The full answer to the P0505 (IAC) code from voltage booster

[Jack B]

As was discssed earlier this week, there seems to be a small percentage of the fuel pump voltage boosters that set a P0505 code. That is a malfunction of the idle air control motor. The indirect source of the problem comes from feeding the booster off of the PDC. When you feed directly from the battery, the problem goes away. Here are all the answers:

1. When ytou turn on the key the fuel pump cycles for 1.3 seconds, that is exact (measured on a scope), the manual says 2 seconds.

2. At the moment the booster energizes the pump there is a 6 Meghz sine wave superimposed on the 12 vdc. The waveform lasts for 2 usec. This is what sets the IAC code in the PCM. This has to be a result of the the capacitance in the booster tuning with the inductance in the line to booster. Typically it is referred to as a tank circuit.

3. The answer is to feed the booster from a source other than the PDC. I chose to pick up the power from the starter motor post. Another possible solution would be a filter cap tuned to a frequency close to 6 Meghz.

 

[Ulysses]

Switching noise. The booster or the pump most likely operates at 6MHz or some harmonic thereof.

 

[Jack B]

I was just visible at initial power-on. It was an over-damped tank circuit.

 

[Ulysses]

Interesting. Power-on only? I thought in your first post the code was set during startup and/or motor running.

 

[Jack B]

The code sets at the initial key-on. If you can get it running without a code (by clearing the code with the key on) it won't set the code during running. The steady-state voltage is pretty clean, only a 4mV ripple.

 

[1TONY1]

I'm guessing this is on the MSD fuel voltage booster ? I recommend the KenneBell over the MSD because of a few differences. This may be another reason to go to the KB.

 

[Bad_Byte]

2. At the moment the booster energizes the pump there is a 6 Meghz sine wave superimposed on the 12 vdc. The waveform lasts for 2 usec. This is what sets the IAC code in the PCM. This has to be a result of the the capacitance in the booster tuning with the inductance in the line to booster. Typically it is referred to as a tank circuit.

3. The answer is to feed the booster from a source other than the PDC.

Seems to me like its the inductance of the pump creating the oscillations. Upon initial supply of DC any coil will oscillate for a time. The short duration of the pulse kinda makes me think this is what's happening.

It would be interesting to see what goes on when the pump is fed from the battery. Is that initial oscillation still present?

I would think that adding a capacitor across the circuit (+ || -) large enough to absorb the initial current and flatten the pulse would help.

 

[Jack B]

BadByte:

Inductance alone does not cause a semi-resonant circuit. It is the right combination of capacitance and inductance and a lack of resistance. I am sure the oscillation is still there when connected to the battery, it just does not directly affect the PDC. The pump theory is good, but, it is isolated thru the conversion of the booster and cannot affect the front end of the booster.

Tony has the right direction, the Kenne Bell booster does have some advantages, this may add to the advantages. I like spare parts, so I bought the Kenne Bell for a spare. I may hook it up and see if it has the same issue. Although is may not hurt the pump, the MSD unit does not have the range that the KB unit does. With the MSD product the output is 15.7 volts at cruise to maintain 17.5 volts at WOT, the KB unit does not have that problem.