fuel pump problem
Thread Starter
8th Gear
Joined: Dec 2008
Posts: 8
Likes: 0
fuel pump problem
Hi Guys, Having a problem getting power to the fuel pump on my 85 vette. I know the pump is good as I removed it from the car and powered it with a battery charger. But before I removed it I connected the battery with a jumper to the alcl port g and had heard it click. So I left the jumper connected and car still didn't start. Disconnected the wire from the battery then touched the battery terminal again and had no power to the pump. I swapped the f/p relay and o/d relay and still no power, also checked the f/p fuse which was good. Checked fusible links all good. Is it possible that I damaged the ecm when I tried to start the car with power connected to the alcl? Or could it be bad relays or the oil pressure switch? The car was sitting for a year. Any Ideas are welcome Thanks.
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Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
Pin G of the ALDL port goes directly to the fuel pump relay and nowhere else, so there is no chance of damaging the ECM by doing this. The relay has to be OFF for this to work.
When you first turn on the ignition the ECM turns on the fuel pump relay (and thus the fuel pump) for 2 seconds to pressurize the fuel rails for starting. It then shuts them off so the fuel pump won't run continuously if you turn on the ignition but don't start the engine. Does this happen?
The ECM turns on the fuel pump relay when it sees DRPs (Distributor Reference Pulses) from the ignition module in the distributor. The ignition module puts out DRPs any time the engine is rotating, which includes cranking and running.
If the fuel pump relay fails for some reason the oil pressure switch acts as a backup and turns on the fuel pump when oil pressure reaches 4 PSI. You should be able to get 4 PSI while cranking, though it will take several seconds. I have heard of a case where BOTH the relay AND the oil pressure switch were bad, but that is pretty unlikely.
This diagram is for my '86 so the ECM pin numbers will be different, but the rest of the connections should be the same (note that this diagram has been modified by me because the one in the shop manual has a bunch of errors and omissions):
When you first turn on the ignition the ECM turns on the fuel pump relay (and thus the fuel pump) for 2 seconds to pressurize the fuel rails for starting. It then shuts them off so the fuel pump won't run continuously if you turn on the ignition but don't start the engine. Does this happen?
The ECM turns on the fuel pump relay when it sees DRPs (Distributor Reference Pulses) from the ignition module in the distributor. The ignition module puts out DRPs any time the engine is rotating, which includes cranking and running.
If the fuel pump relay fails for some reason the oil pressure switch acts as a backup and turns on the fuel pump when oil pressure reaches 4 PSI. You should be able to get 4 PSI while cranking, though it will take several seconds. I have heard of a case where BOTH the relay AND the oil pressure switch were bad, but that is pretty unlikely.
This diagram is for my '86 so the ECM pin numbers will be different, but the rest of the connections should be the same (note that this diagram has been modified by me because the one in the shop manual has a bunch of errors and omissions):
Last edited by Cliff Harris; Dec 29, 2014 at 02:20 AM.
Safety Car
Joined: Nov 2008
Posts: 4,328
Likes: 15
From: Jacksonville FL
I would look carefully at the relay connector. Yesterday I found that 2 wires were almost disconnected and bare of insulation. I thought my fuel pump was going south but I opened up the crimp connection and soldered the wires back to the connector. So far it seems to have solved the issue for me.
