My wall outlet is a 3 prong and I want to convert it to a 4 prong but I only have three wires?
Should I make a jumper in the outlet from the common to the 4th grounding outlet?

Would you please let us know exactly why you want a four prong outlet.

What are you going to connect to this outlet? What voltage and amperage will the load need? What is installed there now, voltage and amperage?

Would you please let us know exactly why you want a four prong outlet.

What are you going to connect to this outlet? What voltage and amperage will the load need? What is installed there now, voltage and amperage?

Sorry, I have a 4 prong 220/240v dryer. The house is 30+ yrs old and has a 4 prong outlet that a previous owner installed in place of the original 3 prong. But the 4th ground post was left un-wired.

That is a potentially dangerous situation, did you have an inspection done before you bought the home? Not that they would catch that anyway.

Your choices are limited here, either replace the receptacle with a 3 prong or run a new cable and keep the receptacle. Connecting the neutral to the ground prong on the receptacle is not something you should do.

Okay, do you have the installation instructions for the electrical requirement for the dryer?

If you do verify whether you need a four wire connection point. If you do, then you must follow the instructions.

That would mean you need to pull a new four wire cable to the receptacle outlet and properly install the receptacle. Most electrical dryers are 120/240 units using 30 amp, which means you would want # 10/4 AWG copper cable.

The dryer almost doesn't matter. If all you have is a three wire circuit run then you can ONLY do as Tev suggests. Re-install a 3-wire receptacle, or run a new circuit and install a 4-wire receptacle.

The receptacles dictates the dryer cord, NOT the other way around.

The dryer's instructions dictate the necessary wiring.

Too add, New construction require 4 wires for Dryers and Ranges, it's a good thing, keeps ground from carrying current.

Too add, New construction require 4 wires for Dryers and Ranges, its a good thing, keeps ground from carrying current.Just to make things clear, older 120/240v dryer circuits were allowed to have two hots and a neutral. A ground was not required. The neutral served as both the neutral and ground. A neutral was always required.
This was allowed under certain specific instances.

Newer codes disallowed the old "3-wire" dryer and range circuit making a dedicated grounding conductor necessary.

If an old 3-wire dryer circuit exists there is nothing saying that you have to change it. It would definitely be a good thing to upgrade to a current "4-wire" circuit, but it is not mandatory.

I agree, for this, 3 wire sounds best.

donf is right also, he should check his dryer instructions. I do believe that most dryers you see can be wired with either plug depending on an internal jumper. The poster should be aware of whether HIS dryer can be prior to making a discission on which route to take.

That's pretty much the reason I said the dryer "almost" doesn't matter.
Every American residential dryer I have ever seen has been 120/240v and convertible as far as the cord goes.

240v European style models are a different story.