What voltage at ECT connector?

sln583 · Jan 9, 2010

There are 2 wires on the connector, one going to pin 7 on the computer, and one labeled signal return.
There is no voltage from signal return, and 3.98 volts from the pin 7 wire.
Is this correct?
Should the pin 7 wire be around 5 volts, vref?

liljoe07 · Jan 10, 2010

signal return is a ground, so it will not have any voltage showing. To get the 5v the sensor needs to be unpluged. If the battery voltage is low, it can effect the reading. Bad grounds also. What is the voltage at PIN 1 at the ECM?

sln583 · Jan 10, 2010

11.95 volts at pin #1. Same as the battery voltage, which I beleive is correct.
The values in my first post are at the connector, KOEO, with the sensor disconnected

Im just a humble civil servant and my grasp of electrical theory is tenuous at best.
I found this explanation of how the Engine Coolant Temperature Sensor works.
Ford Fuel Injection Engine Coolant Temperature Sensor (ECT)
Correct me where im wrong on any of this.
I understand the sensor works on electrical resistance, the higher the temperature, the less K Ohms of resistance. It resists the 5 volts it recieves as vref.

The ECT receives the �Signal Return� voltage from the EEC, then allows a certain amount to return back to the EEC.

This part confuses me.
Where does the voltage(5 volts?) originate?
At Pin 7?
Where does the the certain amount return back to the EEC?
And I guess the most important question is what could be causing me to only have 3.93 volts at the sensor when I should have 5 volts

sln583 · Jan 11, 2010

tmoss · Jan 12, 2010

there is a 5vdc vref regulator in the EEC that supplies the voltage for all the sensors. 3.98vdc sounds low.

sln583 · Jan 12, 2010

It must be something inside the EEC,
I think I educated myself about how the ECT circuit works.
I checked the resistance between pin 7 and the ECT connector and got 1.5 ohms, thats with the computer disconected.
I plugged the computer back in and checked the voltage KOEO at Pin 25 (ACT) got the same 3.98 volts with the sensor disconected.
I checked voltage at pin 26 (VREF) and I get 5.03 volts, thats with all the sensors on that cicuit connected.
Is it the same vref regulator in the eec that generates both vref voltages for pin 26 and pins 7 and 25?
How do I find when I pull the EEC apart?
Could something be causing a voltage drop between the regulator and pins 7 and 25 but not pin 26?
I guess I should sum up the current problem with this car.
Ive had the car for 15 years and it never ran right. Always crappy when it was cold.
Pulled the engine and tranny out about 5 years ago and figered if I replaced most everything, it should run fine.
Didnt happen
Still ran crappy
Last year I decided to make it my mission in life to get the car to run like new.
Started with the codes that were coming up.
Went through all the emmission stuff, found a couple of vacuum leaks and fixed those.
Found a burned up O2 harness that a previous owner had patched back together.
Replaced that and was at last code free, 11s all around. (I ordered a manual harness and installed it, I figured out it was an auto harness they sent me. I repinned the harness and I checked for a burnt trace but the eec didnt have one, could still pull codes no problem)
Car still runs like crap.
I have since replaced or checked every sensor and wire and component on the engine.
One of the things its doing is when its left sitting overnight and is cold, it stalls after I start it.
if I keep my foot on the gas I can keep it running, after a few minutes it warms up and idles on its own, but pops and hesitates untill its at operating temp, but still runs crappy.
I have found a whole bunch of threads on the stalling on a cold start issue, but very few solutions
I have a SNEECIV datalogger and noticed that the temp of my ECT and ACT sensors were reading 55 degrees, when it was below freezing in my garage.
I removed the sensors and tested them at different temperatures and they checked out fine.
So I checked the voltage at the sensor, and when I got 3.98 volts I ended up here.
I just changed out the computer with a Cardone remanufactured one from Rock Auto. That computer has the same low voltage at those two sensors.
Either Im really unlucky and got a bad computer from rock auto, or something fried my new computer.
Wish I had a freind that lived close with a known good computer, but no such luck

AS I side note, I confirmed that the low voltage is causing the car to stall when its cold.
I unplugged both sensors, leaving the computer reciving 3.98 volts at pins 7 and 25.
This is the voltage the computer will see when its 29 deg, or at least thats what my SNEECIV read it as.
It was about 29 deg in the garage and the car started right up and purred like a kitten.
looking at the different voltages from the chart here
Ford Fuel Injection Engine Coolant Temperature Sensor (ECT)
I assume the wrong voltage is causing the EEC to use the wrong timing and fuel tables and the car stalls.
The cold weather really seems to agrevate the problem, perhaps because the voltage is so different at low temps?
I hope I have found the cause of one problem, now just need to find the solution.
Are, my conclusions wrong about this?
What do I do now?

tmoss · Jan 13, 2010

you need to check all the connectors in the wirng path, both supply and return to see if you have a resistance in a connector that is knocking voltage down. With the circuit in use, you can do this with a voltmeter by reading voltage drop across each connector. Keep tabs of the drop at each connector.

sln583 · Jan 13, 2010

I will measure all the sensors that use vref. Measure between vref and sigrtn?
The problem sensor will have 5volts in and less out?
BP sensor
TPS
EVP
Did I miss any?
I test them KOEO?

sln583 · Jan 13, 2010

Im a little slow on the uptake sometimes
You mean on the ECT and ACT circuits?
A voltage drop test?
Found a good article here
How To Perform A Voltage Drop Test - Help With Automotive Circuit Diagnosis

joel5.0 · Jan 13, 2010

Looks like you have a short in the wiring harness. Disconnect ECT connector and ECM's and check resistance from ECT terminal to SIGRTN in the ECT connector + from ECT terminal to a good ground. LUK

tmoss · Jan 13, 2010

litterally I meand you take the low voltage probes and on the same wire, you measure voltage drop on that single wire through the connector(s) from the EEC vref pin to each sensor/MAF. It's not real quick to do, but it shows up connections that are too high in resistance well.

tmoss · Jan 13, 2010

joel5.0 said:
Looks like you have a short in the wiring harness. Disconnect ECT connector and ECM's and check resistance from ECT terminal to SIGRTN in the ECT connector + from ECT terminal to a good ground. LUK

Prob not a true "short" but it very could be a low resistance return path that would load down the vref voltage regulator and lower the vref voltage. I don't know if the regulator has a protective feature in it, but if you run vref regulator at it's highest output, it could cause eventual failure.

joel5.0 · Jan 13, 2010

ACT/ECT signal voltage source is isolated from the other sensors (BP, TPS, EVP) VREF. VREF for ECT is the ECT signal wire, same thing for the ACT. Their temp value depends on how "shorted" to SIGRTN or ground they are through the sensor termistor (lowering the resistance = higher temp = signal voltage drop). If the resistance check between ECT and SIGRTN & ground is <10kΩ ... there's a short that needs to be traced.

tmoss · Jan 13, 2010

yes, the difference is two wire sensors and three wire sensors. I forgot the ECT/ACT are not fed from vref like the others. Still, he needs to do a check of all connections. Since the ACT and ECT are monitored for resistance, he may need to use the ohm-meter function to check connections.

sln583 · Jan 14, 2010

If I connct one end of the volt meter to pin 7 (ECT)on the computer, and run the other end of the volt meter to the negitive side of the battery,then disconect the ECT sensor, and I get 3.98 volts, doesent that take the ECT wiring and connections out of the equation?
Since pin 7 should be the source of the 5 volts, and it is in fact 3.98 volts, dosent that mean the problem has to be before that point in the circuit? Somewhere inside the computer?
Pin 25 (ACT) is also only producing 3.98 volts when I test it the same way, grounded to the negative side of the battery.
When I test pin 26 (VREF) the same way, I get the correct 5 volts.
Dosent this mean the problem is somewhere inside the computer, that the 5 volt regulatorin the eec is working, and sending the correct voltage to pin 26, but there is resistance somewhere between the 5 volt generator and pins 7 and 25?
Am I misunderstanding this?

I checked voltage drop between pin 7 and the sensor and there was none. 0.000

tmoss · Jan 14, 2010

that sounds like a valid test and results. based on Joel's reminder that the power supply for those sensors is seperate from vref, you might have a problem in the EEC. I'm not 100% sure what the voltage to those sensors should be now, but I'd guess it would be 5.0vdc rather than another voltage. Joel, do you know? It could just be monitoring the change in resistance as part of an op-amp input or other control device too. I've not gotten that far into the EEC to know.

sln583 - Can you beg/borrow/steal another ECM to check it out?

joel5.0 · Jan 14, 2010

sln583 said:
If I connct one end of the volt meter to pin 7 (ECT)on the computer, and run the other end of the volt meter to the negitive side of the battery,then disconect the ECT sensor, and I get 3.98 volts, doesent that take the ECT wiring and connections out of the equation?
Since pin 7 should be the source of the 5 volts, and it is in fact 3.98 volts, dosent that mean the problem has to be before that point in the circuit? Somewhere inside the computer?
Pin 25 (ACT) is also only producing 3.98 volts when I test it the same way, grounded to the negative side of the battery.
When I test pin 26 (VREF) the same way, I get the correct 5 volts.
Dosent this mean the problem is somewhere inside the computer, that the 5 volt regulatorin the eec is working, and sending the correct voltage to pin 26, but there is resistance somewhere between the 5 volt generator and pins 7 and 25?
Am I misunderstanding this?

I checked voltage drop between pin 7 and the sensor and there was none. 0.000

Nope..... the only way (which is acceptable to do) is to cut the ECT and ACT wires near the ECM connector leaving space for them to be repaired afterward (this removes the harness possibility out of the picture) and check voltage from the cut wires ECM connector side. Disconnecting the ECT only takes the thermistor (sensor) out of the picture.

Again, if the resistance check between ECT and SIGRTN & ground is <10kΩ ... there's a short that needs to be traced.

joel5.0 · Jan 14, 2010

tmoss said:
that sounds like a valid test and results. based on Joel's reminder that the power supply for those sensors is seperate from vref, you might have a problem in the EEC. I'm not 100% sure what the voltage to those sensors should be now, but I'd guess it would be 5.0vdc rather than another voltage. Joel, do you know? It could just be monitoring the change in resistance as part of an op-amp input or other control device too. I've not gotten that far into the EEC to know.

sln583 - Can you beg/borrow/steal another ECM to check it out?

It should be 5vdc or similar to the other VREF source...

tmoss · Jan 14, 2010

joel5.0 said:
Nope..... the only way (which is acceptable to do) is to cut the ECT and ACT wires near the ECM connector leaving space for them to be repaired afterward (this removes the harness possibility out of the picture) and check voltage from the cut wires ECM connector side. Disconnecting the ECT only takes the thermistor (sensor) out of the picture.

Again, if the resistance check between ECT and SIGRTN & ground is <10kΩ ... there's a short that needs to be traced.

The proper voltage to find at the ECM for ACT/ECT is 5vdc plus/minus ~15%

Why does he have to cut wires - why can't he pull the ECM connector and check the resistance path between the pin connectors with the sensors hooked up? That tests the entire harness and sensor path for each sensor. The temperature vs sensor resistance is pretty easily found online and both vary the same from ~58k ohms at 50F to ~3k ohms at 190f.

sln583 · Jan 14, 2010

tmoss said:
sln583 - Can you beg/borrow/steal another ECM to check it out?

I have 2, the original 8LD and a remanufactured cardone that I just bought.
They both produce the same results.
No one that lives close to me has a stang I can pirate a MAF computer from.

What voltage at ECT connector?

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