L5-30 receptacle for built in vacuum. Receptacle has only 3 leads, one calling for the ground and another for the white. What about the 2 hots (red and black)?

Electric dryers are 240 volt. You must have a 30 amp 240v receptacle to plug it into. The cord on the dryer must have a compatible plug.
The circuit should not be shared with anything else, and should be protected with a 2-pole 30 amp circuit breaker.

It is possible for only one side of a 2-pole breaker to pop. When that happens one of the "hot" terminals would be dead and would account for the weird voltage readings you're getting which are just stray voltages from the one good side bleeding back. So Check your breaker first. You should get 120v on each terminal of the breaker and 240v between them. The cable leading from it should be 3-wire (red, black, white). It may, or may not, also have a seperate (bare) ground wire.

Then check voltages at the receptacle. You should get 120v between each hot side and the neutral, and 240v between the two hot terminals. If not, then check the wiring connections at the breaker and at the receptacle. Switch off the main breaker (or pull the disconnect) before doing that.

If voltages OK at the receptacle, but not at the control board then replace the power cord.

The ground wire should be fastened to a cold water pipe with a clamp designed for that purpose unless your power cord and receptacle have a separate (4th) terminal for the ground.

Explanation.
Inside the rear of the dryer is a row of 3 screws/nuts.
The right and the left of those 3 are "hot".
You will attach the red and/or the black leads to those "outside" 2 screws, doesn't matter which.
The center wire is white wire only, it is a ground.
For the 4th wire, it will attach to the body of the dryer, it is also a ground.
P.S. On both the 3 prong and the 4 prong, the HOT wires ( red and also black ) are straight/flat lugs.
The "L" shaped lug and the round lug (4 prong only) are both ground.
Be blessed.

You really need a 2-pole timer -- 2 wires in and 2 out. In that case the Black/White from the receptacle goes to the 2 "in" terminals and the 2 red wires from the pump go to the 2 "out" terminals. The WHITE wire from the receptacle goes nowhere - tape it off. The Bare wire from the receptacle goes to the GREEN (ground) wire on the pump.
A single-pole timer can be used, but in that case, you ALWAYS have 110 v on one side of the pump. Not safe, and not code compliant.

The big slot is the white wire ground is the green screw small slot is the black wire the 2 screws next to each other is the hot

If I understand correctly, you want to add a photocell to existing receptacle.

Start by looking at receptacle:
Receptacle has 3 different colored screws.
Green screw for bare ground wire
Brass screws with black Hot wire connected.
Silver screws with white Neutral wire.

How to wire photocell to receptacle.
Remove black Hot wire off of brass screw and connect black wire to photocell-black wire. This will power the photocell switch.
Connect photocell-red wire to brass screw on receptacle. This will turn receptacle on-off whenever photocell switch turns on-off.
Connect photocell-white wire to silver screw, and this will complete circuit to photocell switch so the switch is able to operate.
All white Neutral wires are connected to silver screws.

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Are you trying to connect your range to a 3-wire source, using a 4-wire cord? I'm a little confused on your symptoms. Normally you have a 2 HOT leads and a NEUTRAL/GROUND (3-wire) in older homes. In newer homes you have 2 HOT leads, a separate NEUTRAL and a separate GROUND (4-wire).

The following link explains the difference between the two configurations and how they should be correctly wired:

http://www.fixya.com/support/r3575913-installing_a_220_vac_appliance_cord

New construction homes built after the year 2000 REQUIRE the use of a 4-wire plug and are usually readily wired at the receptacle to accommodate. If your existing receptacle already has a 3-wire configuration you do not need to re-wire it, however. The easiest solution would be to purchase a 3-wire power cord that matches the receptacle you have.

Read through the link provided and post back and let me know if you have any questions. I hope this helps you solve your problem.

yes, with a double run of 12-2w/ground you'll end up with 2pairs of 12 for your 2 hot phases(you will need to mark one pair red to substantiate the 2 different phases) and each 12 gauge wire will carry 20 amp each alone, then you'll have 2 pairs of 12 with white insulation white for neutral is fine but the other pair for the safety ground will need to be marked or painted green where ever visible per N.E.C. i.e. main panel or dryer receptacle or hardwired into the unit itself,use 10-3w/ground its cheaper in the long run and you dont have to run a doubleheader

You cannot replace the switch with a switch/receptacle combo because there is no neutral wire. The black and white that you see are both used as hot wires. One is the Line and the other is the Load.

You have to install another 2 wire cable or replace the existing with a 3-wire cable. A 3-wire cable has a black, red, and white, plus the bare ground wire

if its 240volt and you have a 3 wire receptacle, (crows foot) and the new stove has 4 wires:i.e. a RED-BLACK-WHITE-GREEN on the oven unit, your red/black are your 2 hot wires white goes to neutral, and Green is safety ground put the WHITE and GREEN together, as long as the receptacle comes DIRECTLY from the Main Panel in the House, they use a 4 wire systen in some locations depending on local codes/any new installs require a 4 wire plug install now per the N.E.C.

Okay, the normal color coding for a 220-240VAC receptacle is as follows:

White (Neutral) - 0VAC
Black (HOT) - 110 VAC
Red (HOT) - 110 VAC

I suspect that the two black wires from your power panel are the two hot leads in your case. That is...each one should read 110-120VAC. Verify this my measuring each black lead to ground. The copper wire is probably a ground or neutral. It sounds like this is an older house (?).

The following link will give you some advice as well:

http://www.fixya.com/support/r793520-3_prong_4_prong_power_cord_conversion

In addtion, the electric service for major appliances like a Range or Dryer should have its OWN dedicated circuit to prevent overloading. I hope you find this information helpful. If you have questions, please let me know.