Fuel pump power issues
Thread Starter
Intermediate
Joined: Aug 2011
Posts: 47
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Fuel pump power issues
I am working on my 1992 LT1 and have the following problem, hoping someone can get me over this hill.
There is power to the relay via the fuseable link on pin 87. It is hot all the time.
There is no power on 87A (ALDL pin G)
pin 30 is the route to fuel pump
pin 86 gets power for 2 seconds when I cycle the ignition switch then cuts off.
pin 85 is ground and tied to pin 86.
When I jump power to the pump it runs and puts out fuel (new walboro 255).
I am not getting power to the pump (or fuse) when I try to start this thing......I tested the relay by jumping pins 85/86 off the battery and it works fine. I also tried switching the relay next to it over to the FP relay spot......still having the issue. Is there something that should be keeping this relay open past those initial 2 seconds? Any suggestions on what to check next?
There is power to the relay via the fuseable link on pin 87. It is hot all the time.
There is no power on 87A (ALDL pin G)
pin 30 is the route to fuel pump
pin 86 gets power for 2 seconds when I cycle the ignition switch then cuts off.
pin 85 is ground and tied to pin 86.
When I jump power to the pump it runs and puts out fuel (new walboro 255).
I am not getting power to the pump (or fuse) when I try to start this thing......I tested the relay by jumping pins 85/86 off the battery and it works fine. I also tried switching the relay next to it over to the FP relay spot......still having the issue. Is there something that should be keeping this relay open past those initial 2 seconds? Any suggestions on what to check next?
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The ECM turns on the fuel pump relay (and therefore the fuel pump) for 2 seconds when the ignition is first turned on to prime the fuel rails for starting. It then turns off the fuel pump relay so the fuel pump won't run continuously if the ignition is turned on but the engine is not started. The ECM will again turn on the fuel pump relay when it sees DRPs (Distributor Reference Pulses) from the ignition module. The ignition module generates DRPs whenever the engine is rotating, whether cranking or running.
The oil pressure switch is a BACKUP for the fuel pump relay. It is not needed and it is an Internet myth that it "saves" the engine if oil pressure is lost.
This is the fuel pump circuit diagram for my '86. I don't think much changed over the years:
The relay pin numbers/letters changed in several years, so watch for that. The connections are still valid. The diagram in your FSM probably doesn't look like this. I spent a lot of time modifying this diagram to make it accurate.
The oil pressure switch is a BACKUP for the fuel pump relay. It is not needed and it is an Internet myth that it "saves" the engine if oil pressure is lost.
This is the fuel pump circuit diagram for my '86. I don't think much changed over the years:
The relay pin numbers/letters changed in several years, so watch for that. The connections are still valid. The diagram in your FSM probably doesn't look like this. I spent a lot of time modifying this diagram to make it accurate.
Last edited by Cliff Harris; Oct 24, 2015 at 11:59 PM.
Thread Starter
Intermediate
Joined: Aug 2011
Posts: 47
Likes: 0
The ECM turns on the fuel pump relay (and therefore the fuel pump) for 2 seconds when the ignition is first turned on to prime the fuel rails for starting. It then turns off the fuel pump relay so the fuel pump won't run continuously if the ignition is turned on but the engine is not started. The ECM will again turn on the fuel pump relay when it sees DRPs (Distributor Reference Pulses) from the ignition module. The ignition module generates DRPs whenever the engine is rotating, whether cranking or running.
The oil pressure switch is a BACKUP for the fuel pump relay. It is not needed and it is an Internet myth that it "saves" the engine if oil pressure is lost.
This is the fuel pump circuit diagram for my '86. I don't think much changed over the years:
The relay pin numbers/letters changed in several years, so watch for that. The connections are still valid. The diagram in your FSM probably doesn't look like this. I spent a lot of time modifying this diagram to make it accurate.
The oil pressure switch is a BACKUP for the fuel pump relay. It is not needed and it is an Internet myth that it "saves" the engine if oil pressure is lost.
This is the fuel pump circuit diagram for my '86. I don't think much changed over the years:
The relay pin numbers/letters changed in several years, so watch for that. The connections are still valid. The diagram in your FSM probably doesn't look like this. I spent a lot of time modifying this diagram to make it accurate.
I think the relay was just stuck, and needed a nice 650 CCA battery to kick it into gear.
So yea I ordered a new one of those, I am avoiding digging that out again as much as I can
