Resistor for dual walbro fuel pumps

[p5150]

I have two pumps in the tank, and I have an additional relay wired up in the back so that the AEM will turn on a full 12v when the engine load requires it.

For day to day operations, I have the standard 9v circuit (with stock resistor). The problem is that the pumps are pulling too much current through the stock resistor and its getting hot.

How can I go about tackling this problem? I am thinking of wiring in another resistor in series or parallel but Im not sure which way will work?

 

[IJ.]

Parallel would work or you could have the 2nd pump boost activated via a relay and 12v it.

 

[p5150]

I have the 12 v relay set up like that so that it comes on with parameters defined based on boost or engine load. I dont want to wire up the pumps separately because all of the wiring is already done. I was thinking that I could just get another resistor and throw it in parallel to the other resistor to accomodate the additional current draw from the secondary pump.

 

[IJ.]

Sorry I'm having a thick morning here.....

How is it wired using boost/load as a parameter?

If you're running both pumps at once driving them through the stock resistor if it's boost/load dependant how does it run off boost? (no fuel?)

 

[p5150]

I thought about doing it in series but it looks like it will actually increase the reistance to something like 14 ohms which would make it get even more hot?

In parallel it looks like each resistor will share some of the load?
http://physics.bu.edu/py106/notes/Circuits.html

I really wouldnt have trouble figuring this out but I have a case of the stupids today. Feel sick or something

 

[joliroger4]

Two stock resistors in parallel works. I ran like this. Just make sure its not 9V when under boost or you'll drop a ton of fuel pressure.

 

[p5150]

The relay in the back is wired to a switched ground (12v direct to the battery) that comes on when the AEM tells it to. The day to day ops of the circuit is through the resistor.

 

[IJ.]

5150: Hmmmm ok I think I understand what you've done now!

If it were me I'd wire one pump using the stock resistor if you feel you really need the 9v operation, I'd then isolate the 2nd pump to the AEM relay and have it boost controlled as a simple 12v on/off as required.

I hope you are using diode protected relays for all of this.

 

[p5150]

Yeah the 12v relay has a diode in it.

 

[p5150]

I wired in the two resistors in parallel. The pumps sound even quieter than with the single resistor - I guess this makes sense because it is now running at 3.5 ohms for resistance? Well that doesnt make much sense to me. I thought that with the lower resistance that it would make the pumps run harder.

The total resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:

equivalent resistance of resistors in parallel: 1 / R = 1 / R1 + 1 / R2 + 1 / R3 +...

So 1/R = 1/7 + 1/7

1/R = 2/7 Resistance of 2 in parallel is R = 3.5

I measured the terminals of the resistor with my multimeter and both came out to 2 ohms - but the TSRM says that it is a 7 ohm resistor. WTF? Maybe I have a fubared multimeter? It seems to work great with everything else though...

 

[jetjock]

Equal values of resistance in series are additive, in parallel (as the formula shows) divided. For example two 10 ohm in series would be 20 ohms while two in parallel would be 5 ohms. Two in parallel would lower the wattage dissipated only because it'd lower the resistance ie; the pump would be getting more voltage (current actually) than with one resistor. What you need is a bigger wattage rating, not less resistance. If you can't find bigger wattage make it: heat sink the one you have or cool it some other way. Or wire the setup differently.

 

[WeDgE]

Do you *have* to use resistors? If not, have one pump running on 12V for normal duty, then have the 2nd pump activate via AEM when you need it.

??

 

[p5150]

Equal values of resistance in series are additive, in parallel (as the formula shows) divided. For example two 10 ohm in series would be 20 ohms while two in parallel would be 5 ohms. Two in parallel would lower the wattage dissipated only because it'd lower the resistance ie; the pump would be getting more voltage (current actually) than with one resistor. What you need is a bigger wattage rating, not less resistance. If you can't find bigger wattage make it: heat sink the one you have or cool it some other way. Or wire the setup differently.

Well I wired them in parallel and checked the voltage at the pump wire - it comes out to 12v. Im guessing that this voltage decreases as the amperage increases or that the resistors are limiting the current?

Regardless - it seems to be working just fine. The pumps are much quieter and the resistors do not get hot - just warm like they should.

 

[p5150]

Oh - and I just started my car after working on this engine for nearly 10 months. Holy shit it runs.

 

[Mrbaboon]

Oh - and I just started my car after working on this engine for nearly 10 months. Holy shit it runs.

Its a good feeling isn't it? :icon_bigg

 

[p5150]

as an update, I decided not to use the resistors at all. I have a 12v line running directly to the pumps. I really wanted to use the switched voltage setup but finding a resistor that could handle the current that I needed to run both pumps turned out to be more work than I was willing to accomplish.