Member Avatar for viperman224

Hey all,

Went to a local computer show with a friend. Working on building him a ccomputer. Any ways, he got hold of an HP pavilion 7850. The machine had been caught in a lightning storm and the only pieces that worked were the Processor, Ram, floppy and cd-roms. The machine had a PIII 933mhz in it. The motherboard was shot. We bought an HP vectra VL400 MT for 130$ and an axio X5 series case. We got home gutted the HP vectra and the Pavilion. Everything went together just fine. except one problem, the axio case comes with a 480 watt PSU and the motherbaord out of the HP vectra comes up with an error and does not let me boot unless the PSU fan is pluged into the motherboard. I pulled the power supply from the HP vectra and plugeed it in to the Axio case and the machine came up just fine. This is yet another reason I build my computer from scratch normally but my friend is on a budget. The machine ended up being a PIII 933mhz with 382 megs of ram and a 20 gig hard drive. I can't install windows until I trick the HP vectra motherboard to think the fan is running. Is this a proprietory thing or am I just not doing something correctly? I pulled the cover off of the power supply out of the HP vectra and found the cables for the PSU fan come in from the motherboard and go into some kind of circuit and then go out to the fan. My father is an electronic technician and could easily trick the machine if he knew how the circuit worked or what the circuit does. Any ideas?

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Summary and practical follow-ups for the HP Vectra VL400 MT power‑supply/fan problem described by — several members reported workarounds but the core issue is worth clarifying before trying hardware hacks.

Multiple replies in this thread (notably , , , and ) found that the board will refuse to continue POST unless it sees the expected fan signal; some people bypassed that check with a jumper and others suggested creating a dummy load. That behavior is documented for VL400 systems: non‑HP ATX PSUs often lack the fan‑sense wiring the Vectra expects, producing POST error 0051. (h30434.www3.hp.com)

Why the board complains: most PC fan headers provide +12V and a tachometer (speed) output. The tach line is typically an open‑collector pulse used by the motherboard to verify the fan is spinning (many fans give two pulses per revolution). A plain resistor or a powered fan will behave differently depending on whether the board checks current draw or actual tach pulses. (edn.com)

Recommended, safer approach (in order):

  • Fit a known good 3‑pin fan to the board header and mount it so it actually cools the case — this is reversible and preserves monitoring.
  • If you must fake the fan for POST only, first measure the header with a multimeter (check for +12V to ground). If you plan a dummy load, use Ohm’s law (R = 12V / I) and pick a resistor with adequate wattage (P = V * I); for a 0.1 A target that’s about 120 Ω at ≥2 W. Note: a resistor may draw current but will not generate tach pulses, so it may not satisfy MB checks. Typical case/CPU fans draw ~0.1–0.3 A, so use that as a guide. (a-power.com)

Cautions: do not randomly short unknown pins; avoid feeding PSU wires into a motherboard header; prefer using a donor HP PSU or an actual fan for reliable long‑term operation. Given the Pavilion parts came from a lightning‑struck machine, keep an eye on stability — surviving POST doesn’t guarantee long‑term health.

Recommended Answers

All 6 Replies

Member Avatar for kc0arf

Hello,

What I am about to suggest is to be taken with a grain of salt, and I cannot be liable if any damage happens to you or your computer. In effect, you will be performing a hardware hack.

The components that work after a lightning strike themselves are rather astonishing. I would not bet the house on them working for a length of time, as if lightning can over come 3 million volts per meter (the resistance of air before it sparks), then what chance does a small IC chip have of survival? I hope it works out, but if it was me making the purchase, I would have walked by.

Anyways, the motherboard might be looking for a voltage drop across the fan circuit, meaning it could be simply looking to see if current is flowing (the fan is turning, using electricity), and if there is no flow, there is no go. Or, it could be smarter looking for precise measurements for the fan. It might even regulate the speed of the fan based on the heat, and you might later discover that if the circuit thinks the computer is getting too warm, it will proactively shut down on you.

As a hardware hack, you have two choices:

1) See if you can move the fan from one case to the other. Mount it somewhere so it does some good, but have a fan there, maybe blowing the same direction as other fans. Do not have this fan blow against another fan though because it might cause a stalemate in the air exchange, and cause a burn out condition.

2) Measure the resistance of a comptuer fan, and go to Radio Shack, and get a resister to place across the jumpers. The resister will use electricity, and make a little bit of heat, just like a fan would. You might want to use a variable resister, so that if you have to tweak it a bit, you have some leeway.


Either case is risky, and should be considered last-ditch efforts. Anything that you do could burn something else out, or cause other problems. It is up to you on how bold you might want to get.

Christian

Member Avatar for Die Lange

Hey all,

Went to a local computer show with a friend.......etc. etc.

I had the same problem today, there are three fan connectors on the board, look for the Power Fan connector, nearby the main power connector. Just contact the two pins whicj each other who are the most away from the main power connector. Mostly the pins are coloured red and white/yellow at the cable. Its working very well now! :) :lol: :p

Member Avatar for Another Robin

I had the same problem today, there are three fan connectors on the board, look for the Power Fan connector, nearby the main power connector. Just contact the two pins which each other who are the most away from the main power connector. Mostly the pins are coloured red and white/yellow at the cable. Its working very well now! :) :lol: :p

That's awesome dude! Thanks very much. I just tried to make an old HP vectra Vl400-MT work and I had the same problem. All I had to do was install a jumper on the two pins just as you described and it now boots up.
Thanks.

Member Avatar for lookevin

Hi,
I got the same problem here from a salvaged HP motherboard: Error 0051 - Fan error.
I've not tried short the two pins with jumper, but will do so soon. Here's a support page with diagram from a site that I found while trying to find solution to this problem. It show's the position of the fan power supply pin on the motherboard: .

Member Avatar for jalvarezca

That's awesome dude! Thanks very much. I just tried to make an old HP vectra Vl400-MT work and I had the same problem. All I had to do was install a jumper on the two pins just as you described and it now boots up.
Thanks.

It really works

Thanks U, very much....... :cheesy:

Member Avatar for lookevin

Hi,
I got the same problem here from a salvaged HP motherboard: Error 0051 - Fan error.
I've not tried short the two pins with jumper, but will do so soon. Here's a support page with diagram from a site that I found while trying to find solution to this problem. It show's the position of the fan power supply pin on the motherboard: .

ps. I've tried it, and it works! Thanks :)

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