Feedack system voltage?

[FTA1995]

What the voltage supposed to be at the coolant temp. sending unit connector on the side of the water pump on the lt1?
Should be either
5 or 10 volts since its connected to the PCM.
thanks!

also, does the coolant gauge on the dash usually read a little off proportion to the digital PCM readout?

[Injuneer]

The PCM supplies 5.0V. The temp sensor is a thermistor, who's resistance is inverse to temperature. High temp -> 0 resistance, low temp -> 100,000ohms. The exact resistance for each temperature is shown in tables on Shoebox's website and on mine.

http://shbox.com/1/4th_gen_tech2.html#ect

Scroll down to "CLT (Coolant Temp)":
http://www.injuneer.com/ScanMast.html

As a result, the voltage on the circuit, measured across the 2 pins on the harness connector will only show 5V with the harness connector off the sensor. With the harness connected, you might see voltages like these:

170*F 3.0V
176*F 3.3V
195*F 3.4V

Voltage will approach 0 at very cold temps, will approach 5V at very high temps.

In my 94, the coolant temp gauge (from the sensor in the drivers side head) appears to be within 5* of the PCM reading.

[FTA1995]

Great, thanks again.

Whats the range that these thermistors are usually accurate/operable between?

[Injuneer]

The sensor body is matched to the service... IAT is plastic, CLT is typically metal. They function from -40* (C & F) to about 300*F, but I've never seen the table of resistance vs. temp for anything above 212*F. I'd say they are extremely reliable and easy to check. I have multiple IAT and CLT temp sensors in my engine, because I'm running two separate ECU's, and they are all very close in temperature.

The PCM uses the 2-wire sensor, because there is an internal resistor in the PCM. The readings will probably be more accurate than the 1-wire sensor for the coolant temp gauge, which relies on the various engine and chassis grounds.

[FTA1995]

OK i got the sensors out,

im getting readings of 2950 ohms on the single wire sender on the drivers side head,
and 3160 ohms on the one from the water pump.
They seem close enough to be fine, but heres the thing...

After I did my math, which is rusty,
For every degree change indicated there should be a a 114.444.... of Ohm value change. This is telling me that at 3160 ohms, it comes to 71.14 deg F, while the 2950 ohms comes to 75.4 deg F. This will only work for in between 68 and 77 deg on the scale and is obviously a ballpark figure average of change.
YES that is within a healthy level(that 5 deg you were telling me about Injuneer), but taking into effect this is room temp water. Both sensors were tested while in the same shot glass of water after letting sit for 30 min with no movement or anything.

I'm assuming that this difference is not a linear function, because:

It is NOT
Water temp at pump, 190 deg F
Water temp at head, 195.4 deg F

It IS more like
WT at WP 195 deg F
WT at head 180 deg F

Iam at a loss at this one,

and the difference gets worse as the temp climbs etc.

I'm thinking a new water pump sender would be a good idea, but why blow the money if the old one is fine.

as far as voltage supplied at sensors, 5.00v at the PCM to CLT temp, and 11.7 v at the head connector both with the car off, battery voltage at 12.07v.

I've included a pic of the two that I'm testing, one is the pu109 (two wire)
the other is the one wire, can't find the part number anywhere.
sorry about the quality, its a cell phone picture.

What have I missed and did i mess up the diagnostic?
Grounds all seem good, no connector failure anywhere....

Well, its 5 AM here, and this issue isn't going to let me sleep.
I'm going to try a hot water test to see the other variable which i predict will have a larger output gap than the 75 deg test.
If thats the case and the hot test comes out with an overstepping WP sensor, i think ill pick one up tomorrow.

Its the small things that get me sometimes

Thanks guys!

[Injuneer]

You can't interpolate the temperature between the points in the R vs. T table linearly. It is not a linear function.

[FTA1995]

right, i forgot to write that in last post, just changed it before you posted a return.
So I got em on ice right now, and now they switched on me!
The head temp is giving me a higher temp readout that the WP temp
Head : 8630 ohms
WP: 9110 ohms

hmmm
I think one of them is bad,
they're acting as if they have different thermistor qualities.
Any ideas?

[FTA1995]

Well, I found out with another diagnostic tonight that the sender to the dash is bad. I made a little harness adapter for hooking up the one wire to the connector for the two wire, and on the RT tool the temp on the one wire came up with about a 0% difference with the IAT at 48 deg. anything lower or higher the difference was anywhere between 5% and 40% at the extremes. plugging the regular two wire back in, there was less than a .05 deg difference in that than the IAT.
Well i guess its better to spend 13 bucks than 16!

Thanks for all the help Injuneer.