Mystery Gauge

[MikeB]

I bought this coolant temp gauge last year from LMC truck. It was advertised as a direct replacement for a stock 67-72 gauge. The part number is 36-2013, but I don't see it on their website anymore, and they have literally no tech support.

The problem is it reads very low. The needle doesn't even start moving until 150 degrees. And at 175 it is just at the second mark on the left. I've checked it in the truck connected to a new sender, and on the workbench using potentiometers. When I dial in the resistance for 210 degrees, the needle is about mid-scale. As for a shunt resistor, anything less than 500 ohms, causes even less needle movement.

If I can't get this working I'll use the OE gauge. But it reads high! I know that adding just a little resistance in series with the sender will help, but I'm not sure that's the best solution. I'll probably attach a potentiometer to the shunt terminals to see if I can find a value that will solve the OE gauge problem.

LMC #36-2013

[dmjlambert]

It's interesting. I suggest take a temperature sender and wire that gauge up with a battery, and immerse the sender in a coffee can of water and anti-freeze on the stove and go ahead and bring up the temperature to about 240 on a candy thermometer and see how it reads. It may be just fine.

[MikeB]

Originally Posted by dmjlambert

It's interesting. I suggest take a temperature sender and wire that gauge up with a battery, and immerse the sender in a coffee can of water and anti-freeze on the stove and go ahead and bring up the temperature to about 240 on a candy thermometer and see how it reads. It may be just fine.

You may be right about the gauge being OK. I know it's not linear, so the pointer could start moving rapidly once the temp gets to 180.

I've done the hot water test in the past, but all I did was measure sender resistance at various temps. I'll try it using the gauge this time. I have a 12 volt power supply I can use to power it.

However, I do need to find a way to securely hold the sender in a pan of water. Some type of 1/2" NPT bushing setup, maybe. I do know I'm asking for trouble by letting the sender sit on the bottom of the pan right on top of the burner flames.

[Getter-Done]

Some thermostat housing had NPT threads in them.

I am just tossing idea's around.

I have lot's of stored (Hoarded) parts for stuff like this.

[dmjlambert]

I think you can just bend a coathanger to hook on the side of the coffee can or pot, and twist it around the sender, to dip the business end of the sender in the fluid.

[MikeB]

Good ideas guys. Thanks.

[Wrenchbender Ret]

Be sure you have the correct sending unit. Chev. changed the ohms value on their temp gauges in about 1978.
George

[MikeB]

Originally Posted by Wrenchbender Ret

Be sure you have the correct sending unit. Chev. changed the ohms value on their temp gauges in about 1978.
George

The ones I have are supposedly correct for a 69, and cross over to an ACDelco G1852.

I found a chart on the Internet last year showing the resistances a guy had measured for 5 different brands, including his original GM, a new ACDelco, and three others. Resistance values were all over the place. For example, at 180 degrees the senders measured 123, 125, 135, 140, and 160 ohms, with the original and new ACDelco being the farthest apart! So, odds are whatever I buy will be different than my OE sender/gauge combo.

In any case, I'm going to test a couple senders and my new gauge on the stove top. Just need to get a good 0-250F thermometer.

[MikeB]

Installed a bi-metal thermometer in a 3/8" port on my intake manifold. Found that the temperature displayed is only a few degrees (3-4) more that what I measure at the base if thermostat housing.

Used a 2 watt potentiometer in place of a shunt resistor on the gauge, and came up with 98 ohms as a good value to keep the pointer mid scale between 180 and 210 degrees. Best I can tell the final shunt resistor needs to be 5 watt because the 2 watt pot was getting warm. Wish I could find a large selection of OE-type wire-wound resistors, but I'll make something work.

[kwmech]

Where's the resistor that is usually on the back of the gauge?

[68 P.O.S.]

+1, does that gauge not require a resistor?

[MikeB]

The stock gauge came with a shunt resistor, but I don't know the value. I actually found a 5 watt 100 ohm ceramic resistor to use, which is a little chunky.

The original looks like this. It's connected between the top and bottom studs. Bottom stud is ground.

[pjmoreland]

Some reproduction gauges are designed to operate without the stock external resistor. The external resistor on a stock temperature gauge sets the voltage applied to the winding that pushes the gauge toward cold. The sender controls the voltage to the other winding that pushes the needle toward hot. If your reproduction gauge has the resistor built into it internally, then adding an external resistor will decrease the total resistance between the cold winding and ground, which will increase the voltage applied to the cold winding and will make the gauge read cold. Adding resistors in parallel decreases the overall resistance. Try running your reproduction gauge without the external resistor.

[MikeB]

Originally Posted by pjmoreland

Try running your reproduction gauge without the external resistor.

I did that, but the pointer would barely move off the left side. I have spent several hours doing bench tests, and finally gave up. I would like to see an internal schematic, though. My latest configuration should work well, but I can't test it for real until I finish another project that I started.

Not very pretty but electrons don't care! Red wires are power for an oil pressure gauge and a voltmeter from a square body truck.

[pjmoreland]

Here's an internal schematic of a stock temperature gauge.

There's one winding (62-Ohm) that pulls the needle toward C. Its power runs through the external resistor.

There's another winding (48-Ohm) that pulls the needle toward H. Its power runs through the temperature sensor.

Reasons for the needle staying too far toward C:

1) Temp sensor resistance too low
2) Damaged 48-Ohm winding
3) External resistor resistance too low
4) Top terminal of gauge shorted to ground

Reasons for the needle moving too far toward H:

1) Temp sensor resistance too high
2) Damaged 62-Ohm winding
3) Damaged external resistor
4) Poor connection between bottom end of external resistor and ground
5) Passenger side terminal of gauge or sensor wire shorted to ground

Personally, I have seen the following issues:
-Gauge terminals shorted to the gauge cluster housing due to poor alignment between the terminals and the gauge cluster tin holes
-Damaged external resistor (infinite resistance)
-Damaged gauge internal windings
-Sensors that have their resistance go way too low as a result of the current flowing through them (this is difficult to test for because the resistance is correct when the wire is disconnected)
-Reproduction gauges that just don't behave like an OE gauge

[MikeB]

Thanks PJ. That's something I can use moving forward. I assume that's an 85 ohm shunt resistor? Meaning that when shunt resistance = sender resistance, the pointer should be at 50% of full scale?

[pjmoreland]

Originally Posted by MikeB

Thanks PJ. That's something I can use moving forward. I assume that's an 85 ohm shunt resistor? Meaning that when shunt resistance = sender resistance, the pointer should be at 50% of full scale?

When the sender is at 85 Ohms (195°), the needle is not normally at 50%. It is usually just a little bit past the second mark, as shown in this photo. I start to get nervous about the temperature of the engine when the needle is pointing straight upward. I've owned five of these trucks, and that's how all of them have behaved. I've also built and tested about 20 gauge clusters, and they all behaved that way as well.

[HO455]

Great information and timely too! The temp guage in the WMB has been reading low for a couple months now. It hasn't been a concern but, now summer is bearing down on us I need to look it into it just in case.

Here is some information on senders from a Corvette site that may be of some value unfortunately most of the photos are gone. I have some information from the thread downloaded but I is too large to post here.

http://forums.corvetteforum.com/c1-a...t-problem.html