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Thread: 1991 Fox Body Spark Woes

  1. #21


    So that confirms the PIP signal at the ECU is OK, is there an RPM signal now? The test light connected to the white wire (coil negative control) should blink on & off while cranking. The solid light means no switching signal. Now test the trigger signal, which is the SPOUT blue/white wire at the TFI module, and see if the test light blinks while cranking. If not, test at the ECU as well. Need to confirm the grounds are good, connect your voltmeter between the TFI ground wire and the distributor housing, between the block and battery negative post, and between the ECU and battery negative post. With KOEO, they should be no more than 100 mV, but will be higher while cranking.
    Last edited by tcm308; 05-31-2021 at 11:54 PM.

  2. #22


    Also verify that TFI is still selected for the ignition type and perform a TPS Autoset.

  3. #23


    I don't understand how I'm getting pulse signals to the ECU through the Ignition harness, but not to the white signal wire for the Coil to pulse and fire the injectors and spark, and also so the factory tach will work. It's also odd to have the pulse at the ECU, but not have RPM signal in the handheld.

  4. #24


    Also, no. No RPM signal still, anywhere. So wild and really aggravating.

  5. #25


    I remember reading the PIP signal threshold voltage needs to be reached, if too low, the ECU won't recognize the signal. From memory it was 6.5V as seen on a scope. The voltmeter averages the signal, so it needs to be around 3.25V while cranking. You could also perform a bypass test with a incandescent test light. Measure the voltage at the PIP terminal, it should be either 0V or 12V, depending on the position of the Hall-Effect shutter blade. With KOEO, if it's 0V, connect the test light to battery POSITIVE, and touch on & off to the PIP terminal and check for an RPM signal. If you see 12V, repeat the test, but this time with the test light connected to battery NEGATIVE. Do a pin drag test at the J1A connector to verify the terminals aren't spread. Is there 5V reference voltage at the sensors? I'd say the factory tach will show RPM with the J1A connector unplugged.

  6. #26


    When I had the J1A disconnected I could ground the distributor with it out of the car, and hand spin it and got spark, and I'm almost certain a bit of tach play on the dash. So where am I going with this or basically what's this pointing to? Let's say I got a few outcomes, which are different. What outcomes mean what, and what fixes them? If I'm getting low voltages signals at the ECU, why, and how can I fix that? When you say 5V reference voltage, can you be more specific on what reference voltage is, and what sensors you're talking about? Or we still only talking about the PIP/Hall-Effect pulse?

    I really appreciate the pointers, I'm just want to get this over with without having to dump money into it by having a shop diagnose and fix it..

  7. #27


    Testing the 5V reference voltage at usually the TPS or MAP is a quick health check of the ECU. No 5V reference would indicate a shorted sensor, open or short in the wiring, faulty ECU, power or ground problem. The 5V reference circuit is used for communication and sensor inputs.

  8. #28


    I'm using the internal MAP sensor. I can check it at the TPS. The ECU has been swapped out. I'll check it. You said key-on engine-off, then check for 5V at the sensor?

    I'm getting 4.8V at the orange wire on the TPS sensor with key on, not cranking.

  9. #29


    Yes, KOEO. The 4.8V reference voltage should be OK, was that with sensor ground or battery negative? Have you tested the PIP & SPOUT signal voltage while cranking yet?

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