I'm having a fuel issue and have run out of ideas - hoping somebody here can help me think this one through!

My car is a 1995 (the motor is from a '94) and I just recently completed an FMII install with a Link ECU. The other day the car wouldn't start, which I've identified as a fuel issue. Here is what I know so far:

- Checked the fuel pump - applied 12 volts directly to it and it works fine
- Checked the electrical connection at the fuel pump - it's not getting 12 volts when I turn the key to "on"
- Checked the fuel pump relay - it does not "click" at key on; took the relay out and tested - it works (did the test described in Rod's enthusiast manual and took the relay apart and watched it activate when I put 12v to it
- Jumped the F/P and GND in the diagnostic block - fuel pump works fine; fuel pump relay activates
- Swapped the Link ECU for the stock ECU - same issues exist
- Checked the main relay under the hood (good) and all fuses (good also)

I don't know where to go from here. What does the fuel pump relay look for to activate? I believe in the 1.6 cars it needs a signal from the MAF, but in the 94+ cars it is the ECU (which I think I can rule out after trying two ECUs). Would a bad CAS causes this?

Thanks in advance for any help!

 

the relay get its ground at the ECU (Pin 2T). it's gotta be the link/ecu failing to ground it.

 

Thanks Braineack. I agree that the Link is failing to ground the relay...now I need to figure out why. I don't think the issue is with the ECU itself - I've swapped in the stock ECU, and it fails to ground the relay as well.

The ECU isn't seeing something that it needs to. I'm digging through wiring diagrams right now (great info here: Miata Information) to figure out what the ECU is looking for to open the relay, but would appreciate any knowledge anyone can kick from the top of their heads on the topic.

Here is a list of the signals that the ECU needs that I can think of:
- Ignition switch - key in "run" or "start" position
- MAF/MAP signal (MAP in this case with the Link ECU)
- CAS? (does the link need a signal from the CAS?)
- Main relay

What else?

 

Ground the pin at the ECU connector and see if that trips the relay. You dont want to tear the car apart before knowing the circuit is working. Turning the key on and powering the ECU with good ground to the ECU should ground the circuit to prime the pump (stock not sure about the link). If the ECU or Link has lost its ground it wont turn on the pump. Are you sure you are powering up the ECU? Do you get spark cranking for example?

HTH

Ron

 

Thanks Ron. The ECU is getting power - the car runs fine with the fuel pump jumper in the diagnostic block, so it's getting spark. I will check the ground tonight and let you guys know what I find out.

 

I just put ground to the pin in the ECU connector - it triggers the fuel pump relay.

There has got to be some input that the ECU needs in order to ground that connection that it is not getting. I've checked everything obvious - relay, fuses, etc. Any other creative ideas?

 

There appear to be 5 grounds to your ECU. 1k 2a 2b 2c 2d. Be SURE they are GOOD grounds. This is not a job for an ohm meter. I use a headlamp with jumper wires one to batt positive the other to each of the pins at the ECU connector. if the headlamp will light on each pin the grounds are probably ok. This is mostly what is known as a voltage drop test, you can use a voltmeter to check it, but if it lights a headlamp its probably redundant to test further.

1a and 1b are batt positive and keyed positive respectively. keyed power comes from the main relay. both these powers should be checked through the headlight to ground, to be sure they are good.

I dont see any other powers or grounds on the diagram in alldata. if they are good, and the pump wont prime when you turn on the key, the ECU is most likely at fault. Hard to believe both the stock and link are bad, as I said, im not sure if the Link primes the pump tho. I *assume* the pump would then turn on when it sees signal from the cas. The cas has to be ok as you can get spark and injector pulse.

HTH

Ron

 

That's odd. The Lt Green wire is the 2T connection. The relay shares power with a lot of components and triggers the ground to the fuel pump. If you can jump at the diagnostics box and 2T itself, then I have no clue why both your ECUs are failing to do so.

 

Tests complete. I have power at 1a and 1b. I have ground at 1k, 2a, 2b, and 2c. No ground at the 2d pin when the key is in "run" or "start". Looking at page 9 here, it looks like 2d (BLK/BLU) is the ground for the following components:

- Heated oxygen sensor
- Mass Airflow Sensor (replace with the Link's MAP sensor at this point)
- Engine Coolant Temperature Sensor
- EGR Valve Position Sensor
- Throttle Position Sensor

The MAP, coolant temp, and throttle position sensors all get readings on the Link keypad, so they appear to be good. Any ideas on how to identify/isolate the issue to one of these components?

As always, you guys help is sincerely appreciated.

 

Get your multimeter out and set it to AMPS. Then use it to jumper the fuel pump relay in the diagnostic port and note the current the relay pulls when jumpered.

Then set the multimeter to DC Voltage and put one side into the diagnostic port that is for the fuel pump, and the other side of the meter to +12V. Note the voltages at Key Off and Key On Engine Off. Also see if you get a voltage reading for 2 seconds immediately after key on.

Then remove the fuel pump relay and repeat both test again.

Record all results on paper so you aren't relying on memory.

 

Thanks Pat - I did the tests you described and here are the results:

Fuel Pump Relay In
Amps @ Diagnostic Jumpered = .16 (multimeter set to 10A?)
Voltage @ Diagnostic - Key Off = 12.1
Voltage @ Diagnostic - Key On = .4
Voltage @ Diagnostic - Cranking Engine = .7-.9 (slight increase vs key on)

Fuel Pump Relay Out
Amps @ Diagnostic Jumpered = None; FP did not turn on as it did with relay in
Voltage @ Diagnostic - Key Off = 12.0
Voltage @ Diagnostic - Key On = .1-.2
Voltage @ Diagnostic - Cranking Engine = .7-.8 (slight increase vs key on)

Results for 2 seconds after key on were the sames as key on.

Do the results above indicate any problem? I'm hoping they mean more to you than they do to me.

 

Nick

Re read Pats instructions carefully and see if you did this test correctly. If I get what he is asking for,(voltage drop test) you were not hooked up correctly.

Ron

 

Link won't give power to the FP if it doesn't sense an o2 signal. Atleast for 90-93.. I'm assuming its the same case. Might be worthwhile to look into.. - Bryan

 

Here is my 2c worth.

I have a Link ECU in my '92. Months after the initial install I had an issue that the fuel pump would not run unless I jumpered FP/Gnd. I checked all of my ground connections and could not find anything obvious.

It happened after I have unplugged the wiring loom from the Link ECU, without removing the battery negative lead. I also damaged the ignitor, which meant I had no tacho and 2 cylinders lost spark.

I replaced the ignitor, but still no working fuel pump. After much searching I found that one of the wires that I moved as part of ECU install was not fitted correctly in the wiring loom. I corrected this and all was good again.

Good luck.

J