Another Gauge Cluster Question or 3

[Splandman]

My suburban is an 84 has a/c, the wires for the a/c have a green wire going to the connector and a ground wire that grounded on the intake from the connector and it also had a wire with a clip that went to the carb I think it's black.

My temp wire comes out of the main harness with the starter and alternator wires right behind the block but it's not on the firewall, I can't tell what color the wire is its all greased up and weathered but the end of it I can tell its green and it has the spade connector not the peg style.

I may have found it, not sure. It was not hooked to anything and the end is cut off so I can't tell what came on there. It is a green wire and appears to be coming out of the harness.

The wire with the clip that you mention above is the one I have been wasting time on.

 

[PrairieDrifter]

I would guess it's safe to say that's the wire you need if it comes out of that harness. since you had someone hacking up wires putting different gauges in I wouldn't doubt they just cut off the connector for the temp sender.

 

[chengny]

A quick look at the wiring diagram for 1987 C/K series shows 5 green wires coming through the firewall (6 if you have A/C).

Three (or 4 with A/C) go into engine the compartment:

1. A dark green for the coolant temp sensor

2. A dark green for the horns - it splits into 2 wires at the grill

3. A light green for the RH front directionals

4. A light green for the throttle solenoid - also feeds the line side of the LP switch for the A/C compressor

Two others come through the firewall - but immediately head down and back to the rear lighting group:

5. A light green to the back-up lights

6. A dark green to the right rear directional light


The dark green that goes to the compressor clutch doesn't come through the firewall , it originates at the LP switch on the receiver/drier.

 

[chengny]

To confirm that the green wire you found is the temp gauge sensing leg, strip some insulation off the end, turn the key on and touch the bare copper to a good ground. If the gauge jumps up high when the wire is grounded, that is the one.

 

[rich weyand]

To confirm that the green wire you found is the temp gauge sensing leg, strip some insulation off the end, turn the key on and touch the bare copper to a good ground. If the gauge jumps up high when the wire is grounded, that is the one.

I would run the wire to ground through a 150 ohm resistor. If the gauge moves up to the middle of the range or so, that's the right wire. If it's the wrong wire, 150 ohms will keep you from shorting out whatever else it connects to.

 

[chengny]

The sensing leg of any dash gauge is designed to be connected to ground and it only connects to the negative side of the coil. This is my take on how a GM gauge works:


Just as a general discussion on the gauge/sender/ground relationship - and to make sure I have the theory of operation straight in my mind;

The components mentioned above form the "sensing leg" of the coolant gauge located in the instrument panel. They are connected by the green wire (gauge to sensor) and thread engagement (sensor to engine block). To complete the circuit, the engine block is connected to the battery negative post by the main grounding cable.


12 volts is supplied to the post/terminal on the coolant sender (via the green wire). It's source is the negative side of the gauge's measuring coil.

The less resistance in this total combined circuit, the greater the electron flow through the coil wires - and consequently the stronger the force that it (the measuring coil) can exert on the gauge's magnet. The magnet is attached to the lower end of the indicating needle. Something like this:

You must be registered for see images attach

In addition to the sensing leg, there is another - opposing - circuit called the "control leg". It is identical to the sensing leg in every way except that the negative side of it's coil is lead straight to ground (rather than through a variable resistance and then to ground like the sensing leg). Also, it's coil is located on the opposite side of the magnet from the measuring coil's.

So what you have is; two coils located on either side of an indicating needle - AKA the "ray". The ray is pivoted somewhere in the middle and has an integral magnet mounted at the bottom.

These coils create their own magnetic fields - the strength of which is a function of the electron flow (i.e. current) passing through them. When these circuits are energized - by turning the ignition switch to the RUN position - the opposing magnetic fields begin to "fight it out". Each coil tries to pull the magnet - mounted on the needle - towards itself.

Due to the fact that it's negative leg is run straight to ground - resulting in nearly zero resistance, the current flow through the control coil is essentially constant (as is it's magnetic force).

On the other hand, due to the variable resistance generated by the sender (as a result of changes in coolant temperature), the current flow/magnetic strength of the measuring coil can be higher or lower than that of the control coil.

The location of the two coils - to the magnet - is such that:

1. The control leg is always trying to peg the indicating ray to the left (i.e. cold) end.

2. The sensing leg is always trying to peg the ray to the right (i.e. hot) end.

So as extreme examples:

1. If the green wire to the sender is broken/disconnected the control leg exerts the greater force and pegs the needle to the left/cold end.

2. If the green wire is allowed to contact directly to ground (before passing through the sender), the sensing leg has a greater force. This causes the ray to peg out to the right/hot end.


As GM explains it (better and way more succinctly than I can):

VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.

When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil.

When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.

 

[rich weyand]

The sensing leg of any dash gauge is designed to be connected to ground and it only connects to the negative side of the coil.

Yes, of course, if you know it is the gauge wire. But in checking to see if a wire IS the gauge wire, a bit of discretion is in order, and can't hurt.

 

[chengny]

Roger that - sound practice.

But I get the sense - just from reading his posts - that Mr. Splandman isn't the type of person that has a collection of loose resistors sitting on the shelf. But he probably does have a spare fuse - in case we're wrong, and one blows when the cut wire is grounded.

Also, in an effort to help confirm that the "mystery wire" was in fact the lead to the coolant sensor, I provided a list of all the green wires that enter/pass through the engine compartment.

My assumption was; he would check that short list and compare it to his intact wiring. If he found that everything - other than the coolant sensor - was connected to it's associated green wire, he could be fairly confident that the cut wire was the coolant lead. At that point the wire could be safely grounded as a test.

 

[rich weyand]

If he has no resistor, I would use a spare bulb in the lead. Any bulb. All the bulbs in our trucks are low enough resistance to run a temp gauge up to HOT.

 

[Splandman]

But I get the sense - just from reading his posts - that Mr. Splandman isn't the type of person that has a collection of loose resistors sitting on the shelf. But he probably does have a spare fuse - in case we're wrong, and one blows when the cut wire is grounded.

You are correct, I do not have any resistors laying around. I did have several bulbs though.

I believe it is the correct wire. The gauge stays right around 100 while hooked up to the sending unit. I drove the truck four miles. Gauge never moved. Got out and touched the brass part of the sensor and it was hot. Turn truck off back to about the 11 O'clock position. Turn key on with the wire grounded using alligator clips and connecting it to the radiator support and the gauge pegs out at 260 immediately.

I know it's possible the gauge is bad but it seems strange to me that the only two gauges not working in the cluster are the coolant and the oil pressure, which are the same two gauges the previous owners disconnected to run afternarket gauges. I do have another gauge that was in the original cluster laying around.

This is pretty frustrating, I thought once I found the wire it would be hooked up pretty easily.

I have the whole cluster out at the moment so I can put the 4 wheel drive indicator light back where it goes. That is in and lighting up so at least that got done.

 

[chengny]

1. The gauge stays right around 100 while hooked up to the sending unit. I drove the truck four miles. Gauge never moved.

2. Turn key on with the wire grounded using alligator clips and connecting it to the radiator support and the gauge pegs out at 260 immediately.


Those two statements are key.

What they are telling you - with near certainty - is that something isn't right with the sender. It is one of the following two things:

1. Broken

or

2. The wrong application


Also, I'm not sure what this means:

Got out and touched the brass part of the sensor and it was hot.

It kind of goes without saying that - if a sensor is screwed into the cylinder head - it will be hot when the engine is up to operating temperature. But, the question isn't whether the sender heats up or not.

Due to the fact that it is in direct contact with the head, it is going to get hot. But that doesn't tell you anything about it's functionality.

I was wondering - and I don't mean to stir up any trouble - were you ever able to check whether the sender's resistance tracks (downward) as the coolant temp increases? That is the only true test of whether a temperature sender is good or bad.

 

[Splandman]

The gauge stays right around 100 while hooked up to the sending unit. I drove the truck four miles. Gauge never moved.

Turn key on with the wire grounded using alligator clips and connecting it to the radiator support and the gauge pegs out at 260 immediately.


Those two statements are key.

What they are telling you - with near certainty - is that the sender is bad.

Well I am really confused. This sender was just installed. I have 3 different senders at the moment.

Is it possible to test a sender, either in or out of the truck?

 

[rich weyand]

The gauge stays right around 100 while hooked up to the sending unit. I drove the truck four miles. Gauge never moved.

Turn key on with the wire grounded using alligator clips and connecting it to the radiator support and the gauge pegs out at 260 immediately.


Those two statements are key.

What they are telling you - with near certainty - is that the sender is bad.

^^^ x2

 

[chengny]

Is it possible to test a sender, either in or out of the truck?

Maybe you missed the following - it's from the previous page.

Also, I edited my last post - please re-read it. Especially the "wrong application" part.

Until Rich W. enlightened me, I wasn't aware that GM used a different sender prior to 19**. But even given that detail, I still suspect a non-functioning sender. Even using the wrong sender, the gauge might not be perfectly accurate, but it should at least move off the low peg point.


The next thing to check would be that the sensor is providing the correct variable resistance according to the temperature of the coolant. You will need a meter and have it set to ohms. Pull the sensing lead from the contact on the sensor. Using a test jumpers or the alligator clip attachments for your meter (if you have them), connect one meter probe to the sensor and the other to a good clean ground on the engine block. The jumpers/clips are only to allow you to do this hands-free. It takes awhile to do this and you don't want to be trying to hold the tips of two probes on tiny spots for 15 minutes and with the motor running.

Note: Unless you have an infrared gun to take temperatures or are ambitious enough to somehow get a test thermometer to read engine coolant temp - this test will only confirm that the sensor is providing a fairly close resistance. It wont tell you if it's perfect. You will only be able to check ohm values at room temp and operating temp. But honestly, that's enough. If I get good appropriate values at those two points - I call the sensor good.

Before starting the engine: With the engine completely cool - and knowing the ambient temperature - use the chart below and compare the resistance shown on the meter with the value shown on the chart for the air temperature. So, for example, if it's a 70 degree day and the motor is cold, the meter should read about 3400 ohms.

Start the engine and let it idle and warm up. As it begins to warm up watch the meter, the resistance should begin to drop steadily. It will drop quickly at first and then slow down as the coolant temp rises. Eventually the coolant will reach operating temp - which is dependent on the temperature rating of your thermostat. Don't use the exact number of the t-stat - I find that coolant generally runs about 10 degrees under t-stat rating. So a 195 degree stat will probably settle out at about 185. If you don't know the stat rating just use 190 degrees.

You know your truck better than anyone. So when you feel that it has had sufficient time to come up to where the dash gauge would normally stop climbing and settle out (if one were installed and functional) take another resistance reading. If you use 190 degrees as your operating temp, the meter should be reading somewhere around 250 ohms.

As noted above, this is just to check that the sensor is tracking along with the coolant temp fairly closely. If you get anything close to the values indicated on the chart - I would check the sensor off the list of possible problems.

The resistance value for the sensor - that makes your gauge read 120 when at operating temp - is about 1000 ohms. If the meter shows that value at oper temp, you can assume that the gauge is accurate, the wiring is good and that it is the sensor that is bad.

 

[Splandman]

Is it possible to test a sender, either in or out of the truck?

Maybe you missed the following - it's from the previous page.
[/I]

I did read it, just had forgotten. I assumed the sensor was good because I just got it Friday.

However, you were both correct. The sensor was bad, or for whatever reason it did not work with that gauge at all. I put the original sensor back in and let the truck warm up and it worked. Every time I tried using the original sensor I was using the wrong wire until now.

I really appreciate all the help.

Do any of you guys know what the wires in the attached pic go to and where they plug in?

Also, is the oil pressure wire in the engine bay brown? I have found a green wire that I believe is attached to the oil pressure sensor. I think they ran this wire through the firewall to the aftermarket gauge. I am trying to figure out what the original wire looks like so I can splice them and see if it will make the oil pressure gauge work now. Once I get that working all gauges in the cluster will be operational. Even the clock, but it doesn't keep time correctly.