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Old 12-22-2005, 10:24 PM  
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Reference Material: The Art of Electricity

I was on a roof today, and was reminded again about our pesky little friend electricity. No, I was not doing a 60 hertz dance of getting the stuffing zapped out of me either.

I was working in the blinking cold on a roof on a heat pump. I had a component that had failed on the system somewhere because it looked like a glacier. It was failing to go into defrost. So, just as I went to manually trip the unit into defrost, it went by itself into defrost mode, but didn’t defrost. The little reversing valve was supposed to kick in and didn’t. So, I metered the board, and sure enough, there was voltage coming out of the board. I ohmed out the coil on the valve, I had what looked to be the proper resistance. (and it was)

So, I hooked the coil back up to the board, and nothing. Checked the current draw on the coil; nothing. Took the coil and powered it from another source on in the unit, and it switched.

So what does all this mean? This is the same thing that *can* happen on a 6.5TD lift pump circuit (or any other place for that matter) Voltage may be present with a meter, but once a load is placed on the circuit, what little electricity was there with the meter is now gone.

How can this happen you ask? Do I need a new meter or something? This doesn’t make sense. Yes, it doesn’t make much sense from the outset, but read on and maybe this will come clearer.

As we equate electricity with plumbing, think of voltage as pressure. Pressure can be present anywhere in the system. If you have a well pump in your basement, or have seen them in various locations, gauges will indicate system pressure. Pressure is usually very steady with no “load” placed up on the system. Meaning when the toilet is flushed, shower running, etc, etc. Once a load is placed on the system, the pressure will drop. How much the pressure drops is in accordance with how much is able to be delivered, and how much resistance there is in the system itself.

What was happening above in the heat pump illustration was the board had a failure within it’s circuitry. It allowed voltage to appear on the meter, but the meter was not a “load.” How do you ask? The meter is nothing more than a pressure gauge. It was only indicating the pressure of the that part of the electrical system. Once a load was placed upon that circuit, there was either too much resistance or it was incapable of putting out enough current for the load. Just think of your garden hose. It does not deliver as much water as a fire hose. You have limited supply of water and small diameter hose. Both of these factors reduce the amount of water that can come to the end of the hose. The failure in the board did not allow sufficient current to pass through to power the circuit. It would be like having a dirty water filter in the plumbing system. You could have enough water pressure at the outlet of the plumbing system with no load, but once you open the faucet, the resistance offered by the dirty water filter would allow the pressure to drop to zero. Water would only trickle slowly out the faucet instead of with any force.

Electrical circuits are rated to do work at a certain voltage and current. If sufficient voltage and current is not available, then the work that the component is attempting to accomplish will not get done.

The lift pump circuit is the best illustration of this type of scenario. The OPS (oil pressure switch) closes the circuit and provides power to the lift pump. The OPS contacts are under rated and do not provide the best means of powering the lift pump. The OPS contacts can fail, and not provide sufficient current to the lift pump to cause it to pump. If you were to take your meter and check the lift pump circuit coming from the OPS, you would see voltage on your meter. However, once a load was attached to that circuit (the lift pump) the voltage would fall off and the pump would not work. Think of the OPS as the dirty water filter. It provides resistance to the flow of electricity.

I hope that this will help you understand a little bit better about that mysterious thing will call electricity… that thing that mechanically injected folks have not discovered the joy of yet…

Look here-> GM 6.5 Reference Material & FAQ's <-Look here

Diagnostic Links & Common Failures: Turbo System * Lift Pump/OPS * PMD * Stalling

°Please include your vehicle details in your signature line. Did you read the FAQ's and search before posting?
*Bus 1, rest in peace. Bone stock, now just bones. 4.10 rear 10/13 mpg, 182,000 miles with a set of MCI motor coach seats.
*Currently driving an "Canadian Camo" Snow White 2001 5.7L Chevy Express.
*Bus 2, 99 7.3L PSD.
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