ECT & ACT Sensor voltage

[FoMoCo17]

I checked my ACT & ECT Sensor voltage at the 60 pin connector and was only getting 4.65 Volts @ the ECU. I then proceeded to check the grounds and all were less than 1 Ohm. What could cause the ECU to only be giving me 4.65 Volts to these sensors instead of 5 volts. I also checked the sensors that receive a TREF voltage and they were receiving 5.03 Volts. But, the ECT & ACT do not receive voltage from this reference voltage wire. I was under the impression and read that the ECT and ACT also receive 5 volts from the computer. I measured the 60 pin connector and measured this at the ACT & ECT connector (disconnected). I would think that this 3/10's of a volt matters somewhat to what the computer sees since this 3/10's is 1/15 of the total scal reading between 0 and 5 volts. Any help or ideas would be appreciated!

 

[jrichker]

The ACT & ECT feed voltage to their respecitve sensors through an analog to digital converter inside the computer. For that reason, you will not see the full 5 volts at either sensor.

Dumping the codes will normally show a faulty sensor. The ACT & ECT may be on the outer limits and not set a code but cause problems. Use a DVM or multimeter to check them.

Pin 25 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.

Pin 7 ECT signal in. at 176 degrees F it should be .80 volts

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

Here's the table: The values are +/- 15%, so don't get worried if your readings vary a little bit.

Voltages are measured across the two connector pins of the sensor with the sensor connected. Some safety pins used to probe the connector from the rear will be helpful

50 degrees F = 3.52 v
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
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

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
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

 

[FoMoCo17]

Both sensors are new but while installing them, it got me wondering about their voltage fed from the computer since I saw that they were not connected to the TREF wire. I have a Mustang book written by Probst (I think) and it stated that these sensors too are suppose to receive five volts. When I checked mine and only got a reading of 4.65 Volts, I got concerned since I have expereinced other problems that were causing erratic behavior. Just wondering why he would write this if it is not attainable like the TREF voltage which I checked and was indeed 5 Volts?

 

[jrichker]

Both sensors are new but while installing them, it got me wondering about their voltage fed from the computer since I saw that they were not connected to the TREF wire. I have a Mustang book written by Probst (I think) and it stated that these sensors too are suppose to receive five volts. When I checked mine and only got a reading of 4.65 Volts, I got concerned since I have expereinced other problems that were causing erratic behavior. Just wondering why he would write this if it is not attainable like the TREF voltage which I checked and was indeed 5 Volts?

There is no VREF connection to the ACT or ECT. Unlike the TPS, they do not use VREF and only have two wires connected to them. One wire is signal and the other is signal ground. Their operating voltage is supplied by the Analog to Digital Converter chips inside the computer. I do have the computer internal diagram if you have any doubts and can read circuit diagrams.

See the diagram below.

Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

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[FoMoCo17]

I realize they ar enot hooked up to the TREF voltage. However the EEC IV book written by Charles Probst stated they should be receiving 5 volts so this is why I was baffled by this?