Temperture gauge is pegged on high

[scotts72heavyhalf]

So to begin with this is a project truck that has been sitting for 4 years and I have been chasing down poor grounds and corroded connections with great success so far, we got the engine running and noticed no temp reading and when I checked the sending unit it was broken so today I replaced it with a new one a felker brand and as soon as I turned the key on it went stright to the far right pegged at high, it comes down a 1/4 when its cold then right back up. The engine seems to be running a bit warm but no overheating at idle, new thermostat installed and changed the coolent too, the upper hoses are hot so fluid is moving through the system. Bad gauge? or ? it appears that everything is correctly connected but seems like a ground short. Any ideas??

[72chevyk10]

sounds like the sender is the wrong type for the guage.

[scotts72heavyhalf]

it looks correct for a gauge not a light, how do I tell? Can I test the gauge somehow? NAPA part#TS6469 By Ekhert

[67swb72klb]

most times if the guage is pegged the wire is grounded or the motor is realy hot

[VetteVet]

First off you need to verify if the gauge or the sender is the problem. Here's how.

Unplug the sender and turn the key on, With the wire not touching anything, the gauge should read dead cold, now touch the wire to a good ground and the gauge should go to full hot. If you get those readings then the gauge is good and you need the correct sender.

If the gauge continues to read hot with the wire off then the wire might be grounded someplace by being pinched somewhere.

If it reads cold then measure the resistance between the sender terminal and the body of the sender for very low resistance. If it reads less than 50 ohms then the sender is bad or it might be for a different year. Also check the resistance of the sender body to the block for very low resistance like 10 ohms or less. You must have low resistance between the sender body and the center terminal for the gauge to read hot and high resistance for it to read cold.

If I remember correctly I read that the sender should read 500 ohms on the high end when the temperature should read cold and 200 ohms on the hot side when the temperature heats up. Of course the lower the resistance the hotter the reading.

These readings may not be correct but I seem to remember those numbers.

[scotts72heavyhalf]

Thanks, I will check those tomorrow and post a follow-up, My guess is that the green wire to the sender is grounded somewhere, its missing most of the insulation on it.

[scotts72heavyhalf]

Upate on the sender, so I replaced the heatsheild that covers the sender wire and replaced the wire lead conector and now it works but reads higher that it should, truck is not running that warm and it seems that the heater is too cold but tha is another problem I believe. Where would I get the ceramic resistor to replace the smaller on that is 90ohms? Then I think it will be fine.

Of course now I need to replace the fuel sending unit, it does not wook either, too bad it sat for so long but at least it all will be working when were done.

Thanks

[VetteVet]

Originally Posted by scotts72heavyhalf

Upate on the sender, so I replaced the heatsheild that covers the sender wire and replaced the wire lead conector and now it works but reads higher that it should, truck is not running that warm and it seems that the heater is too cold but tha is another problem I believe. Where would I get the ceramic resistor to replace the smaller on that is 90ohms? Then I think it will be fine.

Of course now I need to replace the fuel sending unit, it does not wook either, too bad it sat for so long but at least it all will be working when were done.

Thanks

Here is a list of the readings on the gauge resistors.



as you can see the resistor is about 60 ohms not 90 and I have two clusters with the temperature gauges and they both read near that. Have you read the resistor with an ohm meter? At 90 ohms the gauge will read hotter.

I'm puzzled why the ceramic resistor is 90 ohms unless the gauge is for a 195* engine. Are you sure you got the correct sender for the engine?

[scotts72heavyhalf]

Thanks Vettevet,

Im trying to follow all the suggestions and I have not tried the Ohm meter on the gauge but I will today and see what the reading is, Its the only sender that NAPA showed for the 72C10 with a gauge and this cluster was one that was not installed when we bought the truck as a project so I dont know what truck it had been in a 6 or 8 or even what year, just trying to sort it all out.

Thanks for all your help, with it I have solved most of the issues and am leaning along the way.

Ill post a update.

[Seventy Two]

So having the same trouble as the subject line, i ran the test as per post 5 and determined that i need a proper sending unit. BUT now the needle wont come off of pegged, even after i disconnect the battery!
Any thoughts or suggestions?

[VetteVet]

Originally Posted by Seventy Two

So having the same trouble as the subject line, i ran the test as per post 5 and determined that i need a proper sending unit. BUT now the needle wont come off of pegged, even after i disconnect the battery!
Any thoughts or suggestions?

Pegged how? to hot or to cold.
The gauge will read at it's last point when power is removed so that is normal.

[Seventy Two]

As described in this forum some time ago, i disconnected the lead at the sending unit,turned the key on and the gauge read cold. This is qood. Then i grounded the lead and it pegged the temperture on hot. This is good because it tells me i have the wrong sending unit.
The problem now is the the gauge has remaimed on hot even after i disconnected the battery. Without power why does the gauge remain on hot?

Help!

[VetteVet]

Originally Posted by Seventy Two

As described in this forum some time ago, i disconnected the lead at the sending unit,turned the key on and the gauge read cold. This is qood. Then i grounded the lead and it pegged the temperture on hot. This is good because it tells me i have the wrong sending unit.
The problem now is the the gauge has remaimed on hot even after i disconnected the battery. Without power why does the gauge remain on hot?

Help!

As I said in your post on the other forum, these gauges remain on the last reading when power is removed.
Try reconnecting the battery and removing the sending wire without touching anything with it and the gauge should read full cold.

[Seventy Two]

I tried that. Unfortunately the needle is still pegged on hot.

[Seventy Two]

BTW, this all started after an engine swap, the temp gauge was constantly pegged. Prior to the swap the temp gauge worked just fine. Not sure how I managed to get the gauge to fall to cold during the testing but now again it is pegged on hot.

[72c20customcamper]

Already stated

[tutone]

The threads are the ground for the sensor. No tape and the gage will need a proper ground. I hope that helps and it may not.

[dmjlambert]

What was it like before you touched the wire to ground? With the engine running and at normal operating temperature, did the gauge read cold all the time? I was just wondering what caused you to want to take loose the sending unit wire in the first place. I suppose you could have damaged the gauge, but I'm thinking they are normally pretty tough. If you connect the wire back to the sending unit on the engine and start up the engine, what does the gauge read now?

You can test the sender by using an ohm meter connected between bare metal somewhere on the engine, and the sending unit terminal. How many ohms do you get whit the engine cold, and how many when warmed up?

[Seventy Two]

Typically it has been pegged on hot ever since I did an engine swap. Before that the gauge worked just fine. I was hoping to resolve the issue when I put in my tachometer conversion kit as it came with a new electrical contact sheet for the back of the instrument cluster. During the course on installing the cluster I somehow managed to get the gauge to read cold, so that is when I decided to try testing as per VetteVet suggestions. They make sense. When I got to grounding the sending unit the gauge pegged to hot (as it should have) but now it will not come off of hot, even with the batter disconnected, the sending unit disconnected.

I understand the gauges on these trucks simply freeze themselves wherever they were when the engine is shut off, but the engine has been stone cold.

At this point I got 450 ohms with a cold engine. I will measure the ohms tomorrow after warming it up.

I will be starting it up tomorrow and let you know how it goes.

Thanks

[Seventy Two]

After the engine was nice and warm I measured 350 ohms on the temperature sender. Thought it would be more. At this point I will check the ohms on the temp gauge it self and see what I get.

Its a struggle.

[gmc684x4]

Is the sender for your model of truck ?
Example 96 99 vortec 350 engines use a different range temp sender then the 67 72 trucks

To make one work you would need to install a 67 72 temp sender in intake or thermostat housing

To use in stock location one needs to have the threads cut down as a vortec head has smaller port for temp sender

Your guage is proubly fine i think you have the wrong range of sender?

Sender range and guage have to match or they will not display correctly

What motor transplant did you install?

[gmc684x4]

Here is an example guage has not been powered up in 15 years still reading the temp it was running before it was parked and the engine swaped to a different truck

[Seventy Two]

Mistake in my last post, I measured 350 ohms and thought it would be less not more.

GMC68. It sure looks like the right gauge. The engine swap was a vintage 350 for a 350 crate engine and the dealer new it was going into a 72.

I agree the gauges just stop at the last reading but when it is powered back up it should adjust to the current situation right.

I think Im going to pull the gauge and see if I can bump it off pegged hot and then use jumper wires directly between the battery and gauge to test full cold and full not and see what comes of it. Maybe it is grounded lead.

Hears hoping.

[Seventy Two]

BTW, my needle is pegged on the right side of the H.

[gmc684x4]

User davepl posted this on a 70 shortbox senfing unit reading

http://67-72chevytrucks.com/vboard/s...d.php?t=725468