ACT sensor?

Discussion in 'Fox 5.0 Mustang Tech' started by grey5.0beast, Dec 27, 2006.

  1. Where is the ACT sensor on our mustangs? Also can I test my stock computer? I have bout run out of options on electrical components to look into for my problem
  2. The ACT is located in #5 intake runner on the stock intake.

    Code 24 - Intake Air Temperature (ACT) sensor out of range. Bad sensor, bad wiring. The ACT for Mustangs built before 95 is in the #5 intake runner. It measures the air temperature in the intake to help computer the proper air/fuel ratio.

    Note that that if the outside air temp is below 50 degrees F that the test for the ACT can be in error.

    ACT & ECT test data:

    The ACT & ECT have the same thermistor, so the table values are the same

    Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

    Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer. Here's the table :

    68 degrees F = 3.02 v
    86 degrees F = 2.62 v
    104 degrees F = 2.16 v
    122 degrees F = 1.72 v
    140 degrees F = 1.35 v
    158 degrees F = 1.04 v
    176 degrees F = .80 v
    194 degrees F = .61

    Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.

    Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.
    50 degrees F = 58.75 K ohms
    68 degrees F = 37.30 K ohms
    86 degrees F = 27.27 K ohms
    104 degrees F = 16.15 K ohms
    122 degrees F = 10.97 K ohms
    140 degrees F = 7.60 K ohms
    158 degrees F = 5.37 K ohms
    176 degrees F = 3.84 K ohms
    194 degrees F = 2.80 K ohms

    Code 64 – ACT (Air Charge Temperature) signal low or grounded. Bad sensor,
    sensor wiring on pin 25 shorted to ground, or bad computer.

    Note that all resistance tests must be done with power off. Measuring
    resistance with a circuit powered on will give false readings and possibly
    damage the meter.

    The ACT is located in the #5 intake runner on Mustangs built from 86-93
    . Disconnect the sensor by removing the sensor wiring connector. Measure the resistance across the sensor pins.

    Computer testing is done by dumping the codes.

    Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

    Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.



    IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

    Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

    Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

    For those who are intimidated by all the wires & connections, see for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

    Or for a nicer scanner see – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $35.
  3. I have a scanner, I mean like how to test if it is bad. It is throwing out a bunch of random BS codes annd won't let the fuel pump stop running. But it only happens if it is cold out, that is why I am thinking ACT. I replaced the ECT cuz I broke it it was so cold outside and it was brittle I guess.
  4. Post up your codes.
    I am sure one of these guys will be able to make sense of them.

    Had you made any changes when the problems started?

    good luck
  5. Weather changed basically. No changes were made to the car, just on one particularly cold(for GA) august morning the fuel pump didn't shut off. Ok so the symptoms are fuel pump doesn't stop priming when the key is turned to the on position, it just keeps going, then the fuel starts getting rerouted to the tank via FPR. Then the WOT relay starts clicking as well as the fuel pump relay, then the pump will click off, then back on, then off. And while the pump is going the RPM guage actually jumps as well, and it seems I can hear something under the hood surge with the rpm guage, although the car is not on, could it be a stator or the tfi maybe?? I have no clues and I am really close to blowin my money on a DD even though I know we can figure this out. I have to get this car running right, if anything before the 3rd when I go back to school. Please help!!

    Also might want to mention that if I put my keys in the car in the on position, and go inside and go about my business for say 10 minutes if its around 50 degrees and about 30 min if it is 35 degrees, then the fuel pump shuts off and when I go to start the car, it starts right up and runs fine... I am so confused!!!!!!!

    The codes I pulled most recently were a 95-Secondary fuel pump circuit, common for a SD to MAF converted car because of a wire not hooked up. Which isn't the case b/c mine is stock mass air'ed, then an erratic EGR valve(can't remember the code), not necesarrily related I guess, cuz it would not cause the car to do this, These are a few weeks old, I have to pull the codes again, but its too cold out for a GA boy to be workin on a stang in the dark...
  6. I might want to take a look at your EEC relay and it's input feed (to see if it's inconsistant, which could be like cycling the key on and off constantly).

    Otherwise, the ever-wise JR and Jason will have some very good ideas. When I get something like that, if wire checking doesnt help, I'd almost lightly hit the EEC relay and puter area (being careful with regard to heat) with a hair dryer before you begin. If that helps, you know you narrowed down the source of the issue.

    Good luck.
  7. Start by examining the fuel pressure and EEC control relay wiring and sockets. Look for corrosion, damaged wiring, or evidence of overheating. Wiring looks OK, consider replacing the relays one at a time.

    The EEC relay is mounted on top of the computer and may be difficult to get at. I usually remove the passerger side dash speaker to get better access. Small hands are helpful, so an assistant could be useful here. I have heard of water seeping in and damaging the relay. Look for rust stains and other evidence of water leaks in that area.
  8. thanks for the info, I am trying to get it started now, the key is in it, and the pump has been running for a good ten minutes. I will get at that tomorrow. What, if anything, is under the stock airbox, I hear a weird noise coming from down there too. I see the hvac, but I don't think it is that making the noise, oh and this is koeo as well.
  9. Check and see if you have two relays under the MAF. Supposedly, some of the last of the 91 cars had the fuel pump relay located under there instead of under the driver's seat.
  10. The EVAP cannister is under the airbox a good little ways. The cannister purge solenoid is down there too IIRC.