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When a relay turns into a buzzer, it is caught in a functional loop, much like a dog chasing its tail. The loop is this:
1. The control for the relay causes a small current to flow through its electromagnet.
2. The electromagnet attracts an iron clapper that causes two contacts to touch.
3. The two contacts create a path for larger current to flow to a large load, in this case your fuel pumps.
4. Some other resistance in the path to the relay now becomes too large to support the high current load, so the voltage available to energize the electromagnet falls below what is needed to hold the clapper down.
5. The clapper releases the contacts connecting the high current load, so the small current once again is available to the electromagnet. Go to #1.
The high resistance causing this tail chasing is usually in the red wire circuit for LH fuel relays, not a ground, and not the ECU, but this "usually" is not without exception. I looked at one buzzing LH relay to find it full of water.
The easy way I would check this, is by using a test light at the relay's terminal 30 - the light should stay (reasonably) bright during cranking. I take it one step further in ease - by checking that voltage (with a test light) at the orange AMM wire under the hood. Peel back the boot and probe where the orange wire goes in the AMM. This works for all LH 240's, LH1.0 (1982)through LH3.1.
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Art Benstein near Baltimore
Ever consider what our dogs must think of us? I mean, here we come back from a grocery store with the most amazing haul -- chicken, pork, half a cow. They must think we're the greatest hunters on earth! -Anne Tyler
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