Weakening Power Door Locks

Hey Guys,

Just bought a 99 suburban with 50k miles. Everything looks really clean, no damage history, smooth/quiet ride. The only thing that seems to be wrong with it is that the electric door locks sometimes aren't powerful enough to unlock the rear passenger doors. If I rapidly cycle the power locks several times, they just get weaker and weaker until on about the 5th cycle, nothing happens on any of the locks except for a pittiful sound.

The amount of force is the same on all locks when this is happening, so it's not the sylenoids(sp). It's like the relay or controller that feeds the sylenoids looses power with each cycle. Waiting a few moments, teh first try again is back to normal then tapers off again.

Electrically speaking, I don't see how this is possible.

I found the relay that carries the load for the locks and the contacts were pretty fouled up. I took them apart and polished them with fine sand paper to a nice shiny finish. Thought for sure that would be it, but it didn't seem to help at all. Maybe I'll replace it, just for the heck of it, and see what happens. It can't be more than $5.

I hear one other contact closure under the dash, but only when I unlock. I'll see if I can find it.

Anybody have any ideas???

It almost sounds as if you are drawing enough current from the battery that you run the battery down while using the power locks.  Then when the voltage is low enough you can't use the door locks.  If you wait a few moments the float charge on the battery will return and then you'll have enough voltage to use the door locks.

What is the battery's age/condition?  Does it happen when the car is on (i.e. alternator running the car)?  If the battery is good/new or it still happens when the car is running, then yeah, I'd say that is a strange problem indeed....