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Discussion Starter #1
My 2010 Gen Coupe was having brake issues and I took it to Pep Boys for a diagnostic. They determined that it was the master cylinder.

I ordered the same one from Rock Auto, we replaced it and did all the bleeding before and after installing the Master Cylinder and everything felt fine.

However, if I start the car without putting my foot on the brake pedal initially, when I press the brake pedal to put car in gear (AT) and every time the pedal is pressed, I hear like an electric pump, just like the one you hear when you first start the car; which I'm assuming it is the gas pump, but this sound comes one every time I press the brake pedal and sometimes when I release it.

Now, if I start the car with my foot on the break, then that electric pump will not come on.

Brake fluid is fresh, brake pads are still good and there are not leaks anywhere that I can tell.

I am suspecting that air is still in the lines or abs system and that the pump is coming on when I press the brake pedal to compensate pressure that is lost from the space air in producing in the system.

I'm not a mechanic, I don't know if this is even possible. What causes that whatever electric pump to come on only when I press or release the brake pedal and only when I start the car without pressing the brake???

I was told that the abs had to be bled also but with the scanner; which I took to a shop and they claim they did.


What am I missing?
 

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I'd rather do it myself
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The auto trans models have a vacuum pump that should come on when the vacuum switch closes. The switch is located on the pass side of the master cylinder and power for it is from the ESC module. I would guess either the switch is bad or you are loosing vacuum on the brake booster or this is just normal operation and you have not noticed it before?

Logic is from the ESC module that grounds the pump relay coil through a driver on the module when the switch is closed.

 

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Discussion Starter #3
The booster does have a sensor, you are very right. But I can't find anywhere what type of sensor that it so that I can replace it. Nor can I find that same exact booster anywhere online.

I'm sure that this behavior is abnormal. I've had this car from day 1 and cero miles, so I know it's good and bad habits.

Specially the startup buzzing when I turn the key in the ignition. I don't know if it's the gas pump loading or the abs/brakes loading, but I do know that now it takes longer to stop.
 

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I'd rather do it myself
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18,556 Posts
Dealer wants around $500 for the booster.

I have some vacuum switches. You could tee off the line feeding the booster and connect the switch. It will come on at around 1" Hg vacuum.

There is also a check valve in the hose going from the intake to the booster, make sure that is working as it should. Should be closed when in boost.

https://www.jimellishyundaiparts.com/productSearch.aspx?ukey_make=1033&modelYear=2010&ukey_model=14685&ukey_trimLevel=0&ukey_driveline=0&ukey_Category=0&numResults=50&sortOrder=Relevance&isOnSale=0&isAccessory=0&isPerformance=0&searchTerm=vacuum pump
 
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