Hey everyone,

I reworked an '88 GT harness this afternoon that had an aftermarket security system wired into it (unprofessionally I might add.). I removed about 40 feet of useless wire, cleaned up the C500, and rewired the coil +12v feed and manual starter feed. As I was finishing, I found some other non stock wires protruding from the wire loom on the drivers side of the bay. After about an hour, I had removed a fuel pump relay hot wire, and returned the relay to stock function. The fuel pump still refused to run behind the computer, so I picked over everything to find out why. Here's what I've found:

Fuel Pump Relay harness
Terminal A - 12V signal from computer with key on. Goes to .4 volts when grounded to pin C (this is odd).
Terminal B - Checked continuity to pin G on the ALDL and checks out fine. I can jumper the pump from pin G, so I know the circuitry behind G is fine.
Terminal C - Show's a ground with a bit of resistance (15 m-ohms)
Terminal D - A healthy 12 volts from the battery after I replaced a blown fuel pump fuse.

I have three relays to work with, all were checked out as working with a DC power supply. I've replaced all of the wire going to the relay up to the bulkhead. Continuity checks out on all four wires. I can jumper pins B and D at the relay to start the pump manually. It would seem the issue is with the computer request on terminal A. I did a short bit of reading about some computers needing replacement due to lack of fuel pump request, but the computer is supplying 12v until it is shorted to C. I also tested the pump with an 86 GT computer I had sitting on a shelf.. nothing. I wouldn't think the oil sending unit is messing with the power feed because I'm seeing 12+V at D, which is more than enough to jumper the pump manually. And then there's the fact that the previous owner hot wired the pump to begin with.

I rigged a fused, key on, 12v to the pump so that the owner could make it home (sun went down on us). I'm sort of at my wits end on how to go about getting the computer to command fuel when everything seems to be functioning as it's supposed to. Does anyone have any ideas? At this point it never hurts to ask. Car is equipped with a Hypertech Street Runner prom.

Emc209i,

you definitely have the circuit correct. The voltage to pin A1 of the ECM is applied once the ignition switch is on. Not sure of the ECM internal arrangement. A couple of questions, more than likely stating the obvious:

1. Have you tested the 12V signal from ECM pin A1 on another relay? Not necessarily in the circuit but separately. In doing so will ensure you are getting voltage from the ECM. If the test relay operates then you have issues with the wiring beyond the relay connector.
2. Are you certain all grounds in in tact?
3. You mentioned you had to replace a blown fuel pump fuse - 10A. Could it have blown again? Re-check.

My guess, the chances of having two ECMs bad is pretty slim. Other comments; the Oil Pressure switch provides a secondary path to power the fuel pump in the event the fuel pump relay fails for what ever reason. In theory, the fuel pump should be energized when a predetermined oil pressure is reached. You did mention that you checked continuity from the ALDL terminal "G" to the "B" terminal of the relay. But did you actually apply power?

My 2cents......

quote Originally posted by Emc209i: Terminal A - 12V signal from computer with key on. Goes to .4 volts when grounded to pin C (this is odd).

How and where did you measure this voltage drop... at pin A or at pin C once the key was on? Because there will be a voltage drop as measured at pin C due to the resistance through the relay coil. The other thing you should keep in mind is that you must never measure the resistance of pin A of the relay connector to ground. This will harm the ECM's solid state circuitry.

Some security systems not only cut off the fuel pump but allso power to the ignition.Have you tried cranking the motor?

All appreciated responses, and pluses all around. Let me address each tip..

quote Originally posted by Cajun: Emc209i, you definitely have the circuit correct. The voltage to pin A1 of the ECM is applied once the ignition switch is on. Not sure of the ECM internal arrangement. A couple of questions, more than likely stating the obvious: 1. Have you tested the 12V signal from ECM pin A1 on another relay? Not necessarily in the circuit but separately. In doing so will ensure you are getting voltage from the ECM. If the test relay operates then you have issues with the wiring beyond the relay connector. Yes. I have tested the relay and verafied that it works correctly. I have not tried metering 12V on the B side of the relay were I to manually trip the relay, I'll have to try that. 2. Are you certain all grounds in in tact?Yes, as stated above C checks out. 3. You mentioned you had to replace a blown fuel pump fuse - 10A. Could it have blown again? Re-check. I checked the fuse every few minutes so that I was sure that wasn't the issue. My guess, the chances of having two ECMs bad is pretty slim. Other comments; the Oil Pressure switch provides a secondary path to power the fuel pump in the event the fuel pump relay fails for what ever reason. In theory, the fuel pump should be energized when a predetermined oil pressure is reached. You did mention that you checked continuity from the ALDL terminal "G" to the "B" terminal of the relay. But did you actually apply power? Yes, I primed the pump from G several times. Applying 12v power from D to B also powers the pump, which leads me to believe the issue must be with A curcuit My 2cents...... Thank you

quote Originally posted by Cajun: Emc209i, After giving it some thoughtk, you did mention that you removed the security system installed by the PO. As you may be aware, some auto security systems have a function that disables the fuel system. In the removal of the wiring of the system you may have inadvertently removed some of the wiring for the fuel system. Yes I'm aware of that, and I removed their poor attempt of a bypass. All of the wiring in now is the same as it was stock, which is why this is puzzling. The only wiring I haven't checked is from the 3 pin disconnect on the firewall that runs under the car and to the tank, but there's not much they could have changed there; power, ground, sender (all seem to work as they should)

quote Originally posted by Bloozberry: How and where did you measure this voltage drop... at pin A or at pin C once the key was on? Because there will be a voltage drop as measured at pin C due to the resistance through the relay coil. The other thing you should keep in mind is that you must never measure the resistance of pin A of the relay connector to ground. This will harm the ECM's solid state circuitry. I check pin A when the pigtail was unplugged from the relay. When shorting A to C, the voltage at A would fall off - is this normal to see a voltage drop at A? I'm worried that whoever installed the rats nest of wire already screwed the solid state when they were ripping things apart. Any other ideas.

quote by James Bond 007: Some security systems not only cut off the fuel pump but allso power to the ignition.Have you tried cranking the motor? I'm aware of this, but this is not the case. I already removed all of that wiring to the coil, and bypassed the pump so that the owner could drive home last night. He's been driving around all day and is back in my driveway. I've got everything working in tip top shape, its only the fuel pump relay isn't tripping, and I'm pretty sure the computers to blame. Need more ideas.

[This message has been edited by Emc209i (edited 11-07-2009).]

You shouldn't have jumped pins A to C on the harness with the key in the ON position since this would have resulted in a direct short circuit of the ECM fuel pump relay controller circuit. That will result in blowing one of the fuses to the ECM (not the fuel pump fuse). The wiring diagrams in the '88 service manual are a bit vague when it comes to power routing internal to the ECM, but I believe you would have blown the ECM Ign Fuse if you jumped the two pins. In any case, check all your fuses because one of them will surely have blown. Hopefully that's the case and not that you've caused internal ECM damage.

I'll have to check the ECM fuse, but the owner drove off about an hour ago, and pin C is still seeing 12v. I don't think he would have gotten far without supplying the computer with power. I'm going to try a manual energize of wires A and C,with the relay plugged in Monday (with the ECM unplugged). If the fuel pump starts humming, I'll have isolated the problem to the control circuit. If not, I'll look into both the control and load sides.

Anything I could be overlooking?

[This message has been edited by Emc209i (edited 11-07-2009).]

I once had a problem with the A/C relay, where the contacts in the electrical plug were not making good contact with the relay. One of the spade connectors had loosened up.

It wouldn't hurt to try a continuity check between pins A and C in the relay plug (with the relay plugged in and engine off), to make sure power can get to the relay.

[This message has been edited by Blacktree (edited 11-08-2009).]

Follow up;

I finally finalized the 3800 swap I was working on, so I pulled this car back in to test out some of the ideas above, and to energize the control circuit with the relay plugged in. I unplugged the computer and applied 12V to wires running to A and C on the fuel pump relay. The pump fired up. So I went further "back" to pins A1 and D1 at the computer plugs; pump fired up. This made me mad.

We pulled the 86 GT computer back off the shelf and turned the key on; pump fired up.

The owner explained a bit more about what he knew about the cars past with the previous owner. After the hypertech chip was installed, the car was apparently plugged up to a computer for additional access to the prom. It's my guess the fuel pump request has been removed from the prom... or... the solid state was damaged when the security system was installed.. or I somehow damaged it when testing around (but the pump wasn't working correctly before it came to me, which is why the owner wanted me to fix it}. I explained the following options to the owner: Purchase the correct chip flashing hardware, mail it to Ryan, or stick with the 86 PCM and a rev limiter.

I'll put up some pictures soon. Thanks to everyone who helped out with ideas!

[This message has been edited by Emc209i (edited 11-14-2009).]