94 6.5L No Start - ECM reading ECT incorrectly
 

Hey there everyone! Got a really bizarre issue here.

Got a 1994 Chevy K3500. Everything is bone stock.
No codes present in the ECM. All sensor feedback checks out fine when Data Streaming except for the Engine Coolant Temp. It's reading approx. 75C/167F with the engine cold. Changed the ECT and checked the wires back to the ECM. No short to ground or positive. Checked all +12V and grounds to the ECM. Cleaned all ground locations on the engine. Resistance value is the same at the ECT as it is on the ECM connector(4.7K ohms@ 15C/59F). With the ignition on and ECT disconnected, I get the 5V reference at the connector and the ECM registers -22C/-7.6F. All the other sensors that share the ECT ground are reading appropriately. Swapped the ECM with a known good one and it reads the same. Swapped the EPROM with a known good. No change.

NOW, when I attached a variable resistor block to the ECT plug and monitor the Coolant temp on my scantool while tweaking, I trick the computer in to thinking it's the appropriate temperature(I dial in approximately 430K ohms). Turn the ignition on, the glow plugs go through a full cycle and the truck STARTS!!

Only thing I can think of now is that it's getting 12V on the reference side. But the ECT plug shows 5V...

Anyone experience this problem before?? I have not been able to find a thread regarding a similar issue. I'm not sure where to go from here. Any help is appreciated.

Sounds more like a resistance problem/short in the harness-when disconnected it should read -40 @ the ECM not -22.

I would say that there may be some contamination inside the sensor plug that is allowing a short to the signal wire from the 5v reference-look for corrosion there since the unplugged resistance is about 1/5 of what it should be at the ECM.

Temperature Resistance Resistance
(°C) (Ohms) (±%) (+°C) (-°C)
-40 100,865 4.87 0.7 -0.7
-35 72,437 4.64 0.7 -0.7
-30 52,594 4.43 0.7 -0.7
-25 38,583 4.21 0.7 -0.7
-20 28,582 4.00 0.7 -0.7
-15 21,371 3.80 0.7 -0.7
-10 16,120 3.60 0.6 -0.6
-5 12,261 3.40 0.6 -0.6
0 9,399 3.21 0.6 -0.6
5 7,263 3.06 0.6 -0.6
10 5,658 2.92 0.6 -0.6
15 4,441 2.78 0.6 -0.6
20 3,511 2.64 0.6 -0.6
25 2,795 2.50 0.6 -0.6
30 2,240 2.45 0.6 -0.6
35 1,806 2.40 0.6 -0.6
40 1,465 2.36 0.6 -0.6
45 1,195 2.31 0.6 -0.6
50 980 2.27 0.6 -0.6
55 809 2.23 0.6 -0.6
60 671 2.19 0.6 -0.6
65 559 2.15 0.6 -0.6
70 469 2.11 0.6 -0.6
75 395 2.07 0.6 -0.6
80 334 2.04 0.6 -0.6
85 283 2.00 0.6 -0.6
90 241.8 2.10 0.7 -0.7
95 207.1 2.21 0.7 -0.7
100 178.0 2.31 0.8 -0.8
105 153.6 2.42 0.8 -0.8
110 133.1 2.52 0.9 -0.9
115 115.7 2.61 0.9 -0.9
120 100.9 2.68 1.0 -1.0
125 88.3 2.75 1.0 -1.0
130 77.5 2.80 1.1 -1.1
135 68.3 2.84 1.1 -1.1
140 60.3 2.87 1.2 -1.2
145 53.4 2.89 1.2 -1.2
150 47.5 2.90 1.2 -1.2

The ECT plug is clean. If there were some form of resistance in the harness, the resistance would have changed when I checked the resistance value at the ECM, no? Both the ECT and at the ECM connector read the same resistance value.

It has to be some sort of short between the wires,with the sensor unplugged you only have 1/2 the temperature reported to the ECM that it should be.

According to the chart for that temp to be displayed the measured resistance must only be around 30,000 ohm's instead of an open circuit that should throw a code.

I wonder if at some point someone made a glow plug time extender with a resistor and toggle switch as described here:
extended glows - Diesel Place : Chevrolet and GMC Diesel Truck Forums

Hey I was Just wondering if you ever got this figured out. I have as truck here that is doing the exact same thing and no one even the dealer seems to know anything about it
Thanks

hi there,
I have the exact problem, -7f is the correct lowest temp the ecu can display, I have run complete new temp cct for the ect and the results are still the same. Did the same work as you have, ecm, prom, measurements, all with the same resultsHave you had any success in finding the problem? I have fixed many of these but not run into this b4.
cheers

hi there. have you had any siccess in finding a solution? Have the same exact problem.