Parasitic Battery Draw | 1990 Ford F150
This 1990 Ford F-150 had a parasitic draw that was draining the battery. The first step I take in finding parasitic draws is hooking up my battery disconnect switch. I use this switch because I can close it for everything to power up and than withmy multimeter connected I can open it without ever breaking the connection. This is very important on a lot of vehicles. You can also close it so you can open doors without blowing your multimeter fuse.
When I connected the battery I heard the fuel pump turn on for 2 seconds. This tells me something is commanding the fuel pump to energize like the ecm. After the fuel pump turned off I checked the amperage draw which was 370ma (.37 amps). The most that a vehicle should draw is 50ma, and you would see that maybe on a Cadillac. On most vehicles, I see about 20ma to 0ma.
I figured on this 1990 Ford I would not have to wait 30 minutes for anything to power down so I started pulling fuses. None of the fuses killed my 370ma draw. Usually my next move would be to disconnect the 12v supply off the alternator and than start disconnecting each power feed one at a time either from the starter, battery, or whatever other power distribution connection there is.
On this Ford I went right to the relay center since I heard a relay energize when connecting the battery. What I found was that when connecting the battery the fuel pump relay would energize for 2 seconds and that is what normally happens when you turn the key to the run position. I than disconnected the EEC power relay and the amp draw dropped to 0ma.The connection looked clean so I tested for power at the connection. There was only one 12v supply which tells me the relay must be sticking. If there was two power supplies than I would go after a short to power. I checked the resistance on the load side of the relay that supplies power to the ECM, which should be open but in this case was 0 ohms. A new relay fixed the draw but I still cut the top off the old relay to find the problem. What I found was that the relay was rusted and stuck on.
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