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Sharcnetusa
Username: Sharcnetusa

Registered: 12-2008
Posted on Monday, October 05, 2009 - 08:54 am:   

On the Nuvi 3xx and 6xx series, Pre-boot mode is activated when you press down and hold the on/off power switch.

So, if your Nuvi is powering on and displaying "Pre-boot Mode" all of the time, even if your not pressing the on/off power switch, then you have one of the possible following problems:

1. Your on/off power switch is stuck (shorted).
2. The TVS diode across the switch has shorted.
3. There is no pull-up voltage (3.7VDC) at the power switch. Caused by a failed pull-up resistor or bad connection.

Ok, what is a TVS diode? A TVS (Transient Voltage Suppressor) diode is placed in circuits to clamp transient voltage spikes, which could damage surrounding circuits.

Ok, so why is there a TVS diode across your switch? On your Nuvi's on/off power switch, there is a pull-up resistor that supplies a positive (+) voltage to the switch. When you press the on/off switch, that voltage is shorted to ground as long as your are pressing the switch. Releasing the switch, restores the voltage. That action, cues the Nuvi's microprocessor to boot-up.

However, that transition from high-to-low-to-high can induce a voltage spike that might damage surrounding components, so a TVS diode is placed in the circuit, right at the switch to clamp the voltage spike, therefore protecting the circuitry.

What happens is, the TVS diode can become damaged and either short to ground or become a low resistance to ground. This fools the microprocessor into thinking that the on/off power switch is being pressed all of the time.

SHARCNET-USA can repair this problem by either replacing the TVS diode or replacing the on/off power switch if it's found to be defective.

Contact us for more information.

(c)2013 SHARCNET-USA. This information may not be copied without the express written permission of SHARCNET-USA

(Message edited by admin on February 05, 2013)

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