AggieJustin

Well, glad we found the issue.

My FSM is just a single PDF file that I bought shortly after getting my truck, everything is listed straight-through with no supplement section.

bent valves

Unlike other sensors on the 24 valve engine, the Engine Coolant Temperature sensor is resistive and receives a constant current from the ECM. This current is small, something like 0.4 mA. So when the ECT sensor is at 100*C and is reading 650 ohms the 0.4 mA of current will generate about 0.26 volts. When the sensor is at 0*C and 33K ohms, the same 0.4 mA of current will generate more than 12 volts. The ECM can then sample the voltage level and determine the coolant temperature. This RTD (Resistance Thermal Device) is a thow back to the older 12 valve engine where there were almost no high-tech sensors running on a regulated 5 volt supply.

As you can see at 100*C the small voltage of 0.26 is easy to corrupt with a bad ground. Suppose the ECM ground raises from a true zero volt ground by 0.1 volt. The ECM now thinks the coolant temperature is way above 100*C and will not allow a start.

After thinking this through I am in agreement with you guys it is a ground problem, so get out the scotchbrite and dielectric grease and fix the problem.

Busboy

I wondered how you could scan the page and get it online so quickly, so we are back to installing a sw or a relay as a cheap way out. This almost makes you think you should throw a ECT sensor at it and see what happens.

Red Goat

So what voltage is S165 supposed to be?
Because B+ usually means battery voltage which usually no good as a supply for sensors because it it varies by 3 volts or so during every day usage.
It also wouldn't make sense for it to be +5 volt because you always want the highest voltage possible for detecting water in fuel and that water in fuel sensor is shown in a parallel arangement
With that being said the ground from the ecm on pin 11 looks correct.
My guess is that the ecm would try to push 5 volts to the temp sensor on pin 14 and a full battery voltage to the water in fuel sensor.
But I do not have a truck of that year here to test as mine is an 04.5
Also my buddy lives a thousand miles away and is challenged in the areas of electricity, meters and come to think of it height!
Ooooh I know Im going to hear about that last one!

Busboy

This was how I thought the 2001 worked but as you were typing we discovered it changed in 2001 and the ECM now supplies a ground on pin 11 from the ECU.

Red Goat

Holly smokes!
I had better go to home economics or something and take a typing course!
By the time I hit the submit button I was 6 answers too late!
Anyone know where the ecm is getting its ground at or where to look?

AggieJustin

So tracing through the diagram, the ECM receives ground through pins 30 and 49 (although I thought it was also grounded through the backing plate).

These two pins are wired into the same connection that provides the ground for the starter relay in the PDC, both intake grid heaters and the VP44. This connection goes to the negative post on the primary (driver's side) battery, which then grounds to the frame. The ground from the PCM also joins up here at the ground post on the primary battery. Everything else appears to be ground via other points on the frame.

So I guess the best places to start looking are the frame grounding points for the batteries, then the negative posts on the batteries. If it's really a grounding issue like this, I wonder what other havoc it may be causing.

bent valves

Busboy, I am basing my info on the books I own for my early 24 valve. If later engines use a different sensor than ignore my posts, but if the sensor is still only two wire it must be a current source in the ECM driving it.

Busboy

I guess the pdf format is much quicker than looking in BOTH paper copies but yes my guess is the negative post of the drivers side battery. Corrosion?? good luck!

Busboy

I have a 2000 fsm with a 2001 supplement,(that's what was available for the 2001 when I ordered mine). The 2000 shows B+ on pin 11 on the ECM and the 2001 shows the word "ground" on pin 11 on the ECM. To me that indicates they switched from supplying a "voltage" to supplying a "ground" for the sensors to modify.

Big snow storm here today so I'm staying home.

bent valves

It sure would help if the FSM told us what B+ is. Having two terminals labeled Ground and B+ is confusing if B+ is a voltage source, because a changing sensor resistance will not have an effect on the voltage. If B+ is a constant current the B+ voltage will change with the sensor resistance.

It’s difficult to describe the difference between a voltage source and a current source without a degree in electrical engineering but think of a voltage source as a battery. If I put a 12 volt battery between B+ and Ground a current will flow and this current will change with the change in resistance but the voltage stays the same, 12V. On the other hand if the 12 volt battery is replaced by a constant current source the current is fixed and the voltage changes with a change in resistance. Constant current can be thought of as a fixed number of electrons flowing down the B+ wire per second. The Ground wire from the sensor is not connected to the engine block but is used to return the electrons back to the ECM. The reason automotive circuit’s use a current source is because the connectors and wiring resistance have almost no effect on the signal voltage (B+).

OK, enough geek speak. Clean the grounds.

Red Goat

What I can tell you from working on industrial electronics is there are 15 and 5 volt curcuits both + and -
The auto industry chose the 5 volt version because the 15 volt was A tadd over!
The o2 sensor on a gasser will produce its own voltage albeit not much .5 volt
All other map etc use 5 volt which drags back to the old ttl logic days from the 1950s on machine control but didnt hit the cars untill the late 80s.
Any way what does the ecm do when it thinks the engine is -40 degrees?
It sounds like that is what the ecm is thinking when he unplugs the sensor!
I still think his 44 is going south but in the interest of truck science would love to see where this goes!
Had 4 teeth pulled today!
does diesel go with pepsi?
Or should I just stick wiyh the Crown Royal?Coffee thing aint coffee!

AggieJustin

I was just thinking, I wonder what happens if you have the ECM software that enables high idle and 3 cylinder idle and you pull the ECT plug. Does the ECM really think the temp is that low and try to initiate these programs?

I'm assuming that since he's pulled the plug before and let it idle that it doesn't...unless he doesn't have this code in his flash.

Busboy

Doesn't the high idle and 3 cyl mode come from the IAT?

Busboy

Tylenol 3....... and the Crown Royal