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Old 07-25-2009, 10:13 AM   #1
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Default Fuel Pump Circuit

Does anyone know where the FP circuit is/the components used/are on the V3 mainboard?

might help me diagnose an issue I have with a mates MS....

Cheers guys
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Old 07-25-2009, 12:05 PM   #2
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Q2 and Q19 are the transistors, they looked **** solders too.... fingers crossed!!
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Old 07-25-2009, 12:35 PM   #3
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Pretty simple circuit:



Q2 is what drives the pump. When the ECU applies +5 on "FP-1" it turns on Q2 through R16. Q2 then conducts to ground through R40. That's the main drive circuit.

Why the heck is it going to ground through a resistor? That's where Q19 comes into play. That part of the circuit is a current limiter, to protect Q2 from blowing up if you try to drive too big a load with it.

As current from the fuel pump relay passes through R40, a small voltage develops across the resistor in proportion to the amount of current flowing through it. That voltage is applied to the base of Q19, and if it rises too high, Q19 will turn on, conducting Q2's base drive signal to ground, turning off Q2. The turn-on voltage for Q19 in this configuration is going to be about 0.7v, so the current limit point is about 700ma across R40. Q2 is capable of safely passing 1000ma, so this is a nice, conservative limiter.

If Q19 is internally shorted (or solder-blobbed from collector to base) then it'll hold the base of Q2 low and prevent it from turning on.
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Old 07-25-2009, 01:07 PM   #4
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Quote:
Originally Posted by Joe Perez View Post
If Q19 is internally shorted (or solder-blobbed from collector to base) then it'll hold the base of Q2 low and prevent it from turning on.
Cheers Joe.

Sometimes it turns on, other it doesn't. a gentle tap of the case and it bursts to life...
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Old 08-01-2009, 11:03 AM   #5
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Problem solved.... the rock hard susupension, (stock shock and VERY low springs) are to blame IMO.

the CPU had rattled loose..... not obviously, but gentle tapping of the unit in operation with the handle of a screwdriver caused the FP to cycle on and off, Pushed the CPU down and it moved 2-3mm.

Fixed! only took a month to diagnose!!
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