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Discussion Starter · #1 ·
hi, i could use some help. my 84 has had a few elec. bugs ( minor melt down under dash ) most i have resolved but i cant get the temp gage to work. it has a 75 350 in it and no more e.c.m. most all not used wires have been removed except the green ones.i have tried grounding all these to peg the gage and get nothing. however one wire that comes from the firewall plug has power when the key is on. i have always thought that the sending unit operates with a ground. my buddy touched it to the sender unit and the gage went up to around 200degrees.is this correct ? and poss.that its the wrong sending unit?if this is not correct i will have to wire the gage sender terminal direct to the sender unit on the block. exactly where does this green wire come from on the harness? thanks for any help. carl
 

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In my 85 (gone now) I believe the temp wire came from a loom near the front of the intake manifold.

Doug
 

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i have tried grounding all these to peg the gage and get nothing. however one wire that comes from the firewall plug has power when the key is on. i have always thought that the sending unit operates with a ground. my buddy touched it to the sender unit and the gage went up to around 200degrees.is this correct ? and poss.that its the wrong sending unit?if this is not correct i will have to wire the gage sender terminal direct to the sender unit on the block. exactly where does this green wire come from on the harness? thanks for any help. carl
Hey there,

Almost all of the gauges (temp, oil, fuel, and tach) need three connections: a 12v power supply (+) & (-) in addition to the connection they measure through. The reason why one of your dash harness leads is hot with key ON is that this is the wire that feeds all of your gauges their initial power. They share a common 12V feed as well as a common ground.

this graphic has been coming in handy these days:


So what happens is that the gauge is energized with 12v to power on. It then measure the resistance to ground through a third connection -> sender -> ground in engine block. If your sender and gauge are not calibrated to work together across the range of temps than you won't be seeing any readings that mean anything.

I cannot speak for cars later than 82, but it has been my experience that the sender for temp gauge resides in an engine head. On my 80 V6 it resides between cylindes 2 & 4. On my 70 Big Block and a 82 Monte LS V8 (dont know size) it was between 1 & 3. These are one wire jobs and I cannot speak at all to cars with ECMs or multi-pronged senders. Also, there is a difference between temp sender and temp switch. The sender has a gradual lessening of resistance as it heats. The switch simply hits a point to say "I'm hot!!!" to turn on a light.

my buddy touched it to the sender unit and the gage went up to around 200degrees.is this correct ?
Doesn't sound right nor does it sound like the wisest diagnostic method. You want to test your gauge? Okay... do this:

Gauge at rest, no power


Gauge with power


Hook up to give gauge power red = (+), black= (-)

First give the gauge 12v power. This is the gauge on my bench connected to a motorcycle battery. When it has power, the needle will deflect to the left and rest below 100F. If your gauge does this than there is at least some hope it works as it is reacting the way it should to power. Shorting the gauge to ground and this point will show needle pegging to the right.

To test how your sender acts with the circuit you need to hook it up this way:


When the gauge is powered up, the third connection (green) goes to sender's terminal and then a ground is wrapped around the body of the sender and then connected to the battery ground, I use a stripped piece of 14gauge romex for home wiring. At room temps the gauge still shouldn't move at all. but if you gently heat the sensing end of the sender you should see the temp gauge rise. I suggest a hot water bath to do this, but the careful application of a match far away from the end of the sender can also work. If you want to be scientific I'd heat a pan of water to boiling, and while it boils on the heat dip the sender into the bath and watch where the gauge needle ends up. You should read around 212F because under non-pressurized conditions water will not get any hotter that 212 while still liquid. If your gauge reads higher or lower, that you probably have the wrong sender or a faulty gauge and/or sender.

Hook up to test gauge with sender red = (+), black= (-), green = (variable resistance to ground, higher temps = lesser resistance)


I do not fully understand the method you are working with to investigate your problem. I cannot say if your gauge or sender are faulty or even correct for the application. But if you follow these steps, you should be able to get a better idea and report back to us.

Hope this helps.
 

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You can find your wiring diagrams here on the site >>http://elcaminocentral.com/showthread.php?t=27190
Go to the link, then the sheet for your year. It will give you the wire color, where it goes to as well as from. (Schematics) Print them, and view them while trying to trace them. Makes the job alot eaisier
if you have a sense of direction, and know what your looking for.
(I would think that if I knew anything, but as you know, What do I know) :dontknow:
 

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Discussion Starter · #5 ·
thank you guys. michaelj you are awesome. posting pics.yes i understand the way the gages share - & + and have each there own sender . i think i should wire that temp. gage by it self.this is the second cluster& pod i have tried. how i dont want to do this again you dont know but i suppose i need to pull the pod & cut the temp. gage out of the printed circuit board.any body think of a better way to do this? can it be done by only removing the cluster ? thanks carl
 
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One thing to check; if your 1975 350 engine had just an idiot light connected on the dash, there would be two connections to the temp sender. The sender is just an on/off switch. If the dash had a sweep gauge, there would be one wire. The sender would present a variable signal to ground.
 

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to CF:
If you didn't want to cut the PCB, you could pop out the spring terminal clamp that holds the PCB to the pod plastic. Then just crimp or solder a loop connector to a wire, screw it to the proper terminal post on gauge housing, and finally feed the other end of that wire through the firewall. Be sure to insulate the wire well at possible leak points (heat shrink over the loop connector and wire where it passes through the pod plastic past the PCB.)

All in all, CF I do think you're gonna have to remove the pod one more time. At least these pods are much easier to get at than the dash of a 70-72. You have to move the whole dash in order to access the gauge pod. Been there, no fun.

to peteyx72:
Are you sure about the two wire temp switch? what is the second wire for? the electrical circuit normally grounds into the engine head or block (making its return to battery via frame.) Electrons are waiting at the idiot light for the go ahead to make the journey to ground at the switch. I cannot see a reason for a return journey (i.e. second connection on switch) unless some other function is being triggered, like kicking on an electric fan as well as turning on the idiot light. Or maybe it's ECM related?

The sender shown above is actually a temp switch for idiot lights from my 80 V6, which is a plane jane carb car - no ECM at all. I have also performed this swap on two Big Blocks (adding a temp gauge) and their idiot light switches also only had one connection. As Coyote says... "but what do I know?" Anything is truly possible and I'm always eager to learn.
 

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Discussion Starter · #8 ·
do you guys think it is best to remove the temp gage , since i have 12v @ the assumed green sender unit wire, and wire the gage to 12v,on w/ key ground and sender?also the spring connecters mentioned above. are they removable or should i just pull the whole pod and do this the right way by cutting the p.c.b. thanks you guys. carl
 

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The Green wire that comes from the temp. switch on the Motor CAN NOT BE HOT with 12 volts. Voltage on that Green wire comes from the Insturment Cluster. If that wire is not connected to the cluster, as you have stated, it will be a dead wire.
Now on the other hand, the other end of that green wire it was cut from, or at,
would indeed have voltage, and it comes from the Cluster Plug or connection if you will, that plugs into the back of the cluster. The only way the wire from the temp switch can be hot, if it's been cut, is if another wire has been attahed to it at some point that is hot. Look at the Schematics as I suggested. We are all trying to help you, but to work, it has to be right. You don't want to hook up wires that are not correct,
and find your Elky Burnt to the ground later because of a short. As stated many / many
times in other posts, check it twice, check it three times, fix it once, and fix it right.
NOTE:::::: Other green wires would be Inst. lights / high beam indacator light / a right turn / Direc, sw as well a an Ign wire.
 

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Discussion Starter · #10 ·
thanks again Coyoteon2,i will just remove the pod & cluster ,remove the spring clips and wire it direct. i agree something has shorted the orig. feed wire. i dont fit to well under that dash board so i am not going to even try to find the short. thanks carl
 

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Your Welcome Carl, I just didn't want you to make a costly goof (maybe).
You understand my point what I was saying in that "You have a switch, (temp), that the body of the switch is ground, (screwed into block), only one wire (green) coming from that switch through the firewall, WHERE WOULD IT GET IT'S 12 VOLTS FROM?,
if it is only hooked to the switch, and ends where it is cut. "ONLY from a power source that has been tied to it somewhere between the cut and the switch" Don't think that would be a good idea, and have the temp gauge work properly, hence a short, hence "BAD NEWS". So I agree with you in wireing it seperate if getting under the dash is not an option. But also unplug that green wire at the switch and insulate it, it's hot remember as you stated, and run a new wire to the switch from the Gauge.
Hope this helps and makes sense to you, and you get your issue resolved.
CUZ what do I know :dontknow:lol
 

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CF
You only need to pop the ONE clip that holds the far left terminal of the gauge, as seen from behind (green alligator clip shown here.)

You need all of the rest of those clips for the various other connections of power, ground and leads for the other gauges.

I realize now that this original passage is faulty based on the discussion above, but I'm leaving it unedited because it accurately describes how the gauge circuit SHOULD work.

Hmmm, after reading Coyote's post a second time "Where would get its 12v from?" It got me to thinking... from the gauge. The sender lessens its resistance so that more and more current can flow to ground. The current is there waiting to be unleashed (like anarchists at a G20 rally.) It is the sender that opens the nozzle of the hose. The gauge, therefore, acts more like a amp meter showing how much current is flowing to ground through the sender, but calls it a temp reading. So if you take a volt reading from the sender wire directly to ground you should see voltage. Yes?

So I did a test using the setup shown above. Instead of my green lead shown running to the sender, I connected a multimeter to it and then grounded the meter to battery ground. Guess what? 8.75 volts shown. So the gauge has an internal resistor to bring 12v down to the 9v range (that battery is a little on the weak side.)

Conclusions. If you are seeing a full 12v on your sender wire something is wrong. You've stated that there was some sort of melt down with your harness, so I would follow Coyote's advice and wire in a direct link to sender from the gauge. After the original sender wire is disconnected, measure it again for voltage. There should now be none, but if there is we've further evidence of bad mojo going on and you had best insulate that wire or risk a short and/or fire.
 

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Upon a third and fourth read:

f it is only hooked to the switch, and ends where it is cut. [/COLOR]"ONLY from a power source that has been tied to it somewhere between the cut and the switch" Don't think that would be a good idea, and have the temp gauge work properly, hence a short, hence "BAD NEWS".


The original sender wire is cut? And still seeing voltage? How? Why? Bad Bad Bad!
 

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Upon a third and fourth read:



The original sender wire is cut? And still seeing voltage? How? Why? Bad Bad Bad!

Yeh Baby!! Now you got the picture.
But I'd STILL be concerned where that voltage is coming from on that Green wre.
CUZ What do I know? But i do want's to lern :secret:
 

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Discussion Starter · #15 ·
from the get go i didnt know witch green wire was the right one so before anything i checked to see if any was hot. one of the 2 of them not hot i suspected to be the wire from the sender to the gage. so i grounded each and nothing on the gage. my friend touched the other green wire the hot one to the sender and the gage read around 220 degrees. he did not know it was hot.but it seems it is connected to the sender and something else hot. thanks to all of you guys . i know this went on a little bit long thanks for all your patience. carl
 

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Length is not the issue my brother, getting it right is.
 
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To michaelj:

This thread hit home since I just this week swapped out the temp switch on my '72 for a gauge-type sender. The OEM switch had a two wire connector, one (key) hot and the other returned to the gauge. The new sender has just one terminal and grounds thru the threads.
That is the way it works on my '72.
 

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Discussion Starter · #18 ·
well guys i did exactly as you said(poped out the one spring connecter and went direct from the gage to sender) and the gage works. i probaly need to get the sender for a 87 305 m/c because it reads around 220 until the t-stat opens and then drops to 200. i dont remember what t- stat is in there now, thanks again to all of you. carl
 

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Glad to hear it, mate. Glad to be a help.
 

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This thread hit home since I just this week swapped out the temp switch on my '72 for a gauge-type sender. The OEM switch had a two wire connector, one (key) hot and the other returned to the gauge. The new sender has just one terminal and grounds thru the threads. That is the way it works on my '72.
Hello Peteyx72,

I believe you and my being wrong has long since ceased to be a novelty :smileyb:. I just couldn't what the reason would be for two terminals on a temp switch would be. So I did the thing I should have done originally and I pulled out the wiring diagrams for your 72 and my 70. Sure enough! There they are. The temp switches show two terminals. One obviously leads to the light/gauge and the other leads to the TCS relay. I had forgotten all about the TCS relay (because my 396 while original had been modified by a previous owner for performance. Hence removing things that don't make it go faster.)

My thought is that the key hot wire you mentioned is the wire coming from the light and is waiting for the the temp switch to get hot enough to complete the circuit to ground and send current flowing, turning on the light.

I don't quite know how the two prong sender works with TCS. There must be a two stage connection internally to the switch. I know that the TCS (transmission controlled spark) is temperature controlled. From the Hayes manual "...the temperature switch reacts with the vacuum advance solenoid to allow full vacuum advance whenever the engine temp is below 82 degrees..." & "...the TCS system is not functional and should have no affect on engine operation unil the engine has warmed up to these operating temperatures."

So there it is, the answer to the question of two prongs. There is a secondary purpose, the pre-computer, infant attempt at emissions control called TCS, a subject rarely talked about. I wonder how many working TCS systems have survived on the early 70 models. I know mine is long gone.

I hope you didn't take offense, Peteyx. None was intended. As I stated before, my being wrong has ceased to be a novelty.
 
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