Temperature Gauge Pegs Out

[crazy4offroad]

Between year breaks and the difference between diesel and gas powered trucks, tracing out the pinouts on your cluster bulkhead connector is going to be a challenge.

 

[sonicbluezx3]

Is it really that much different? I thought that the printed circuit was the same for all years. Also, it seems odd to me that all of the other gauges work just fine other than the temp gauge. I wish I knew which wires were which so I could just run new wires or something. I'm not too great with wiring and whatnot.

 

[chengny]

The sensing wire is probably grounded somewhere between the instrument cluster and the sender.

The coolant temp gauge pins out in the lower right hand corner of the clip (looking at the back of the dash) - pin #9/wire 35. It is a dk green wire.

After it leaves the gauge pc board it splits. One lead goes to the sender via the main harness connector on the firewall the other goes to the ignition switch.

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"I wish I knew which wires were which so I could just run new wires or something."

That might be the way to go. Cut #35 just before it passes through the firewall connection block and run a new wire out to the sender.

Before you go that route however, you should do a thorough visual inspection of that part of the wire that is exposed in the engine compartment.

The wire exits the loom behind the distributor and is generally run through the throttle linkage bracket and then along the the LH valve cover.

Also be certain that it really is grounded before going through the hassle of running a new lead:

Disconnect the wire from the sender (be sure it's not touching a ground) and pull the harness plug out of the dash interface. Using a multimeter, test for continuity to ground. One lead of the VOM to a good ground and the other to either the connector that goes on the sender or pin 9 at the dash connector. Zero ohms or steady beep mean the wire is dead to ground.

Might as well check it for continuity as well. Just attach one lead to the sender end of the wire and the other to pin 9. You should get low ohms or a beep if the wire is intact. O/L means it's broken.

 

[sonicbluezx3]

Okay i figured out its definitely the wiring. I looked in the engine bay and i didnt ser anything wrong with the wire. Im wondering if i hooked something up wrong when i put everything back together. I dont see how since it all goes together only one way, but who knows.

I ran a series of alligator clips from the snipped green wire right behind the connector to the gauge cluster and ran it directly to the temp gauge. I ran the truck for a bit and it moved so it appears to be the wiring. Nothing to do with the cluster since i had it all plugged in.

Im guessing it goes to the ignition so that it kills power when the truck is off. Problem is i dont know if its the ignition lead or the sensor wire that is the problem. Kinda cramped back there to follow the green wire.

 

[sonicbluezx3]

Just in case anyone wants to know, the top post on the coolant gauge goes to (-) ground, the connector on the right (looking at it from the back) is the sensor wire, and the connector on the left is (+) positive.

 

[oneluckypops]

So did you trace out the cluster to determine which terminal is what on the gauge? What about when you test those 3 terminals at the plug in for the cluster, are they getting the correct values?

Im still beleiving that your sending unit wire is grounded out, rather its from different pin locations in the cluster, or if something has happened to the wire during the installation process.

I see no reason to cut the wire before it goes to the ignition switch, so long as your key is off the circuit should be OPEN, simply un plugging the sending unit and the instrument cluster should be enough to test for a short to ground. But then again I am having troubles understanding why the sensor wire is even running to the ignition switch in the first place? What makes the gauge read cold when the key is off is the 12 volt ignition wire, when the key is off ignition voltage is 0.00v with the key on it should be system voltage.

 

[chengny]

"But then again I am having troubles understanding why the sensor wire is even running to the ignition switch in the first place?"

Me too 1luckpop. I have never had to think about it. And all I know is that it runs to the ignitoin switch. That lead can never be hot right - I mean it shouldn't be? That wire is basically a straight shot to ground (through the variable resistance of the sender). Maybe the ignition provides a direct ground when the key is in the off position.

I think, if I remember right, that while the oil pressure gauge just hangs where it was when the key was turned off, the coolant temperature indication pegs low. But that doesn't make sense either - a direct ground should make it peg high. Ah, but that's only when the other side of the ciol is hot.


Here is the schematic that includes the ignition switch and the internal connections at different positions, but I can't read it:

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[oneluckypops]

"But then again I am having troubles understanding why the sensor wire is even running to the ignition switch in the first place?"

Me too 1luckpop. I have never had to think about it. And all I know is that it runs to the ignitoin switch. That lead can never be hot right - I mean it shouldn't be? That wire is basically a straight shot to ground (through the variable resistance of the sender). Maybe the ignition provides a direct ground when the key is in the off position.

I think, if I remember right, that while the oil pressure gauge just hangs where it was when the key was turned off, the coolant temperature indication pegs low. But that doesn't make sense either - a direct ground should make it peg high. Ah, but that's only when the other side of the ciol is hot.


Here is the schematic that includes the ignition switch and the internal connections at different positions, but I can't read it:

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Well even so when the key is off the coil in the gauge is dead meaning OPEN circuit, open circuit would peg the gauge to cold on both the sensor wire, 12 volt wire, and the ground wire. Those diagrams are sure not very clear at all, i cant make them out either.

 

[sonicbluezx3]

So would it be ok to skip the ignition side and just run it from the cluster straight to the sensor?

 

[chengny]

What you could do (and this would leave you with the option of returning the wiring to a factory condition) is temporarily cut the lead to the ignition switch.

Find a section of that lead between the split and the ignition switch that has some slack. Cut it and then continue with troubleshooting your temp gauge issue.

If it doesn't help (or causes more problems) you can reconnect it with a butt splice, bullet/spade connectors or solder and insulate with heat shrink tubing.

If cutting the lead to the igniton allows the gauge to function properly you have two options:

1. Leave the wire cut

2. Investigate what is happening within the ignition switch that is causing the gauge to peg and repair it

 

[sonicbluezx3]

Well i cut the wire right behind the connector to the gauge cluster and just tested it with alligator clips directly to the temp gauge and it seemed to work okay. I cant even remember but how long does it take normally for the gauge to move above 100 degrees?

 

[chengny]

At idle, from a cold start, in cool weather - maybe 5 minutes.

Finally found a legible print of the wiring connections to the ignition switch. Unfortunately it does not show internal connections at the different key positions:

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Also found this brief (partially correct) explanation:

TEMPERATURE GAUGE - STOP IT FROM PEGGING ON START

The temperature gauge is infamous for "pegging" (violently moving to its maximum range) when the engine is started.

Why does the gauge "peg"? The factory wiring applies a full 12 volts across the temperature gauge when the ignition key is turned to "Start". This momentary connection is called the "Bulb Test" position and shows at a glance that critical warning lights are functional. Unfortunately it appears GM made a mistake and connected the Bulb Test feature to the Temperature Gauge instead of the Temperature Warning light.


So, as best as I can determine - this is why the sensing lead to the temp sender is also wired to the ignition switch:

When the key is turned to the start position, the dark green lead from the temperature sender (also the tan/white lead from the brake system) is grounded. This causes a full sweep of the gauge (and the brake warning light to illuminate) proving full functionality of that vital indicator.

 

[sonicbluezx3]

The legible print doesn't show up btw.... but thanks for the other info.

So this seems like a gauge sweep to me.... what mine does is it doesn't return back to the original position. Is this saying that the green wire doesn't need to go to the ignition? It can just go straight to the gauge?

 

[chengny]

The legible print doesn't show up btw....


What - doesn't it open for you? Or, it opens but isn't clear?

Could someone let me know what the problem is and I'll repost it or clean it up. It's the best schematic of that part of the electrical system I've ever seen - hate to lose it.


Opens right up and looks great from my end.

 

[crazy4offroad]

Looks good to me too. His browser is probably resizing it to fit the screen. He either needs to click the screen for the bigger view or press Ctrl and + to zoom in. Or save it locally and open/view it.