Power to the fuel pump - Ghost in the machine
12-18-2010, 07:24 PM
#1
Pro
Thread Starter
Power to the fuel pump - Ghost in the machine
OK, Got a weird one here and we're at our wits end.
So today was the day we finally decided to start the car. Exciting as it hasn't run for months.
When we put the key to the acc or on position, the alarm horn sounded once. I evidently have alarm issues. Not a huge surprise as doors and hatch are still off the car, center console switches aren't hooked up, etc, etc, etc. I'll figure out the alarm once we have her 100% back together.
I bypassed the alarm at the dme as per Clarks Garage. Seemed to work as I now have spark.
Try to start the car and no fuel. Turns out the fuel pump is not running. Check the fuse and everything else. Fuel pump won't run. We put 12v directly to the fuel pump from the battery and it kicks on. Turn the key and the car fires right up.
Here's where it get's weird.
My meter shows 12.2v at the battery. With the key in the on position, I have 0v at the pump. I understand that the fuel pump in these cars only runs at crank and when running.
At crank, I have 11.86v at the pump. At idle, I have 13.6v at the pump.
But, no matter what I do, the fuel pump does not run unless I jump it to the battery. It will not run at crank even though I can see the volts at the plug for the pump.
I cannot see why the pump will run from the 12v at the battery but will not run from the 12v that is coming from the harness.
Am I missing something?
So today was the day we finally decided to start the car. Exciting as it hasn't run for months.
When we put the key to the acc or on position, the alarm horn sounded once. I evidently have alarm issues. Not a huge surprise as doors and hatch are still off the car, center console switches aren't hooked up, etc, etc, etc. I'll figure out the alarm once we have her 100% back together.
I bypassed the alarm at the dme as per Clarks Garage. Seemed to work as I now have spark.
Try to start the car and no fuel. Turns out the fuel pump is not running. Check the fuse and everything else. Fuel pump won't run. We put 12v directly to the fuel pump from the battery and it kicks on. Turn the key and the car fires right up.
Here's where it get's weird.
My meter shows 12.2v at the battery. With the key in the on position, I have 0v at the pump. I understand that the fuel pump in these cars only runs at crank and when running.
At crank, I have 11.86v at the pump. At idle, I have 13.6v at the pump.
But, no matter what I do, the fuel pump does not run unless I jump it to the battery. It will not run at crank even though I can see the volts at the plug for the pump.
I cannot see why the pump will run from the 12v at the battery but will not run from the 12v that is coming from the harness.
Am I missing something?
12-18-2010, 08:07 PM
#2
Pro
Thread Starter
Updating my own thread my confusion grows.
I am assuming that the two pins in the fuel pump connector represent pos/neg.
So, to test the volts at the pump we run two jumpers to the pins at the harness side of the fule pump connector when the car is idling (fuel pump wired straight to battery). We connect those two jumps to a light bulb and nothing. It appears our 13v won't run a bulb. BUT if we take power from one pin on the fuel pump plug connector and ground to the frame, the bulb lights.
So now we jump one wire from the harness side of the connector to its opposite on the pump side of the connector (where it would normally plug in). We then jump the other pin on the pump side of the connector to the frame and the car starts and runs.
Where's a guru when you need one?
I am assuming that the two pins in the fuel pump connector represent pos/neg.
So, to test the volts at the pump we run two jumpers to the pins at the harness side of the fule pump connector when the car is idling (fuel pump wired straight to battery). We connect those two jumps to a light bulb and nothing. It appears our 13v won't run a bulb. BUT if we take power from one pin on the fuel pump plug connector and ground to the frame, the bulb lights.
So now we jump one wire from the harness side of the connector to its opposite on the pump side of the connector (where it would normally plug in). We then jump the other pin on the pump side of the connector to the frame and the car starts and runs.
Where's a guru when you need one?
12-18-2010, 08:29 PM
#3
Rennlist Member
Join Date: Jul 2001
Location: Brisbane, Australia (Formerly: Sunnyvale, CA)
Posts: 2,120
Likes: 0
Received 3 Likes
on
3 Posts
Power to your fuel pump is controlled by the DME via relay G5. For safety, the DME will cut power to the relay (and hence switch off your fuel pump) if a number of conditions aren't met. To test, do the the following:
Jumper two wires from terminal 30 on relay G5 to terminals 87 and 87b on relay G5. This basically bypasses the DME control of the relay, and your fuel pump will be getting power as long as your ignition is in the "on" position, whether the car is riunning or not. (Obviously, don't leave your car like this for any length of time with the engine not running).
If this solves your problem, then there are two possibilities: 1) The DME relay itself is shot, and needs replacing. b) (more likely in my opinion) the DME is not seeing all the conditions it needs to keep power going to the fuel pump -- quite likely alarm circuit related.
But first things first -- try the jumpering test and see where you stand.
Jumper two wires from terminal 30 on relay G5 to terminals 87 and 87b on relay G5. This basically bypasses the DME control of the relay, and your fuel pump will be getting power as long as your ignition is in the "on" position, whether the car is riunning or not. (Obviously, don't leave your car like this for any length of time with the engine not running).
If this solves your problem, then there are two possibilities: 1) The DME relay itself is shot, and needs replacing. b) (more likely in my opinion) the DME is not seeing all the conditions it needs to keep power going to the fuel pump -- quite likely alarm circuit related.
But first things first -- try the jumpering test and see where you stand.
12-18-2010, 08:35 PM
#4
Rennlist Member
Join Date: Jul 2001
Location: Brisbane, Australia (Formerly: Sunnyvale, CA)
Posts: 2,120
Likes: 0
Received 3 Likes
on
3 Posts
BTW, I've actually soldered in a bypass switch to this relay, so that I don't get stranded if the relay fails, which is not uncommon. You can also use spade connectors to replace the relay to do the jumpering -- some people carry a set in their glove box. Or just a spare relay.
12-20-2010, 04:40 PM
#5
Pro
Thread Starter
Looks like we found it. Fuel pump appears top ground through the harness near the taillights which were not installed. Grounded at that point and the ghost appears to be gone. Car is starting and running. Alarm issues are still there but assuming it is related to doors, hatch, etc not being wired up. Thanks for the help.
12-20-2010, 05:08 PM
#6
Pro
Join Date: Sep 2010
Location: Upstate, South Carolina
Posts: 546
Likes: 0
Received 0 Likes
on
0 Posts
FWIW, the reason why you read the voltage there and got nothing in response was due to potential. Voltage will read in potential even though a circuit cannot complete. I just stumbled upon this thread and I was going to suggest you check the ground...
12-20-2010, 11:44 PM
#7
Advanced
Join Date: Oct 2010
Location: Nova Scotia Canada
Posts: 61
Likes: 0
Received 0 Likes
on
0 Posts
Check the ground wire in the back of the hatch. It is the ground for the fuel pump. If its off or a bad connection the pump will not run and will show marginal voltage if any at all.
