New guy with a question

[Chub380]

Hey guys Iam new here thanks so much for all the info Here's a little background on me I have had 3 trucks in these yeAres an 83 shortbed 4x4 a 87 reg bed 4x4 and now this 77 reg bed k10 bought the truck from original owner who had it since new he hasn't driven it for about 10 years so far I did a tune up cleaned the carb changed intake a valve cover gaskets change front and rear diff fluid transfer case fluid fixed speedo fixed gear indicator so on sat I was working on finding a vac leak on the heat/air system found it got everything bolted back up grabbed my daughter and headed to town to pick up some missing bolts that I had found while working and as I was looking at the gauges the temp gauge pinned all the way past right like about 6 o clock so I wasn't really to worried because if it were a prob I think it would have went up slower so when I got home I took the wire off the sending unit and the needle went back to cool then grounded it and it pinned to the right I am thinking that's normal correct? This should mean the gauge is ok and the sending unit is bad? That's what I was thinking so I did a little digging and realized you could check the resistance on the sender so I did and it was changing not sure if the values were right or not tho any way I was thinking of just changing the sender because. It's a good place to start a tom ever see one go like that or does anyone have anything to add I really appreciate any input regarding this before I tear into it on sat well thanks again chub

[Irish_Alley]

The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω
let the engine cool get an ohms reading then start it up checking it then plug the sensor in and see where the needle points. it should point to the right line regarding the oms reading. you might need to get a new sensor and what i did was put it in a pot on the stove and warm it up getting a ohms reading as it warmed up then plugged it into the gauge to see if the gauge was reading right

[Chub380]

I will try that ,I know the values were dropping but I don't think that much tho I will try again thanks for the values to look for in relationship to the gauge

[Chub380]

Ok really confused now tried the resistance test tonight here are the results motor off for 4hrs but ambient temp pretty high for Montana this time of year about 90 even toward night let it warm up awhile and it settled a 125/26 so Iam guessing the sensor is bad? Well here's the kicker before when I unplugged the wire from the sending unit with the key on the gauge went back to cold now it's locked on hot no matter what so I am at a loss is it grounded some where. Any help would be appreciated also while on this subject where does the volt meter pick up its current as that is not working either does anyone know if there is a fuse for these gauges on a 77 thanks for the help

[bd]

You're describing an intermittently grounded temp sensor wire.  The factory engine temperature sensor is located in the left cylinder head between the #1 and #3 spark plugs.  The dark green wire that connects the sensor to the dash gauge generally runs from the firewall bulkhead connector located below the w/wiper motor, around the back of the brake booster to the rear of the intake manifold, then along the left side of the intake toward the front of the left valve cover, up over the top of the valve cover and down to the sensor. 

Inspect the full length of that green wire for any chaffing to ground.  Pay particular attention to anyplace the wire makes a tight bend around metal brackets, and check for melting/burning of the wire insulation in proximity to the intake and where the wire drops down to the sensor past the exhaust manifold.

If you decide to replace the temperature sensor, be sure to acquire the correct service replacement.  Many of the available sensors are incorrectly calibrated and provide an inaccurate temperature reading.  Refer to Correct temperature sender and connector by Rich Weyand.

Your fuse box should have a 4-amp fuse adjacent to the lower flasher marked "CLSTR FEED" that powers all of the gauges and voltmeter.  However, the voltmeter uses an unshared ground lead that runs from the cluster connector to a ground splice in the harness that is then separately grounded to the cab sheetmetal.  Make sure you have good electrical connections between the gauges, printed circuit board and cluster harness connector.

[Chub380]

Thanks on the way to work this morning I hit a bump and the gauge reset with I disconected so maybe it's just sticky also was going to pick up a sending unit at napa after work so it may not be the right one I will have to do some checking thanks again

[Chub380]

Okay here is an update replaced the sending unit and put all new coolant and new radiator hoses still had factory originals anyway the gauge still doesn't work took the dash apart today to get to the gauge had it out it is there any way to test the gauge or should I just get another I am just hoping it's not in the wiring harness is there any way it could be the printed circuit ok just hoping to get it back together and working thanks again for any tips chub

[Chub380]

Ordered a new gauge I am hoping it fixes it

[bd]

Have you looked for that grounded wire?

[Chub380]

Are you talking of the 2together  under the gauge cluster

[bd]

Based on your description so far, I'm thinking the dark green wire that connects the temperature gauge to the sender is scraped, cut or burned and touching ground somewhere between the gauge and the sender.  The problem could be under the dash if the wire runs too close to a steel bracket or the brake pedal arm, or it could be under the hood.  It could also be grounded by some bare metal resting up against the printed circuit on the back of the instrument cluster. 

Take some time with a flashlight and follow the harness from the back of the gauge cluster to the firewall plug behind the fuse box.  Then continue from the firewall plug on out around the back of the power brake booster and along the driver's side of the intake manifold to where the wire drops down over the top of the valve cover to the sender.

Pay real close attention to anyplace the wire makes a bend around metal brackets, or runs close to sources of high heat like the intake and exhaust manifolds, or if the wire is darkened and feels brittle.

[Chub380]

Thanks a lot I have been wandering about the printed circut ;under the hood the wire goes into a loom near the back of the engine I put a small loom on the rest of it in case it had torn insulation even tho I didn't see any tears ect could putting the new gage in hurt it if there is a short somewhere I really appreciate you trying to help thankS again

[bd]

The symptoms you described don't match a short circuit, so installing a new gauge shouldn't damage the gauge.  You need to look closely at the wiring under the dash that connects to the cluster to see if it is rubbing against metal somewhere.  And, while you are there, inspect for anything that might be pressing against the back of the cluster and printed circuit.

[Chub380]

Ok will do , you don't happen to know which of the three pins on the gauge represents the green wire if I knew that I could check continuity from the clip in the engine bay and the terminal connector in the gauge cluster that should tell me if the green wire is bad correct? The funny thing about this is I physically watched the gauge pin when it happened over a week ago I will look in detail more this weekend about the things you described thanks

[Chub380]

Thanks bd for all the help got the new gauge in and everything works also got a new voltmeter and it's all good to just got finished putting new brakes on the back today