John Burns Posted February 15 Share Posted February 15 Hi all! It has been wayyy too long since I've been on these boards. I am trying to get my factory temp gauge to work properly. I searched the site and there is almost an overwhelming amount of information about these gauges. I didn't see anyone with my specific issue. Here goes: I have a the factory temperature gauge inside an otherwise highly-modified gauge cluster! The temp gauge goes to 210 and sits there- the motor runs at 165. I think I might have had an idiot light sending unit. I got a "correct" Lectric Limited sending unit and now the gauge never comes off of 100 (I'm not using teflon tape!) I pulled the gauge and made sure everything is properly connected. I bench tested (and tested in the car) using various resistors and it behaves properly. (100 ohms reads about 160°, and 47 ohms reads something like 250°.) I called Lectric Limited and the guy told me my gauge won't read unless the temp is over 180°. Is that true? He said the factory gauge is only there to tell if the car is running hot. My 68 Camaro factory gauge certainly doesn't operate this way. Is there a sending unit that will enable the gauge to work below 180°? Am I missing something obvious? I'm not an electrical expert, but I've done this long enough to be dangerous. This one is hurting my head! (One other thing I'll mention: the resistor on the back of the gauge is a wire type that is 90 ohms. Most posts discuss a ceramic resistor that reads 80-83 ohms. I can't image a 7 ohm difference being the issue. My guess is this wire resistor is a leftover from 1969 that they used up on early 70s Montes. This car was built September 69.) Help - Thanks! John Quote Link to comment Share on other sites More sharing options...
jft69z Posted February 15 Share Posted February 15 Something doesn't sound right with what Lectric said, about only reading over 180, makes no sense. What is the resistance reading of the Lectric sensor, sitting at room temp, etc? You've basically function checked the system with resistors to sweep the dash gauge, that's a good sign. I'd maybe suggest you give Shannon a call at 'Redline Gauge Works'. He may be able to help steer you in the right direction. https://redlinegaugeworks.com/ If you call him, let us know what he said. On a side note... since you're in Delaware, any chance we could talk you into coming to the Eastern meet in Carlisle this year, mid June? Found this too, said his resistor was 90 ohms: https://www.chevelles.com/threads/70-ss-temperature-gauge-resistor.53172/ https://www.chevelles.com/threads/70-ss-temp-gauge.1018762/page-2?nested_view=1&sortby=oldest 4 Quote Link to comment Share on other sites More sharing options...
DragCat Posted February 15 Share Posted February 15 Quick question, are you using the "original wire harness" for your dash gauge? Just wondering because of the note from Lectric Quote Link to comment Share on other sites More sharing options...
DragCat Posted February 15 Share Posted February 15 3 hours ago, jft69z said: On a side note... since you're in Delaware, any chance we could talk you into coming to the Eastern meet in Carlisle this year, mid June? Not that it helps you now, but I know someone who could bring you a standalone gauge for testing in Carlisle. Might be cheaper just to order one from Summit/Jegs test and return. 1 Quote Link to comment Share on other sites More sharing options...
Dtret Posted February 15 Share Posted February 15 2 hours ago, DragCat said: Not that it helps you now, but I know someone who could bring you a standalone gauge for testing in Carlisle. Might be cheaper just to order one from Summit/Jegs test and return. No test and returns. That’s just wrong. Quote Link to comment Share on other sites More sharing options...
Scott S. Posted February 15 Share Posted February 15 Hi John, I had similar issues finding the right sender for my '70 as well. The first sender i had was an "idiot light" unit that reacted the same was. It took a couple tries... Apparently the quality of sending units has really declined... But I found one made by Dorman that works good. Best of luck in your search, Scott. 2 Quote Link to comment Share on other sites More sharing options...
John Burns Posted February 15 Author Share Posted February 15 5 hours ago, jft69z said: Something doesn't sound right with what Lectric said, about only reading over 180, makes no sense. What is the resistance reading of the Lectric sensor, sitting at room temp, etc? You've basically function checked the system with resistors to sweep the dash gauge, that's a good sign. I'd maybe suggest you give Shannon a call at 'Redline Gauge Works'. He may be able to help steer you in the right direction. Hey hey- thanks for the reply! The resistance at 70° is 1200 ohms, but speaking to Mike at Lectric Limited, he told me that room temp isn't a really good indicator of how the sender is going to operate in the operating range. He was a very helpful too and expanded on the "won't work under 180°"... it is more like: you can't really trust the reading below 180°. So I broke out the wife's pots and boiled some water to test the resistance. At 160° I got 165 ohms. This resistance isn't low enough to move the gauge. (At 200° I got 105 ohms.) So I called Redline and I'm not sure if I spoke to Shannon, but whoever it was was very very helpful! He said they recommend NAPA's sending unit. I'm not saying the Lectric Limited unit is wrong or bad- I think there is a lot of variation in the sending units and 50+ year old gauges. I prefer factory gauges and I know they are rarely dead nuts accurate, but they are consistent. Redline also gave me this chart: 100° (bottom scale) - 355 ohms 160° (1/4 scale) - 122 ohms 210° (1/2 scale) - 83 ohms 3/4 scale - 65 ohms 250° (full scale) - 55 ohms Anyway, I'm off to NAPA to pick up the sending unit. I'll let you know if it works! 3 1 Quote Link to comment Share on other sites More sharing options...
John Burns Posted February 15 Author Share Posted February 15 11 minutes ago, Dtret said: No test and returns. That’s just wrong. I won't I promise! Bad karma for sure! 2 Quote Link to comment Share on other sites More sharing options...
Scott S. Posted February 15 Share Posted February 15 Hope they have what you're looking for...... There's just so many things at a NAPA store I'd like to drag home. lol Quote Link to comment Share on other sites More sharing options...
John Burns Posted February 15 Author Share Posted February 15 6 hours ago, jft69z said: On a side note... since you're in Delaware, any chance we could talk you into coming to the Eastern meet in Carlisle this year, mid June? I will put this on my "I want to do this" list and see if I can make it happen. My "I want to do this" list keeps expanding and available time keeps shrinking!! Quote Link to comment Share on other sites More sharing options...
jft69z Posted February 15 Share Posted February 15 2 minutes ago, jrb70454 said: I will put this on my "I want to do this" list and see if I can make it happen. My "I want to do this" list keeps expanding and available time keeps shrinking!! Well if nothing else, reserve the room ASAP, because the available block of rooms is dwindling fast. Then, cross off all the things on your list that are above Carlisle, simple.😁 For more life tips, just ask, I have plenty of them . 1 1 Quote Link to comment Share on other sites More sharing options...
jft69z Posted February 15 Share Posted February 15 22 minutes ago, jrb70454 said: Redline also gave me this chart: 100° (bottom scale) - 355 ohms 160° (1/4 scale) - 122 ohms 210° (1/2 scale) - 83 ohms 3/4 scale - 65 ohms 250° (full scale) - 55 ohms Anyway, I'm off to NAPA to pick up the sending unit. I'll let you know if it works! That's good info to have! I'll be using that when I get this customers 69 Z/28 running soon. One of his complaints was the temp gauge doesn't work, and I already purchased a Standard Motor Products sensor, TS-6. If that doesn't work, I'll look at a NAPA sensor, but it could very well be the same thing, though they list Echlin as their brand of choice, part # TS6469 I'll use those above values with a resistance decade box though, and run it thru the range to check the gauge itself. 1 Quote Link to comment Share on other sites More sharing options...
DragCat Posted February 15 Share Posted February 15 2 hours ago, Dtret said: No test and returns. That’s just wrong. Guess I forgot the 🤣 on the end. Oh well 🙃 2 Quote Link to comment Share on other sites More sharing options...
Dtret Posted February 15 Share Posted February 15 40 minutes ago, DragCat said: Guess I forgot the 🤣 on the end. Oh well 🙃 lol. 😂 2 Quote Link to comment Share on other sites More sharing options...
John Burns Posted February 16 Author Share Posted February 16 17 hours ago, jft69z said: That's good info to have! I'll be using that when I get this customers 69 Z/28 running soon. One of his complaints was the temp gauge doesn't work, and I already purchased a Standard Motor Products sensor, TS-6. If that doesn't work, I'll look at a NAPA sensor, but it could very well be the same thing, though they list Echlin as their brand of choice, part # TS6469 I'll use those above values with a resistance decade box though, and run it thru the range to check the gauge itself. Interesting about the 69 Z/28- I have a 68 Camaro and I swapped the gauge sensor into the Monte to see what it would do. It did read but it was very low. I have an AC Delco sensor G1852 on order from Amazon. One review says "This sensor measures about 550 ohms cold and about 30 ohms hot which is what GM gauges of that vintage require." It is supposed to be delivered late today- and it is supposed to snow tonight (grrrrrrr). I did the hot water test on the NAPA unit and the resistance was similar to the Lectric Limited, so I didn't even try it. I'm going to write a "final conclusion" post when I figure this out (I'm going to get this right or die trying! ). Maybe because my car is so early, it needs a different sensor than later FGMCs? 2 Quote Link to comment Share on other sites More sharing options...
jft69z Posted February 16 Share Posted February 16 10 minutes ago, John Burns said: I'm going to write a "final conclusion" post when I figure this out (I'm going to get this right or die trying! ). Maybe because my car is so early, it needs a different sensor than later FGMCs? Good, it sounds like you'll be finished long before I get this car done. Some of those links I added earlier also made reference to the resistor on the back of the gauge. Sounded like different colors were used as a 'calibration' for the system, but I didn't read that much into it to see if it was accurate. Something else to consider though, if trying to fine tune the reading. 1 Quote Link to comment Share on other sites More sharing options...
Chevy Dave Posted February 16 Share Posted February 16 After installing a reproduction gauge and a new ACDelco sender, I read 210 degrees with an actual 185 degrees. I then replaced the sender with a Lectric Limited one and everything now works perfect. I think your Lectric Limited sender is defective. 1 Quote Link to comment Share on other sites More sharing options...
John Burns Posted February 18 Author Share Posted February 18 My simple conclusion & fix: I replaced the factory 90Ω resistor on the back of the gauge with a 100Ω. I simulated the sending unit with resistors from 220Ω to 47Ω and it behaved properly. (Temperature increases as resistance decreases.) Here is what I learned from this adventure: (And please feel free to rebut, correct or clarify my ramblings- I want to document this before I forget it in a few days... I have a couple of other projects coming up. I'll refer back to this to refresh my memory.) The initial problem with my gauge is that it would go to 210° and sit there- even though the engine was running at 165°. I'm pretty sure this is how an idiot light sending unit behaves with a gauge. I installed a Lectric Limited sending unit and the gauge wouldn't come off of 100°. The resistance wasn't dropping enough in the sending unit to move the gauge needle. I tried a sending unit out of my 68 Camaro and at best it would just come off of 100°. I tested 3 different sensors using hot water and watched the resistance behave properly, but none of them would drop low enough to operate the gauge. My gauge needs a 100Ω resistor vs. the factory 90Ω- but why? I'd guess the age of the mechanism in the gauge is the culprit, but I have no idea why changing the resistor corrected this. These gauges are hardly an exact measurement! There are variations in the gauges and sending units. BUT, once they are correct, they are very reliable as a reference- you get to know where the needle is when the engine is running normally. If it starts to creep up (or peg hot), you have issues! No wonder they didn't have actual numbers on temp gauges in the 60's. I'd bet the temp gauges with numbers gave mechanics fits. I don't buy "don't use teflon tape" on the sending unit. Unless you don't know how to apply teflon tape, use it. I bought 2 new sensors and they came with tape already on the threads. Take a resistance reading after you install the sensor if you aren't sure. If you aren't used to measuring resistance with a multimeter, take your time! You have to dig into the contact points with the leads and make sure the number you come up with is repeatable. Measuring voltage is quick- take your time with resistance. Taking the temp gauge out of these cars is a painful experience. Do as much troubleshooting with the sending unit first. If you determine you need to remove the gauge, take your time. Tiny screws and nuts that are hard to access, delicate parts... walk away for a while if you get frustrated. If your gauge doesn't work- what are the symptoms? It sits at 100° and won't go up as engine temp increases: 1) Make sure the wire is on the sensor 2) Test the gauge at the sending unit. With the key on (engine doesn't have to be running), ground the sending unit wire and see if the gauge goes to full hot. If it does, then the gauge is most likely wired correctly. 3) Test the sending unit. Measure resistance ohms (Ω) using a multimeter between the post of the sending unit to ground. Measure this with the engine cold (you will get a high Ω reading). Get the engine hot and measure it again- the resistance will drop to somewhere in the range of 120Ω to 150Ω at an operating temp of 180°. 4) Test the gauge using resistors in place of the sending unit. (You can get a pack of a 1000 resistors with a wide range of resistance for $10 from Amazon. Also make sure you have a bunch of alligator clip test leads ($6 for 20 on Amazon). My gauge needed very low resistance to operate. Lower than the sending unit was able to reach. As I mentioned above, I replaced the 90Ω resistor on the back of the gauge with a 100Ω resistor. The gauge reads too high. (This seems to be the most common problem I read about.) 1) Make sure you don't have an idiot light sending unit. My first sending unit had very high resistance (1700Ω) at room temp. I haven't verified if this is correct for an idiot light sending unit or if it was broken. It would respond eventually as the engine warmed up moving the gauge to 210°, but this was incorrect. 2) The resistance from the sending unit is too low in the operating range. This seems to be a common issue with replacement sending units. Someone makes a variable resistor that can be installed in-line with the sending unit wire to increase the resistance up to 50Ω. OR decrease the resistor on the back of the gauge (opposite of my cure for a low reading). I'm assuming this will work, but I didn't try it. The needle flops around. The gauge either doesn't have proper voltage or ground. 4 Quote Link to comment Share on other sites More sharing options...
jft69z Posted February 18 Share Posted February 18 Thanks for the follow up John, great work! 1 Quote Link to comment Share on other sites More sharing options...
1970mcss Posted February 18 Share Posted February 18 Great follow up and writeup on your problem. I'm not good at testing with a multimeter but always learning 1 Quote Link to comment Share on other sites More sharing options...
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