Question Computer in an Endless Startup/Shutdown Loop

Jan 1, 2021
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I have a HP Pavilion h8-1090t computer. I have a problem where the computer starts up and runs for about 15 seconds and then shuts down. It keeps repeating this loop endlessly. During this 15 second startup the two multimedia drives run and the fans run, but there is no video and no audible sounds, such as beeps. I've disconnected all of the peripherals, drives, and RAM from the motherboard. The only thing I have left connected to the power supply is the motherboard and the CPU and I'm still having the startup/shutdown cycle problem. I've reset/cleared the CMOS data and that did not fix the problem. I've reset the heat sink on top of the CUP with new thermal paste and that didn't fix the problem. I've inspected the motherboard and I cannot see any apparent damage or anything that would be shorting it out. I've checked the voltages on the 24 pin connector and they are all exactly what they are supposed to be except for possibly the gray wire (Power OK). The gray wire voltage is 3.27 volts and I'm not sure that this is high enough to signal the computer that it is alright to continue the startup cycle. I've seen some information online that indicates that the gray wire should have a voltage of 5 volts.

Can anyone confirm that a gray wire voltage of 3.27 volts is adequate to start the computer and that the PSU is not the problem? If so, does anyone have any suggestions on how to determine if the motherboard or the CPU is the problem?

Any help would be appreciated.
 
Jan 1, 2021
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The following link may be helpful:

https://www.lifewire.com/how-to-manually-test-a-power-supply-with-a-multimeter-2626158

Reference the included pinouts: e.g., the Note in Step 9 > the 24 pin link, pin 8.
Thanks for the reference. However, I already had a table for the 24 pin connector voltages. The problem is I can't find a table that definitively tells me what the voltage should be for pin 8 (the gray wire). The table you referenced just says "Power On" and doesn't specify a voltage. The table I have seems to indicate that pin 8 (gray wire) should have a voltage of +5. As I mentioned in my original post, the other pins on the 24 pin connector all have the proper voltages.
 

Ralston18

Titan
Moderator
The voltages are all standardized and +5 volts is the voltage for the gray wire.

However: that is not absolute as there are allowed/ acceptable voltage tolerances.

https://www.lifewire.com/power-supply-voltage-tolerances-2624583

Tolerances are needed simply because it is very difficult to maintain an absolute fixed voltage either in the power source or the voltage being demanded by the component.

Precision electronic devices often have very tight tolerances + or - maybe 0.1 % . And to accurately measure those voltages then a multi-meter or other diagnostic tool is needed that must be able to measure those voltages and also be within its' own tolerance levels. Accuracy matters.l

If the gray wire is 5+ volts and you are measuring 3.27 volts per your original post then the voltage is out of spec.

The voltage should be within tolerances that being + or - 5% of 5 volts.

In other words a minimum of 4.750 volts or a maximum of 5.250 volts per the above linked table.

The measured 3.75 volts is below minimum and by quite a bit provided that the multi-meter is accurate and properly calibrated.

As for the cause: the power switch may have a problem. Too much resistance for what ever reason. Check the resistance of the switch.

The ohms measurement should be zero.
 
Jan 1, 2021
3
0
10
0
The voltages are all standardized and +5 volts is the voltage for the gray wire.

However: that is not absolute as there are allowed/ acceptable voltage tolerances.

https://www.lifewire.com/power-supply-voltage-tolerances-2624583

Tolerances are needed simply because it is very difficult to maintain an absolute fixed voltage either in the power source or the voltage being demanded by the component.

Precision electronic devices often have very tight tolerances + or - maybe 0.1 % . And to accurately measure those voltages then a multi-meter or other diagnostic tool is needed that must be able to measure those voltages and also be within its' own tolerance levels. Accuracy matters.l

If the gray wire is 5+ volts and you are measuring 3.27 volts per your original post then the voltage is out of spec.

The voltage should be within tolerances that being + or - 5% of 5 volts.

In other words a minimum of 4.750 volts or a maximum of 5.250 volts per the above linked table.

The measured 3.75 volts is below minimum and by quite a bit provided that the multi-meter is accurate and properly calibrated.

As for the cause: the power switch may have a problem. Too much resistance for what ever reason. Check the resistance of the switch.

The ohms measurement should be zero.

Thanks for the suggestions. However, I found an article about Power Supply trouble shooting that gave tolerances for the gray wire voltage and they are as follows.

The Power_Good signal has tolerances that are different from the other voltages, although it is nominally +5v in most systems. The trigger point for Power_Good is about +2.4v, but most systems require the signal voltage to be within the tolerances listed here:
SignalMinimumMaximum
Power_Good (+5V)3.0V6.0V

Replace the power supply if the voltages you measure are out of these ranges. Again, it is worth noting that any and all power supply tests and measurements must be made with the power supply properly loaded, which usually means it must be installed in a system and the system must be running.

This would seem to indicate that a voltage of 3.27 volts should be adequate to send a "power OK" signal to the motherboard.
 

Zerk2012

Titan
Ambassador
You said you removed the memory. It has to have a stick of memory to boot.
Try 1 stick if no boot then use the same stick in anouther slot till you tried all slots. If you have anouther stick try it the same way.
 

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