9 replies to this topic

[Keith Novak]

I'm trying to debug an issue with my 99 where the fuel pump runs continuously, when the key is in the acc position w/o a jumper in the diagnostics box.  It's burned out a fuel pump and that gets both expensive and frustrating.

 

I'm sorting my way through the electrical system and I realize this car has once been rewired, and then changed back by someone else, so it's a bit of a murder mystery story. 

 

I've traced the power for the fuel pump back to the FP relay behind the dash.  Then I traced the green wire from the relay over to the large flat RHS connector, where it goes into the dash.  The wire diagram says power plant module, but I find no actual module.  It just plugs into the dash. 

 

Then I traced it from the dash to plug, to a connector that seems to be for the passenger airbag kill switch next to the cigarette lighter, bottom center of the console.  That switch, and the lighter are missing from my car.  Does missing that switch screw up the fuel pump circuit somehow so it runs in accessory as well as the start/run positions?  Why on earth would airbags be related to the FP relay?

 

As fascinating as it is sorting through wire diagrams circuit by circuit, it's getting a bit irritating.  Any clues would be greatly appreciated!

[tom1977]

Airbag modules can be required to kill the fuel pump in collision activation, similar to an old ford inertia switch concept. Would have to look at full schematic to communicate a fix better. Can call in an evening this week and see if this can be figured out.

[davew]

You need to start by understanding how a relay works. The idea is to use low power wires to control a high powered item.

 

You generally have 4 wires, Main power in, power out (goes to the pump), switched power and ground (which may also be switched)

 

The main power in is what powers the pump. Should be hot only in RUN position. If power is there when it should not be, you have an extra power source wired in between the switch and relay.

Power out goes to the pump, should only have power when the relay turns on. Quick diagnosis, unplug the relay, does the pump stay on? If it does you have an extra power source wired into the circuit from the relay to the pump.

Switched power should be the smaller wire. It should have power in the run position.

The ground wire is controled by the ecu in this case. If the ecu sees the correct peramiters, it will ground that wire and turn the relay on, thus powering the ecu The main peramiters being key in run or start and airflow in the AFM. There are others but these are the main ones to be concerned about. I can not see it being an airbag module, since most SM,s don't have one anymore.

 

By understanding the circuitry and doing simple voltage checks with certain components plugged in and unplugged, you should be able to determine what part of the circuit has the fault. Then go to the wire diagram and start thinking about where someone may have modified it.

 

Diagnose, do not guess.

 

dave

[Keith Novak]

Thanks Dave.  I've been struggling with the wire diagrams a bit since they don't show the FP circuit as it's own system, but there is a little bit of it on different schematics like the cooling fan circuit.  I'm now wondering if I have the right diagrams since all the references to the Powertrain Control Module (ECU) are behind the RHS of the dash, but it's on the left near the fuse block.   I also found a number of wires had been cut and respliced, so I have the dash out trying to figure out what was altered.

 

I did find that the FP wire in the diagnostics box is the light green wire and grounded for "on" so I was trying to trace back where the normal "on" signal comes from so with the dash out, I'm trying to track down the circuits by continuity.  I may have found the wrong light green wire at the airbag disarm.  I've found some tests in the FSM for both the relay (not sure I understand the open/short circuit tests though) and the ignition switch.  My thinking is perhaps the relay is faulty or the ign switch is faulty.  I know that the ignition power had been cut into and spliced back together so rather than another power source supplying the FP, it's not switching on/off correctly.

[Tom Hampton]

Keith-
 
I don't know what diagrams you have, but I have the 1995 FSM with wiring diagrams on my website:

http://www.tomhampton.info/downloads/

 

There is a Fuel Control System schematic on page 1137 of the PDF, such as it is. 

[Keith Novak]

Thanks Tom.  This is actually for my 99 that I bought last year and the FSMs I've found on-line don't have the diagrams.  I've found one from Mitchel Repair online, however that has some oddities like computer on the wrong side.  The 95 schematics do help though because at least it shows me all the parts of the circuit on one page, so unless they got very creative on the 99, there shouldn't be weird interfaces to other systems.  I now figure either the relay is bad and staying closed, the ECU is keeping it closed (possibly due to an ign switch issue which I can test), or there's power from somewhere else and that's the easy wire to trace.

 

  Since I didn't build this car, it's enough different that I'm going through the process of identifying what everything is again.  I find plugs and boxes in the car, and then have to go figure out what they are in the diagrams or grainy scanned photos in a pdf file.  Then I find stuff on the diagrams and I have to go find where it is in the car.  The good news is, I'll know this thing top to bottom by the time I put it back together. 

[Tom Hampton]

Ah...got it. 

 

I always like to start at the beginning.  In this case the IGN switch.  What volts are actually coming out of the switch.  then work my way down from there. 

 

Assuming a similar design to the '95....

 

The ign switch drives the coil to the MAIN-relay, which provides power to the FP-relay.  So, in order for your FP to be "ON" both the main relay and the FP relay need to be in the closed state....either stuck that way or energized. This is only supposed to be true in the ON position, not ACC. 

 

Two failed relays seems unlikely...it happens, don't get me wrong.  But, if your main relay were failed, then your ECU would get energized, and then energize the FP relay.  you can test that by pulling the main relay (like Dave suggested for the FP relay).  Alternatively, the +B terminal in the datalink connection is connected to the main-relay switched-output. 

 

If your FP runs with both the Main and FP relay's removed...then light it on fire. 

 

Good luck!

[Bench Racer]

1999 Miata Start at page 8 of 32, Engine Performance Circuits of which there are 3 pages. Top of page 8 shows fuel pump, fuel pump relay, main fuse block/main relay and wire colors. If these diagrams are better, it's a place to chase stuff around.

 

http://www.yorbalind...iring/99sys.pdf

 

I chased several electrical issues for my 1.6 through the Yorba linda electrical diagram. Worked well for me and I'm no EE.

[Keith Novak]

That's the one that had me confused, Bench!  When you trace the light green wire over a couple pages, it goes to a big connector on the RHS which does have a light green wire that goes the the airbag kill switch. 

 

I think I have it now.  The main relay gets an always hot power, and a run or start only power.  The run/start power to the main, causes it to close and sends power to the fuel pump relay.  If the main is the one stuck closed, that will mean the FP relay gets a constant power when always hot is on.  The ECU or the diagnostic box has to ground the circuit to close the FP relay, but if the ECU isn't smart enough to apply the ground just at switched power because the power itself is only supposed to be applied when switched at the main, it still closes the FP.

 

Now I just have to reassemble enough car to test it with power. 

[Keith Novak]

It's been a while since I was doing a bunch of work on the car, and this problem may never be completely solved, but the lack of jumper needed looks like it's the ECU.

 

The FP no longer runs in the accessory position, and is now switched between acc and run.  It still doesn't need a jumper though. Did all the checking, wiggling, and probing fix something?  Did dropping the steering column on the floor "fix" the switch?  Was is somehow related to the kill switch wiring which I changed?  I may never know, but it works in the right position now.

 

As for the FP that keeps running...  The main relay which switches between always hot and run checks out since the fuel pump only runs when you switch it to the run position.  The FP relay isn't the problem.  Trying a good relay doesn't fix it.  This means the exciter wire for the relay isn't switching correctly.  That is switched from 2 places.  You can ground it in the diagnostics box, or the ECU sends a signal.  Since the diagnostic box is not required, the exciter wire in the ECU is grounding the circuit and powering the relay.