Fuel pump sound influenced by hazard lights

GTamas

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Just noticed yesterday that the fuel pump sound changes when the hazard lights are flashing. Is that normal? If it's a problem, what could be the cause?
 

cobrajeff96

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How old is pump and how long since fuel filter change? It kind of seems like the extra electrical load on the system (and hazard lamps are not that big of a load) is causing a higher than normal voltage drop and the pump is only barely keeping up with it. So it could actually be the alternator not supplying enough amperage to the entire system at idle. Stock alternators were never that good on 79-04 Mustangs to begin with. For example, you can roll up the windows for just a second when they are already fully up and you should see all lights get weak and dim. That's the voltage drop on the system.
 
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How old is pump and how long since fuel filter change? It kind of seems like the extra electrical load on the system (and hazard lamps are not that big of a load) is causing a higher than normal voltage drop and the pump is only barely keeping up with it. So it could actually be the alternator not supplying enough amperage to the entire system at idle. Stock alternators were never that good on 79-04 Mustangs to begin with. For example, you can roll up the windows for just a second when they are already fully up and you should see all lights get weak and dim. That's the voltage drop on the system.
Fuel pump, alternator are 30 years old. :) The filter was replaced last year. So I guess I should measure the alternator output?
 

cobrajeff96

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Well, the GT alternators of that year car I think are about 110 A, the Cobras were 130 A.

It might be nothing if the car still performs as it should. But 30 years on the original pump and alternator is a good run, but a long run. Might be time to test and possibly replace.
 

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I would say the most important thing is if it effects the pressure. If you don't have a fuel pressure gauge you can add one easily to the fuel rail on the schrader valve and see. I would not be surprised if a lot of cars did this but not enough people have had the hazard lights on and then listened to the pump.
 
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Checked the voktage today on the battery terminals:
12.8V with engine off
Dropped to 12.23V with first key position
Up to 14.78V with engine running
Turning hazard lights on, I did not see fluctuation.
Turned all lights, radio (aftermarket amp), Max AC on it dropped to 14.69V but steady.

Seems like it is fine?
20240526_130512.jpg
 

cobrajeff96

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Seems like a high alternator output voltage. Not insanely high, but normally it should be in the neighborhood of 13.8 - 14.2 (+/- 0.2v). In general, the hotter the output the less lifespan you get out of things like light bulbs, pumps, etc. The voltage regulator might be suspect.
 

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