trouble with temp guage - The 1947 - Present Chevrolet & GMC Truck Message Board Network

nturner · 03-19-2004, 02:43 PM

I'm having trouble getting my temp guage to work on my 72 Cheyenne. Brand new cluster and new 350/330hp crate motor. The hole for the sensor on the new motor is smaller than the stock motor so we need a new sensor. The old stock sensor shows 45 ohm so we need a new one the same with 45 ohm resistance with 5/8 threads. So, when we hook everything up, we get zilch. Does anyone have a wiring diagram of this circuit ( could also use a diagram of the a/c circuit ) or have a clue as to what we may be overlooking?? Thanks

jhow66 · 03-19-2004, 05:46 PM

With switch on take wire off sensor ground it and see if gauge goes to full right. Are you checking it with motor at full temp? At full temp gauge should read to the 1st mark --approx. 180* Was your harness for gauges or lights?

bpmcgee · 03-19-2004, 06:18 PM

Nturner,

In my experience, there IS no 5/8 diameter sensor that is the right resistance for our gauge. If there is, I would DEARLY like to know the part number.

I'm in exactly the same situation. My solution has been to design a small adapter that sits on the back of the gauge and converts it to work with new senders.

To test your gauge and circuit, just turn the ignition on and touch the sender wire to a good ground -- gauge should go full hot.

Brian

nturner · 03-19-2004, 06:51 PM

To jhow 66 --- the harness was the engine harness, we didn't change the light harness as it looks good. When we started with the truck wires were going everywhre, and we still havn't figured out why the a/c stays on whenever the fan is switched on even tho we're asking for heat. Maybe this is how it's supposed to work unless we don't know about a switch somewhere. Thanks ... To bpmcgee -- What kind of adapter did you use??? I was thinking all we needed to do is find a 5/8 with similar or slightly less resistance and maybe wire in a resistor of the difference. What are your thoughts --- could the guage be recalibrated to work with something else?? THANKS, Brian!

bpmcgee · 03-19-2004, 07:17 PM

Nturner,

The adapter is a small circuit board that I designed to effectively "recalibrate" the gauge. Right now I have a prototype board that's pretty ugly, but I have to install it and see how the practical application corresponds to the bench testing.

The problem is that the newer senders are not only significantly HIGHER resistance, but also have a much steeper curve. We spent a week or two working on it, and this is the best solution I could come up with.

Brian

nturner · 03-19-2004, 07:30 PM

Hey Brian --- we're also thinking of 2 ways to fix the problem -- thinking out loud. First, how about an adaptor to the block so it'll take the original 72 sensor (3/4 " ?) and secondly get a guage from a new chevy truck and retrofit it to the 72 guage faceplate.. Might be worth a look.. If you come up with anything or if we do, let's keep in touch. Noel

bpmcgee · 03-19-2004, 07:47 PM

Noel,

If I'd realized this problem before I put the heads on, I certainly would have had it re-tapped for 3/4" NPT. As it is, I don't think you'll be able to find an adapter that takes it UP.

As for the gauge, I suppose that's a possibility. It would be interesting to see if later model gauges would be able to fit into the existing dash and circuit board.

Here's the technical info: These gauges are a Wheatstone Bridge circuit. This circuit requires 4 resistors wired in a specific way. Two of them are internal to the gauge, one is a ceramic resistor on the back of the gauge, and the fourth is the temp sender. If you found a gauge that had internal resistors of about 80-90 ohms (as opposed to 40-50 ohms of ours), that would work in the circuit. Whether or not it could be made to fit, of course, is a different matter.

Brian

jhow66 · 03-19-2004, 11:34 PM

nt' if you took out a no gauge cluster and replaced it with a gauge cluster and used the same harness to plug into your new one that is your problem (wiring is not the same). Do a search under dash wiring--someone shows how to change it. As to your AC there is a switch on the back of the box that changes the air flow that cuts the AC on on off--out of adjustment in the on all the time . Do a serch under AC wiring--there is a picture showing where it is.

Duallie · 03-20-2004, 01:38 AM

Just a small sidenote: the older temp senders are 1/2 pipe thread. I don't have a newer sender to measure.

bpmcgee · 03-20-2004, 03:02 AM

Duallie,

I think you'll find the old one is 3/4 NPT and the new is 5/8 NPT.

Brian

da-burb · 03-20-2004, 08:28 AM

There is one that fits that smaller opening. What I do remember about it is, the correct sending unit was from a Volvo, late 60's or early 70's. It was the correct ohms for our gauge and a smaller fitting for the head. It was on this board so you still might be able to find it. I found this one but it mentions 3/8 threads. http://67-72chevytrucks.com/vboard/s...threadid=50015

What I also remember was this guy spent a fair amount of time at NAPA going through the parts book looking for the correct size and ohms range. If the volvo one isn't the correct one, then you might have to do the same.

Duallie · 03-20-2004, 04:00 PM

bpmcgee: 3/4 pipe thread is over an inch in diameter, and the older senders use a 7/8ths wrench, which is smaller than 1".

Another sidenote: there is no 5/8 NPT.

tom hand · 03-20-2004, 05:46 PM

the older ones are 1/2 npt and the newer ones are 3/8 npt. Most aftermarket intakes have a 1/2 npt hole in the water jacket so the simple solution would be to put it there

bpmcgee · 03-20-2004, 05:47 PM

Quote:

Originally posted by da-burb

What I also remember was this guy spent a fair amount of time at NAPA going through the parts book looking for the correct size and ohms range. If the volvo one isn't the correct one, then you might have to do the same.

Da,

That guy was me, and I haven't found one yet. I haven't tried the Volvo one -- any idea more details?

Brian

Rakort · 03-20-2004, 07:19 PM

If you need to see the difference between the idiot light dash connector and the gauge dash connector I have made an AutoCAD drawing of the entire electrical schematics for trucks with either system. I'd be glad to post a pic of the schematic for the connectors if desired.

vlturner · 03-21-2004, 02:05 PM

Ok here is some very good info on what the problem was. When you look at the current wiring diagram, you will not that it reflects a ( temp sw ) and is connected to green wire that travels throught the firewall connector to the ignition switch and then on to the gage cluster. If you look at the diagram at the cluster connector, you will see it on the second tooth in of the connector, and the first as a ground. The diagram is only listing the temp light option, and not a gauge. In order gor the gauge to work, their has to be a individual wire that travels from the cluster to the temp sender "WITHOUT" a connection at the ignition switch.

Now I am going to look into the volvo sender that was referenced in the message threads, but the problem is that the sender must have the proper base resistance at the fixed start temperature, and the proper swing through the temperature band. from around 180 - 225 degrees. Our current solution was that we got the temp swithc and idiot light working on the normal wiring harness, and added a wire to the cluster connector with a stock copper connecter to the first position that shows a ground connection on the base diagram and ran it directly to the tempo sensor we located in the water passage at the themostat housing. We had a spare gauge cluster that was sent with the connector and 2 inches of the wire still in tack. I just pulled the green gage wire, denoteded it with som red heat shrink on each end, and routed through the firewall to the gauge. All is now well. Both the gauge and light are functional.

What I think though is that there was some factory modification of the ignition switch that it had no internal connection to positive voltage on the gauge version, and they moved the connector in the shell to the position that connected to the gauge. But once the ignition switch was changed out, this problem started to show up. This was the optional guage could use the existing wiring loom without any serious changes on the assembly line.

If someone will send we the autocad diagrams they have of the wiring harness, I use visio, and can make the revisions to fix the diagram. I wouls also like to add the AC circuit and fix the compressor always on. I will start a thread on the AC issues.

Now that I have a little time in the wiring diagram, I can answer most of the question people are stuck on. I do have a little background in complicated electronic circuits.

My email address is vance.turner@sbcglobal.net

Thanks for the help

03-19-2004, 02:43 PM	#1
nturner Registered User Join Date: Nov 2003 Location: Stockton, CA Posts: 31	trouble with temp guage I'm having trouble getting my temp guage to work on my 72 Cheyenne. Brand new cluster and new 350/330hp crate motor. The hole for the sensor on the new motor is smaller than the stock motor so we need a new sensor. The old stock sensor shows 45 ohm so we need a new one the same with 45 ohm resistance with 5/8 threads. So, when we hook everything up, we get zilch. Does anyone have a wiring diagram of this circuit ( could also use a diagram of the a/c circuit ) or have a clue as to what we may be overlooking?? Thanks __________________ Vagabond '55

03-19-2004, 05:46 PM	#2
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	With switch on take wire off sensor ground it and see if gauge goes to full right. Are you checking it with motor at full temp? At full temp gauge should read to the 1st mark --approx. 180* Was your harness for gauges or lights? __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

03-19-2004, 06:51 PM	#4
nturner Registered User Join Date: Nov 2003 Location: Stockton, CA Posts: 31	To jhow 66 --- the harness was the engine harness, we didn't change the light harness as it looks good. When we started with the truck wires were going everywhre, and we still havn't figured out why the a/c stays on whenever the fan is switched on even tho we're asking for heat. Maybe this is how it's supposed to work unless we don't know about a switch somewhere. Thanks ... To bpmcgee -- What kind of adapter did you use??? I was thinking all we needed to do is find a 5/8 with similar or slightly less resistance and maybe wire in a resistor of the difference. What are your thoughts --- could the guage be recalibrated to work with something else?? THANKS, Brian! __________________ Vagabond '55

03-19-2004, 07:30 PM	#6
nturner Registered User Join Date: Nov 2003 Location: Stockton, CA Posts: 31	Hey Brian --- we're also thinking of 2 ways to fix the problem -- thinking out loud. First, how about an adaptor to the block so it'll take the original 72 sensor (3/4 " ?) and secondly get a guage from a new chevy truck and retrofit it to the 72 guage faceplate.. Might be worth a look.. If you come up with anything or if we do, let's keep in touch. Noel __________________ Vagabond '55

03-19-2004, 07:47 PM	#7
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	Noel, If I'd realized this problem before I put the heads on, I certainly would have had it re-tapped for 3/4" NPT. As it is, I don't think you'll be able to find an adapter that takes it UP. As for the gauge, I suppose that's a possibility. It would be interesting to see if later model gauges would be able to fit into the existing dash and circuit board. Here's the technical info: These gauges are a Wheatstone Bridge circuit. This circuit requires 4 resistors wired in a specific way. Two of them are internal to the gauge, one is a ceramic resistor on the back of the gauge, and the fourth is the temp sender. If you found a gauge that had internal resistors of about 80-90 ohms (as opposed to 40-50 ohms of ours), that would work in the circuit. Whether or not it could be made to fit, of course, is a different matter. Brian Last edited by bpmcgee; 03-19-2004 at 07:49 PM.

03-19-2004, 06:18 PM	#3
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	Nturner, In my experience, there IS no 5/8 diameter sensor that is the right resistance for our gauge. If there is, I would DEARLY like to know the part number. I'm in exactly the same situation. My solution has been to design a small adapter that sits on the back of the gauge and converts it to work with new senders. To test your gauge and circuit, just turn the ignition on and touch the sender wire to a good ground -- gauge should go full hot. Brian

03-19-2004, 07:17 PM	#5
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	Nturner, The adapter is a small circuit board that I designed to effectively "recalibrate" the gauge. Right now I have a prototype board that's pretty ugly, but I have to install it and see how the practical application corresponds to the bench testing. The problem is that the newer senders are not only significantly HIGHER resistance, but also have a much steeper curve. We spent a week or two working on it, and this is the best solution I could come up with. Brian

03-19-2004, 11:34 PM	#8
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	nt' if you took out a no gauge cluster and replaced it with a gauge cluster and used the same harness to plug into your new one that is your problem (wiring is not the same). Do a search under dash wiring--someone shows how to change it. As to your AC there is a switch on the back of the box that changes the air flow that cuts the AC on on off--out of adjustment in the on all the time . Do a serch under AC wiring--there is a picture showing where it is. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

03-20-2004, 01:38 AM	#9
Duallie Registered User Join Date: Feb 2001 Location: Nikiski, Alaska Posts: 477	Just a small sidenote: the older temp senders are 1/2 pipe thread. I don't have a newer sender to measure. __________________ Just as if I knew what I was doin'

03-20-2004, 03:02 AM	#10
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	Duallie, I think you'll find the old one is 3/4 NPT and the new is 5/8 NPT. Brian

03-20-2004, 08:28 AM	#11
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	There is one that fits that smaller opening. What I do remember about it is, the correct sending unit was from a Volvo, late 60's or early 70's. It was the correct ohms for our gauge and a smaller fitting for the head. It was on this board so you still might be able to find it. I found this one but it mentions 3/8 threads. http://67-72chevytrucks.com/vboard/s...threadid=50015 What I also remember was this guy spent a fair amount of time at NAPA going through the parts book looking for the correct size and ohms range. If the volvo one isn't the correct one, then you might have to do the same. __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

03-20-2004, 04:00 PM	#12
Duallie Registered User Join Date: Feb 2001 Location: Nikiski, Alaska Posts: 477	bpmcgee: 3/4 pipe thread is over an inch in diameter, and the older senders use a 7/8ths wrench, which is smaller than 1". Another sidenote: there is no 5/8 NPT.

03-20-2004, 05:46 PM	#13
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	the older ones are 1/2 npt and the newer ones are 3/8 npt. Most aftermarket intakes have a 1/2 npt hole in the water jacket so the simple solution would be to put it there __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

03-20-2004, 07:19 PM	#15
Rakort Registered User Join Date: Jul 2001 Location: Neenah, WI Posts: 327	If you need to see the difference between the idiot light dash connector and the gauge dash connector I have made an AutoCAD drawing of the entire electrical schematics for trucks with either system. I'd be glad to post a pic of the schematic for the connectors if desired. __________________ Rakort Parts for Sale: Pictures Awesome Elec. Schematic Available: Picture Awesome Elec. Schematic Available: Picture

03-21-2004, 02:05 PM	#16
vlturner Registered User Join Date: Mar 2004 Location: foo Posts: 1	The problem was found with some insight Ok here is some very good info on what the problem was. When you look at the current wiring diagram, you will not that it reflects a ( temp sw ) and is connected to green wire that travels throught the firewall connector to the ignition switch and then on to the gage cluster. If you look at the diagram at the cluster connector, you will see it on the second tooth in of the connector, and the first as a ground. The diagram is only listing the temp light option, and not a gauge. In order gor the gauge to work, their has to be a individual wire that travels from the cluster to the temp sender "WITHOUT" a connection at the ignition switch. Now I am going to look into the volvo sender that was referenced in the message threads, but the problem is that the sender must have the proper base resistance at the fixed start temperature, and the proper swing through the temperature band. from around 180 - 225 degrees. Our current solution was that we got the temp swithc and idiot light working on the normal wiring harness, and added a wire to the cluster connector with a stock copper connecter to the first position that shows a ground connection on the base diagram and ran it directly to the tempo sensor we located in the water passage at the themostat housing. We had a spare gauge cluster that was sent with the connector and 2 inches of the wire still in tack. I just pulled the green gage wire, denoteded it with som red heat shrink on each end, and routed through the firewall to the gauge. All is now well. Both the gauge and light are functional. What I think though is that there was some factory modification of the ignition switch that it had no internal connection to positive voltage on the gauge version, and they moved the connector in the shell to the position that connected to the gauge. But once the ignition switch was changed out, this problem started to show up. This was the optional guage could use the existing wiring loom without any serious changes on the assembly line. If someone will send we the autocad diagrams they have of the wiring harness, I use visio, and can make the revisions to fix the diagram. I wouls also like to add the AC circuit and fix the compressor always on. I will start a thread on the AC issues. Now that I have a little time in the wiring diagram, I can answer most of the question people are stuck on. I do have a little background in complicated electronic circuits. My email address is vance.turner@sbcglobal.net Thanks for the help



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