I have a vector VEC051C 3000 Watt inverter, the vent fans run continously without load, can it be modified to cool only when, the temperature increases..

No they do not have any internal cooling devices. they rely on ventilation alone.
I will suggest that you read up on reviews for this item if you are thinking of buying one.
They have not gotten great reviews.

Hello, I hear you. Was the reason to supply 240 volts? When you try the inverters separately, do you get 120Volts output? Let us look at the units working individually first and go from there.

Inverter run on batteries sir, and if these batteries are not charged, the run time {the number of hours it will work} reduces.
S0, you will need to connect the inverter to a power source.Theses will allow your batteries to be charged and enable it to run for a long period of time.
Make sure your batteries are still OK.i.e working properly

Two possible problems; input voltage too low, or the output load too high. If input remains at 12v or higher and it shuts off with no load, then the inverter is bad.

With the car voltage at 13.8V and you have less then a 400 Watt load the inverter is probably bad. The output voltage is not adjustable by the user.
Good Luck,
Gil

I can't answer your question but I strongly suggest you urgently contact the manufacturer or dealer in order to clarify this most important point as a 3kw inverter isn't cheap and shouldn't need replacing very often.

Check that ground is correctly connected. If nuetral is provided, ensure it too is connected. Your total load is 780 watts. Check power in and ground out of the supply side. There may be a fuse in the inverter on the output side also, replace if necessary. If fault light does not go out after these checks, you will need to contact Black and Decker who manufactures the vector series now.

your inverter needs to cool at all times , if the fan does not work,try buying a small standing fan to blow cool air to you machine, it is possible that your heater is making the inverter to over heat please avoid using the heater on the inverter

Inverters use high power transistors to create an A/C voltage from a D/C input. These transistors are the common point of failure when the units are not cooled properly or used beyond their rated capacity. (Actually, you should always buy an inverter that’s rating for continuous use is well below what your actual need is). What’s most likely happened is that these have shorted out and need to be replaced. Unfortunately, replacement parts alone will likely cost nearly what a new unit costs.

Sorry to be the bearer of bad news; but you basically have a nice paperweight with some pretty lights and fans on it.

Have you looked at the fuses within? I have two in the one I am trying to fix!!
Peter