 12v goes into the coil where it is stepped up to high current not voltage then into the distributor and is transfered to the spark plugs.

The voltage that is on the negitive side of the coil, IS AC, always has been, always will be (for conventional coils). The byproduct of being AC voltage is due to the colapse of the feild within the coil, which is when the coil actually fires, when the signal from the ignition module is dropped, the coil stops charging and then current then travels from the secondary through the cap, wires and to the spark plug. You can actually try this, with a coil and battery, and some wire, by holding the negitive wire to the battery negitive and then pulling it away, the coil will discharge.

I don't know if Radio Shack still sells the "electronics handbooks" but there were a few different ones, that explain how electronic circuits worked and had designs for different uses and triggers in them, maybe take a look for these, they were pretty inexpensive too, about $6 last time I bought one.

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The Raven :Under Construction
"James" 1985 GMC Jimmy, 3.2L turbocharged intercooled hybrid 13.873 @ 99.08

"Speed Costs, How fast do you want to go?"

What you are looking for is a current sensor. There are many different types of current sensors but basically you add this sensor in series with your tach wire. It will add about .01 ohms of resistance in the line which is really negligible. The sensor picks up when current is traveling through it and closes a switch to as isolated circuit (that should be for your fuel pump. If the current sensor output can’t handle the current draw of your pump (10-15 amps), then add a relay to switch the pump on. A relay typically has about 80 ohms resistance across the coil so it should draw about .17 amps.

Do a search for an “AC Current Sensor” that only needs a small current to trigger the switch. Check the resistance in your tach line to get an idea of how much current actually passes (it should be a very small amount). I’m not sure how much the AC current sensors are, but the DC current sensors are about $30 (AC should be a bit more).

Aaron

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OK, a simple fix from a self-proclaimed electronics guru:

If you've got a working circuit off of the oil pressure switch, I'd stick with that, but improve a little bit.

You state that your relay cuts in and out at idle, You can take care of that with just a capacitor across the relay coil.

Just hook an electrolytic capacitor across the relay coil side (not the contacts). Make it at least a 15volt cap,
and as far as a Farad value, bigger is better. Just be sure to hook the NEG side of the cap to the grounded side of the relay.

This won't work if your current setup cuts out at idle and stays out, just if it's cutting in & out as the engine lopes.

Hope this helps

-Tim

quote Originally posted by The_Raven: The voltage that is on the negitive side of the coil, IS AC, always has been, always will be (for conventional coils). The byproduct of being AC voltage is due to the colapse of the feild within the coil, which is when the coil actually fires, when the signal from the ignition module is dropped, the coil stops charging and then current then travels from the secondary through the cap, wires and to the spark plug. You can actually try this, with a coil and battery, and some wire, by holding the negitive wire to the battery negitive and then pulling it away, the coil will discharge. I don't know if Radio Shack still sells the "electronics handbooks" but there were a few different ones, that explain how electronic circuits worked and had designs for different uses and triggers in them, maybe take a look for these, they were pretty inexpensive too, about $6 last time I bought one.

Ignition Coil - This is the part that makes high voltage (approx. 20KV for a stock coil, and up to 40KV for a high performance coil) for the spark plugs from the low voltage (12V) that is supplied to it by the car. It is basically a simple transformer operating on the principle of "mutual inductance". The coil stores up energy over a relatively long (for ignition systems) period of time and then releases it suddenly to the spark plugs via the distributor and HT wiring.

Coil operation - when the points close, current through the coil primary increases from zero to a maximum value (determined by circuit resistance) in an exponential manner, rapidly at first, then slowing as the current reaches it's maximum value. The rate at which the current rises is determined by the coil inductance and the circuit resistance. At low engine speeds, the points are closed long enough to allow the current to reach a level limited only by the total circuit resistance, ie, a DC value. At higher speeds, the points open before the current has time to reach this maximum value. In fact, at very high speeds, the current may not reach a value high enough to provide sufficient spark, and the engine will begin to miss. This current through the coil builds a magnetic field around the coil. When the points open, The current through the coil is disrupted, and the field collapses. The collapsing field tries to maintain the current through the coil. Without the capacitor, the voltage will rise to a very high value at the points, and arcing will occur. The time for the field to collapse will also increase. With the capacitor, the current provided by the collapsing field will discharge through it, limiting the voltage at the points, and the current/field will collapse very rapidly, having a discharge path to ground through the capacitor.
The coil, capacitor, and resister form a tuned, oscillator circuit. When the coil is completely discharged, the capacitor is completely charged. Now, the capacitor will try to discharge to the coil. Without resistance, there is nothing to limit the coil or capacitor discharge current, and the cycle will repeat, ie, the coil will charge, then discharge to the capacitor, which will charge, then discharge to the coil, etc. With the resistance, however, the current is "dampened," and the amplitude of the oscillating current is reduced rapidly, dropping to negligible within 3-4 cycles.

When the magnetic field of the primary coil collapses, it cuts through the windings of the secondary coil, producing an output voltage. The magnitude of the output voltage is determined primarily by the windings ratio and by the speed at which the primary field collapses. A slow collapse will produce a lower output than a rapid collapse. Until the arc occurs at the plugs, the output of the secondary is nearly an open circuit, allowing the voltage to reach a peak before current is produced. As soon as the spark occurs, the resistance is reduced, and current flows through the plug gap, maintaining the arc. The primary and secondary windings are isolated from each other, so that no current in one flows through the other. However, the secondary is connected to the primary at the point where the primary connects to the points and capacitor, and there is no direct path for the return of the secondary current other than through the capacitor. As a result, the capacitor is part of the secondary as well as the primary. There is an oscillation in the secondary, just as there is in the primary, for the same reasons.

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Tuners of the Quickest 4.9
12.51@118 mph

4.5 (RSR)Hi Perf. Caddi-V8
In place and running.

Correct me if I'm wrong but rather that the tach signal being AC voltage(changing from + to -), isn't it really just a series of negative pulses. I mean you could use the AC setting on a volt meter to measure it because rather than measure both positive and negative it would just measure the negative pulses. This may be WAY out to lunch but if someone could clairify it would be greatly appreciated.

If the signal has negative pulses it is DC because that is only half of the sine wave and not a complete cycle and ac voltage does not have polarity + -.

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Tuners of the Quickest 4.9
12.51@118 mph

4.5 (RSR)Hi Perf. Caddi-V8
In place and running.

Great description on how a POINTS system works, too bad it has very little to do with a fully electronic system such as what teh Fiero uses, and 99% of swaps.

It is common acceptance the coil negitive when measuring voltage on this terminal, that it is A/C, or rather the byproduct is A/C and this is what you use on your DMM to measure it accuratly.

It took me a while to figure out why, and it's difficult to describe, but it is the way it works.

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The Raven :Under Construction
"James" 1985 GMC Jimmy, 3.2L turbocharged intercooled hybrid 13.873 @ 99.08

"Speed Costs, How fast do you want to go?"

I should have probably said "changing from push to pull" instead of + to -?