Fuel Pump Circuit Checklist

[pbanders]

Recently, I responded to a posting from 24flys who was having fuel pump problems. I gave him a checklist to go through to find the problem. I've had some time to review it and I have corrected some errors and added some additional checks. Here it is, if you find any errors or omissions, please let me know and I'll correct them.

1. Check power supply to the relay board

- Pull the plug on the relay board that is next to the firewall and connect a digital multimeter (DMM) to pin 12 on the plug. You should read +12V, if so, disconnect the DMM and put the plug back into the socket. Zero volts on pin 12? You have a wiring fault to the battery.
- Remove relay 74 and use a DMM to probe the relay socket on the board for relay pin 30. You should read +12V continuously, if so, wrap a test wire around pin 87 and plug the relay back in. Zero volts? You have a relay board trace fault.
- Connect a DMM to the test wire from pin 87 of relay 74. With the ignition key in the "off" position, you should read zero volts. Read +12V? You have a stuck relay. Turn the ignition key to "on". You should read +12V, if so, remove the test wire and go to step 2 below. Zero volts at pin 87? Turn the key to the "off" position, remove the relay, turn the key to the "on" position, and probe the relay socket for relay pin 86, you should read +12V. If you read +12V, probe the relay socket for relay pin 85, you should read ground. Ground present? You have a faulty relay. No ground at pin 85? Turn the key off, remove the plug on the relay board next to the firewall and check pin 10 on the plug . Ground present? Then you have a relay board trace fault. No ground at pin 10? You have an open ground wire, check the ground connection point on the firewall in front of the relay board. Zero volts on pin 86? Turn the key "off", remove the plug on the relay board next to the firewall, turn the key back to "on", and check pin 8 on the plug. If you read +12V there, you have a relay board trace fault. Zero volts at pin 8? You either have a wiring fault to the fuse box (fuse #8) or you have a fault in your ignition switch - check to see if you can turn on other key-controlled accessories.

2. Check the power supply and ground to the ECU, and the ECU fuel pump control connection

- Use a DMM to probe pin I on the small white plug on the back left side of the relay board. Turn the key to "on", you should read +12V. Zero volts? You have a relay board trace fault. If you read +12V on pin I, turn the key to "off", remove the ECU from the bracket and remove the wiring harness plug connector from the ECU. Turn the key back to the "on" position and check pins 16 and 24 on the ECU, you should read +12V, if so, turn the key to "off". Zero volts? You have a wiring harness fault.

- The ECU is grounded through the wiring harness. In the middle of the harness, centered over the rear (near the engine bay rear firewall) of the engine case, are three pairs of white wires breaking out of the harness which are attached to the engine case via a three-way male spade connector. Remove the air box assembly to check the condition of these connectors, making sure there is a good clean ground (shiny metal). After checking the condition of the ground connections, check pin 11 of the ECU plug for ground. No ground? You have a wiring harness fault.

- Connect a DMM to pin III on the small white plug on the back left side of the relay board, and connect the other side of the DMM to pin 19 on the ECU plug and look for continuity. No continuity? You have a wiring harness fault. Good continuity? Leave the DMM probe on pin III, plug the ECU socket back into the ECU and re-install the ECU on the bracket and re-install the air box assembly.

3. Check the ECU fuel pump control operation

- Remove the fuel pump relay #74 from the relay board. Turn the key to the "on" position. You should see the pin the DMM is connected to go to ground for 1.5 seconds, if so, replace the relay and go to section 4 below. No ground for 1.5 seconds? You have a faulty ECU fuel pump control circuit.

4. Check the power supply and ECU control of the fuel pump.

- Pull the plug on the relay board that is next to the firewall and connect a DMM to pin 14 on the plug. You should read +12V, if so, disconnect the DMM and put the plug back into the socket. Zero volts on pin 14? You have a wiring fault to the battery.

- Check the condition of fuse 91 (red, 25 amp). If blown, replace it. If it blows when you turn the key to "on", you have a short either in your Auxiliary Air Regulator, relay board traces, or wiring to the fuel pump.

- Remove relay 75 and use a DMM to probe the relay socket on the board for relay pin 30. You should read +12V, if so, wrap a test wire around pin 87 and plug the relay back in. Zero volts? You have a relay board trace fault.

- Connect a DMM to the test wire from pin 87 on relay 75. Turn the key to the "on" position. You should see +12V for about 1.5 seconds, if so, turn the key to the "off" position and go to the next step below. Zero volts? Turn the key to "off", remove the relay, and turn the key to the "on" position. Probe the relay socket on the board for relay pin 86, you should read +12V. Zero volts? You have a relay board trace fault. Turn the key to "off" and probe the relay socket on the board for relay pin 85. Turn the key to "on", you should see ground for 1.5 seconds, if so, you have a faulty relay. No ground? You have a relay board trace fault.

- Pull the plug on the relay board that is next to the firewall and connect a DMM to socket for pin 13 on the relay board. Turn the key to the "on" position. You should see +12V for about 1.5 seconds, if so, turn the key to "off" and reconnect the plug and go to the next step below. Zero volts? You have a relay board trace fault.

- Jack up the car on the passenger side and access the fuel pump. Disconnect the electrical plug to the pump. There are two female connectors in the plug, the one with the brown wire is ground, and the one with the red/black wire is the power supply from the relay plate. Check the ground connection. No ground? You have a wiring fault, the ground point is located in front of the relay board on the firewall, check the connections there. Good ground? Probe the power supply connector in the plug with your DMM, and have an assistant turn the key to the "on" position. You should see +12V for about 1.5 seconds. Zero volts? You have a wiring fault. If you do see +12V, turn the key to the "off" position, put the plug back into the fuel pump and have your assistant turn the key to the "on" position. If the fuel pump does not run for 1.5 seconds, you have a faulty fuel pump.


Brad Anders


[This message has been edited by pbanders (edited 06-14-2001).]

[24flys]

brad,

thanks for the update, I'm still trying to get it running. Hopefully, I'll be able to tell you its running soon, otherwise, its going into the river! just kidding, I wouldn't do that to a 914.


scott

[greenwood]

hey Wayne
you should make this a tech article so it doesn't get buried in the BB archives...just a thought.

jeremy

[pbanders]

I'll send it on to Wayne later, good idea. I would like for it to stay up here for a bit to get feedback on errors and omissions.

thanks,

Brad Anders

[DDS]

Hi Brad:
Thanks for taking the time to put this together.

Dave

[pbanders]

OK, I know I'm psycho for doing this, but I'm also converting this document into a set of flow charts. They're much easier to follow and understand. I should have them done in a couple of days and I'll post them to my web site. Visio makes the job pretty easy.

Brad Anders

[pbanders]

OK, I got motivated and got them done and posted. Here's the URL:

http://members.rennlist.com/pbanders/FPChecklist.htm

Brad Anders

[24flys]

brad,

I found some problems. everything checks out up to where I tested pin 87 on relay 75(fuel pump relay)( 3rd from the last paragraph). I didn't see 12v at all. I searched pin 86 for 12v with the key on and had zero volts. I do have ground for 1.5 seconds on pin 85 once the key is turned on. So do you think I have a faulty relay or a relay board trace fault??

I'm not sure how to fix this. I do have the bottom of the relay board exposed and all the connections looked good and I tested several for continuity.

scott

[pbanders]

Originally posted by 24flys:
brad,

I found some problems. everything checks out up to where I tested pin 87 on relay 75(fuel pump relay)( 3rd from the last paragraph). I didn't see 12v at all. I searched pin 86 for 12v with the key on and had zero volts. I do have ground for 1.5 seconds on pin 85 once the key is turned on. So do you think I have a faulty relay or a relay board trace fault??

I'm not sure how to fix this. I do have the bottom of the relay board exposed and all the connections looked good and I tested several for continuity.

scott

The +12V that is sourced to the socket for pin 86 on relay 75 is the +12V that comes from pin 87 of relay 74 and provides power to the ECU. If your ECU is getting +12V (sounds like it is), then there is a fault in the relay board trace between the socket for pin 86 and the trace that goes between pin 87 of relay 74 and the pin for terminal I of the white plug in the lower left corner of the relay board. To verify that the trace is bad, pull relays 74 and 75 and the white plug on the lower left corner. Check the continuity between pin 86 on relay 75 and pin 87 of relay 74 - you should get continuity. If not, your trace has a fault in it. I haven't done it, but I've heard that you can solder the crimped connections on the board if they're faulty.

If the relay were faulty you'd still see +12V on pin 86, but nothing on pin 87 when the ECU takes 85 low for 1.5 seconds after turning the key to "on".


Brad Anders

[24flys]

they really need to make these cars easier to understand. I'm finally back to solving my problem(or at least trying to). brad, I checked for continuity between pin 86 on relay 74, pin 87 on relay 75 and terminal I. i have continuity. Are relays 75 and 74 interchangable. I have a red fuel pump relay which may not be the right one?? I'm not sure what's going on with this car(as you can tell!). I'll have fresh paint on it in 2 weeks but it will be next summer before I get it running. I guess I'll look good towing it around!!! let me know if you have any more advice! thanks


scott

[pbanders]

Originally posted by 24flys:
they really need to make these cars easier to understand. I'm finally back to solving my problem(or at least trying to). brad, I checked for continuity between pin 86 on relay 74, pin 87 on relay 75 and terminal I. i have continuity. Are relays 75 and 74 interchangable. I have a red fuel pump relay which may not be the right one?? I'm not sure what's going on with this car(as you can tell!). I'll have fresh paint on it in 2 weeks but it will be next summer before I get it running. I guess I'll look good towing it around!!! let me know if you have any more advice! thanks


scott

All of the black round relays are interchangable. There is also a silver metal can relay that is nearly the same (may be a 911 part). The only red one that I'm aware of is the door buzzer (has a red plastic bottom). I'd suggest that you try switching out the relay. You can use the relays that drive your headlights (under the black plastic covers next to the headlight motors) if you don't have any spares to test with.

Brad Anders

[Stuart Lehr]

thanks for the checklist Brad- now my upcoming weekend will be a little busier- that is when I'm not watching the F1 and CART races!