Low Temp Reading And Tc Lockup Problem

furious-C · June 25, 2009

1994 Chevy K1500, 350 with 4 speed automatic 4L60E.

The temperature has been reading low for a while on this truck. The previous owner had changed the sensor and thermostat but it did no good at all, and so he thought the problem was with the gauge in the dash. I tested the gauge by grounding out the wire off the sensor on the side of the block which maxed out the gauge so no problem there, right?

I'm thinking the wrong sensor was changed out, the one on top of the engine.

I'm also wondering if this low temperature reading would keep the torque converter lockup from engaging. It apparently stopped engaging about the time the temperature issue came up...

I've felt the upper radiator hose, and it feels closer to 200 than 145....

Any help is appreciated!

Thanks!

C

Mike G · June 25, 2009

First off, welcome aboard!

Secondly, yes. When I just had the trans done in my truck, the guy called and said it was locking up intermitently. He said it could be temp sensor, speed sensor, brake switch or trans solenoid. Of course he was the expert and I told him to take his time on the diag. Ended up being the trans shift solenoid, so he just put a new one in.

--Mike

furious-C · June 25, 2009

First off, welcome aboard!

Secondly, yes. When I just had the trans done in my truck, the guy called and said it was locking up intermitently. He said it could be temp sensor, speed sensor, brake switch or trans solenoid. Of course he was the expert and I told him to take his time on the diag. Ended up being the trans shift solenoid, so he just put a new one in.

--Mike

Thanks for the reply! OK, so the temp sensor is a start. I'll check it out here in a couple of weeks when I get a chance to work on it further. I'll be driving it long distance on the interstate, so I'd like for it to lock up when needed!

bill190 · June 26, 2009

There are typically TWO temperature sensors, one for the engine computer and another for the instrument cluster gauge.

It is possible that these two sensors would give different readings for a given temperature. So very important the correct sensor is installed in each spot.

Then the engine computer does all sorts of things relating to all sorts of variables like engine temperature, engine load, etc. So I would not be surprised if the transmission problem is related to this.

Then be careful when ordering the parts at an auto parts store to get the correct sensor(s).

furious-C · June 26, 2009

Yeah, these 2 sensors really complicate things. Especially if they have identical threads and use the same connectors. I noticed the one on the side of the block, which goes to the temp readout on the dash, has a 1 wire hookup but the connector can utilize 2 wires...

I didn't pay too much attention to the one on top of the intake manifold. I'll try to check the resistances of both (they go off resistance, right?) and try to find what they should be in my Chilton's manual. I think I've seen it in there before....

After I determine the sensors are correct, or after I correct them, if the problem persists then am I to assume the gauge cluster is bad? I ground out the wire going to the temperature sensor on the side of the engine and the dash read full temp, so I thought that was a good indication the dash was functional. Would that be a good test for the temp gauge itself?

Thanks for your time!

bill190 · June 26, 2009

Do not ground out or apply voltage to a gauge as this might damage it. Instead use a resistor of the appropriate resistance (ohms) for testing. If there is just one wire to the sensor, this would be ohms to ground. (See below)

Both temperature sensors on my 92 use resistance, but I had some time today and poked around. They *are* different!

The computer temperature sensor is 100,000 ohms (resistance) @ 40 deg. F / 70 ohms @ 266 deg F.

The instrument gauge temperature sensor is 1400 ohms @ 100 deg F. / 55 ohms @ 260 deg F.

So I figure around 70,000 ohms difference at 100 degrees F. between the two different sensors.

Again, this is with my 92. It is very possible they changed this for 94? No telling! Also I noticed something about the computer coolant temperature sensor for California being different?

Bottom line, be sure to get the right part for each sensor. The way to know for sure what they should measure (ohms @ certain temperature) is with a factory service manual for that year/model.

As to getting a resistor of a certain amount for testing say the low / high and accuracy of your instrument gauge, you probably will not find a 55 ohm resistor at Radio Shack.

But could take five 10 ohm resistors and one 5 ohm resistor and connect them in "series" and this would then be 55 ohms.

Or you could take two 100 ohm resistors and connect them in "parallel" and this would be 50 ohms. Then add a 5 ohm resistor in series and you would have 55 ohms.

You can use an ohm meter to measure each resistor to see what its resistance is. Or learn the resistor color code if you have good eye sight!