# Parking Brake Relay... Risk of frying HU?



## hopper33 (Mar 21, 2011)

I have seen numerous posts and videos on the web about making a parking brake bypass by using a relay to allow for watching DVDs while the vehicle is in motion. They almost all say to connect a wire from the 12v relay to the HU's remote turn-on lead, which also can be connected to an amp.

My question is will this wear out the HU?.

I've been told the HU's remote turn-on wire is used to turn on amps, etc... (using very little power), but is not meant to power a relay (suggesting the HU will wear out quickly due providing constant power to the relay for the parking brake bypass.

Thanks for any input.


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## flexdmc (Aug 14, 2009)

There are typically 4 legs to the relay. ground, power does the switching or on/off of the relay. The switched legs would be used to control the parking brake wire whatever it is. When you connect the relay I'm guessing that the wire is then connected through the parking brake. My installer did mine however I'm familiar with relays.


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## flexdmc (Aug 14, 2009)

And I don't think it will ruin the unit as you don't have a huge influx of power going to the unit.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

> *Quenching Diodes: Anytime that a relay coil is driven by a circuit that is not specifically designed to drive a relay, you should use a quenching/suppression diode connected in parallel with the relay coil. The diagram below will show the connection of the diode. Initially, you may think the diode serves no purpose because the voltage applied to the relay cannot pass through the diode. This is true when the relay is energized. The diode comes into play when the power source is removed from the relay coil. When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF). The back EMF can easily reach 200 volts. The diode will absorb the reverse voltage spike. This voltage, if not absorbed by the diode, will cause premature failure of switch contacts and may cause the failure of power switching transistors. You can use virtually any type of rectifier or switching diode (i.e. 1N4001, 1N4002, 1N400x... or Radio Shack part #s 276-1101, 276-1102, 276-1103, 276-1104). *


Relays


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## hopper33 (Mar 21, 2011)

So, no... It won't fry from powering the relay, but could from a kickback? Suggested installation of a diode?

If the diode stops current from going a certain direction, wont it stop the HU from getting a signal that sasys the parkimg brake is "engaged?"

Thanks for the responses!

Anyone have a different opinion or other thoughts?


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## flexdmc (Aug 14, 2009)

I would imagine that circuitry inside the radio already but I guess an additional diode wouldn't hurt


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

flexdmc said:


> I would imagine that circuitry inside the radio already but I guess an additional diode wouldn't hurt



That's possibly why you have 38 posts VS my ~6500... :laugh:

Someones got to keep people from spreading imaginary things guessed upon... :laugh:


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## sqshoestring (Jun 19, 2007)

I'm not entirely sure what you are saying here but a relay is used to power something from another source of power. If the coil takes too much power then get a smaller relay. I use a 10A relay to run my amps and fans, it takes far less power to run the coil than a typical 30A bosch. Newer HU are limited in the power the remote can supply, but most all can run a 30A relay with no problems. Some relays have the diode in them.


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## IBcivic (Jan 6, 2009)

When the current flowing is DC, a simple rectifier diode is often employed as another form of snubber. The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor). The diode is installed so that it does not conduct under normal conditions. When current to the inductive load is rapidly interrupted, a large voltage spike would be produced in the reverse direction (as the inductor attempts to keep current flowing in the circuit). This spike is known as an "inductive kick". Placing the snubber diode in inverse parallel with the inductive load allows the current from the inductor to flow through the diode rather than through the switching element, dissipating the energy stored in the inductive load over the series resistance of the inductor and the (usually much smaller) resistance of the diode (over-voltage protection). One disadvantage of simple rectifier diode used as a snubber is that the diode allows current to continue flowing, which may cause the relay to remain actuated for slightly longer; some circuit designs must account for this delay in the dropping-out of the relay. This delay often leads to greatly decreased life of the relay contacts due to arcing.


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## flexdmc (Aug 14, 2009)

I've used a small 5V relay from radio shack but I don't know if it had the diode inside, doubt it.
maybe now I know why my car caught on fire... not enough posts... doh!
I kid I kid


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

flexdmc said:


> I've used a small 5V relay from radio shack but I don't know if it had the diode inside, doubt it.
> maybe now I know why my car caught on fire... not enough posts... doh!
> I kid I kid


No but seriously, I wasn't tryin to diss ya bro, but as you can see, I've been doing this a little while.. I'm sure you prolly have too, but my 6500 posts here pale in comparison to the 10yrs on say my Subaru site.. 

I'm not tootin my horn here, jus sayin.. Often times there is no reason to guess on anything if you know where to look.. that's all.. 

Cheers


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## Timelessr1 (Feb 12, 2010)

Am i missing something from the OP post? He is trying to bypass the parking brake for a double din headunit? 

what headunit? Each headunit has a certain way to do it...some are easier then other. Pioneer you need to remove a pin from the plug and reposition it and ground it out. Kenwood's you just ground out the parking brake wire, etc. Why are you trying to use the radios remote lead to bypass the parking brake?


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Timelessr1 said:


> Am i missing something from the OP post? He is trying to bypass the parking brake for a double din headunit?
> 
> what headunit? Each headunit has a certain way to do it...some are easier then other. Pioneer you need to remove a pin from the plug and reposition it and ground it out. Kenwood's you just ground out the parking brake wire, etc. Why are you trying to use the radios remote lead to bypass the parking brake?


Newer Pioneers require the HU to not see ground, THEN seeing ground on the PB wire.. hence using the relay on the remote turn on lead.. 

The HU PB wire doesn't see ground until a source is selected, then the remote wire comes on, powering the relay, grounding the PB wire..


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## Timelessr1 (Feb 12, 2010)

Aaron'z 2.5RS/WRX said:


> Newer Pioneers require the HU to not see ground, THEN seeing ground on the PB wire.. hence using the relay on the remote turn on lead..
> 
> The HU PB wire doesn't see ground until a source is selected, then the remote wire comes on, powering the relay, grounding the PB wire..


when you say "newer" Pioneers...are you refering to the z120bt? or like the 130nbt? the reason i ask is b/c i just installed a z120bt like 2 weeks ago, and the bypass was exactly the same as the earlier z110bt unit.


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## 96jimmyslt (Jan 31, 2011)

What head unit OP?

I just put the RCA loom (GPS bypass) on ground, and the parking brake (fused black speaker and power loom) on ground and it did the trick.

Maybe the newer ones are different??

BTW my head unit is an avic f700BT

Kinda hard to say without knowing the head unit. 

Regardless, probably a lot more info on the head units forums, such as avic411 for my head unit.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Timelessr1 said:


> when you say "newer" Pioneers...are you refering to the z120bt? or like the 130nbt? the reason i ask is b/c i just installed a z120bt like 2 weeks ago, and the bypass was exactly the same as the earlier z110bt unit.


I just did an AVH-P3300bt and that's how the bypass works.. I guess I shouldn't have said "newer" but "different"


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## Timelessr1 (Feb 12, 2010)

oh damn....look at that...learn something new everyday! Looks like you can actually buy the relay all wired up and ready to go for that unit too. 
eBay.com.sg: PIONEER DVD BRAKE BYPASS HACK AVH-P4300DVD AVH-P3300BT (item 130480983047 end time Mar 31, 2011 22:17:50 SGT)

I wonder which radio the OP has....


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Timelessr1 said:


> oh damn....look at that...learn something new everyday! Looks like you can actually buy the relay all wired up and ready to go for that unit too.
> eBay.com.sg: PIONEER DVD BRAKE BYPASS HACK AVH-P4300DVD AVH-P3300BT (item 130480983047 end time Mar 31, 2011 22:17:50 SGT)
> 
> I wonder which radio the OP has....


Yup, or you could wire one yourself, youtube has the vids on how..


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## hopper33 (Mar 21, 2011)

The HU is the Pioneer AVH P4300. The DVD needs to see a grounded parking brake to work.

Thanks for the help... still trying to sort this stuff out.

I already have the bypass relay in place and it works, but I am going to add an amp. I was wondering how to add it without risk of frying the HU from to much strain of powering the bypass... or at least minimize the risk.

I had heard you can splice the amp's remote turn-on wire and the bypass power wire to the HU's remote wire and it should be fine. But after hearing the HU might get over worked from the bypass relay, saw a suggestion of a second relay for the amp put on after bypass relay.

I am just looking for the safest way to hook up the bypass relay and the amp to the HU, to avoid blowing or frying something

Thanks for all the help!!


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## Timelessr1 (Feb 12, 2010)

It sounds like you might want to hook up an additional relay BEFORE your bypass relay. This "pre-relay" will use your headunit's remote lead as the trigger for this relay...then the + feed from this relay can power the "bypass relay" and your maps without any issues since the current draw wouldnt be put on the head unit at all....just sayin


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