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Converted 49 GMC to 12 v negative ground system with alternator. Put voltage reducers in front of both gas and temp gauges. Gas gauge fine. Temp gauge not reading. Engine is 230 CI out of a 62 Chevy. Replaced engine temp sending unit with new one that has a 'G' terminal which I think is for gauges and 'R' terminal for a light used on later models(same as was on engine originally). Have tried swapping [/LIST] both terminals to the single wire to the low temp side of the temp gauge with no reading on gauge. Gauge shows continuity when checked with ohm meter. Also will peg to high temp side if I shift polarity on back of gauge. Pretty sure gauge is good, but do have another old gauge that I can try. Have refaced my gauges so hoping to keep from replacing or buying new 12 volt temp gauge. Everything else is working fine and have had no other wiring issues. Hoping to get dash and wiring done so can move on to rest of truck. Putting together from pile of parts so new challenge every time I work on it.

jwc1949gmc,

Welcome to the Stovebolt. I looked at a diagram of your truck...I didn't know they had an electric temp gauge.

The wiring diagram shows the temp gauge power in series with the fuel gauge power...both off the ignition switch. If you have a voltage reducer ahead of the fuel gauge, and the wiring is as I described, you now have TWO reducers in front of the temperature gauge.

If this is actually how the truck is wired, it would make the temp gauge read incorrectly. I would verify the wiring, and for sure would stick with the stock sending unit if you still have it. A new sending unit would probably not be calibrated for the stock temperature gauge, reduced voltage or otherwise.

Stuart

I looked at a wiring diagram that shows the feed off of the gas gauge also. Tried it with a feed from the gas gauge and no second voltage reducer. Decided to go with a straight line off of ignition switch with voltage reducer, so have only had one reducer at a time in front of the temp gauge. I will try to find a temp sending unit from 49-50 series truck to see if that will work. Like you, I have been concerned that the new sending unit is not compatible with the temp gauge. Do the sending units come in 6v or 12v? Also have read that Ohm reading changed sometime in last 60's to 60-90 ohms vs a 30 ohm sending unit on original. Have not been able to verfiy this info however. Thanks for the ideas and I will post solution when found.

jwc1949gmc,

With electric temperature gauges, it's important to have the gauge itself properly grounded in the dash assembly. The case must be paint free and bonded to a ground, or maybe there is a separate grounding screw provided. The same holds true for the fuel gauge.

A temperature sending unit would be rated for resistance (Ohms), but not for voltage. It would be assumed that to work properly, the sending unit would be installed in a system that had the proper supply voltage the gauge was designed for.

Sending unit ratings for the fuel system were 30 ohms for the AD rigs and 90 ohms for modern systems....I don't know if this is applicable to temperature sending units or not.

Stuart

You have the wrong sending unit. That is for a light. G is green light for cold & R is red light for hot. Get a sender for a gauge & it should work. The Standard products # is TS-6, but any brand should work. It has 1 term that is flat on top & the connector comes in from the side.

What really sucks is when you put a 261 engine into a GMC and have an electric gauge. The temp ranges on the Chebbie and GMC gauges are different and the old GMC gauge does not read correctly. The sensor is an NTC resistance (negative coefficient) unit and no matter what I have tried to use in inline resistors won't make the gauge read correctly. Anyway, I've gotten used to the fact that when the gauge reads in the 140 degree range, the engine is fine. Would love to have definitive solution to this delimma!

49geezer,

I'm lost on this one. If you stick a 261 Chevy into your GMC, why not just put the original GMC sending unit back into the Chevy engine and no one will be the wiser.

Is your actual problem getting a 6 volt gauge to read properly after you convert your system to 12 volts. Does installing a voltage reducer ahead of the temp gauge gum up the works and make the gauge inaccurate?

Stuart

Stuart, my gauge is electric and the old sender on the 228 was gas filled. It was gone when I got the engine. Since the electric gauge is 12v and works, it appears to be simply a problem of getting a sensor that more closely matches the GMC gauge temp range and not the Chevy gauge temp range. If you add resistors of any value to the sensor wire, they either peg the needle or make no difference. Think that's because of the NTC sensor as one is increasing resistance rather than reducing resistance. I guess one solution would be to get a 12v Chevy gauge, cover it with a brown decal to match the GMC gauge colors and see if that works. Anyway, not a big deal as the truck runs fine and doesn't overheat.

I tried the TS-6 sending unit noted above, checked the gauges for proper grounding, tried the second gauge that I have and still could not get a reading on my old 6v gauge. Power to the gauge after the ceramic voltage reducer was 5.8 volts so wouldn't think that would be the problem. Finally got aggravated and put after market temp gauge under dash. No modifications were needed to mount the new gauge and sending unit, so can work on getting old gauge working properly later. The maintenance manual for 1947-1954 GMC Models 100 thru 400 shows that The electric type gauges were used on Series F350-24 and all other series using the capillary type unit. My truck is a 150 series so not sure if someone modified or just came from factory that way. I had gauges from two different 150 series and both were electric temp gauges. The manufacture ID plate shows both vehicle were made in mid to late 1949. Right now, I have a working after market temp gauge and will work on old gauge/sending unit later. Thanks to all for ideas and help.
1949GMCjwc
Willie

I got a TS-3 sender off Ebay. Gonna try it after the Winchester ATHS show this weekend to see if it more closely tracks my gauge than the TS-6 or whatever is in my truck now.

Hey 49 Geezer
Thanks for the update. Let me know if the TS-3 sender works. I would like to just have the original gauges, but at least have working temp gauge now.

Well, the TS-3 wasn't the right size from the supplier. 3/8" threads rather than the 1/2" threads. Got another one from him though instead. Have put it beside the other 3 senders I have and let them all get to the same room temperature, then measured their respective resistances. One varies by about 80 ohms less than the TS-6 model. Since these are NTC resistors, as the temp falls, the resistance rises. So, I guess logically, if I install the one with the lower resistance, that would equate to a higher temperature on the gauge and may possibly work for me and my situation. Will try the install this weekend to see and report.

I never did get any reading from the two sending units that I tried. It seems that I read somewhere that the later model units ran 80-90 ohms while early model ran about 30 ohms. The parts store that I checked could not tell me what the resistance was on any of the sending units. Really curious to see what works for you.
Thanks

Maybe 80-90 ohms when the engine water temp was 180 degrees. At about 70 degrees in my garage, the sensors ran from 640 ohms to 725 ohms approx.

TS-6 368 ohms @ 100 degrees. 80 ohms @ 220 degrees. TS-3 is Mopar. 120 ohms @ 100 degrees. 135 ohms @ 220.

Well, didn't get the TS-3. Got a TS-7. Set it up on my work table with a TS-6 and another Echlin unit. Most were reading about 650 to 670 at 70 degrees F ambient. Checked my older unit and it was up in the megaohm range.

Wrenchbender: Where did you get your data on the ranges and temps? Do you have a listing somewhere that shows this info? If so, I'd be interested in a copy.

I got it from the Standard Motor Parts catalog. I,ll look at tomorrow & see which catalog it is. I don,t have any way of E-mailing a copy.

WB; Did you get a chance to check the catalog? If the info is usable, I'll pay you to send me copies of the pages.

It,s called an Illustrated parts guide. It shows a photo of most parts & gives some of the applications. It tells the values & describes the use. This one is Standard/Blue streak & is from Carquest but other stores handle that brand also. Napa,s are different part numbers. This doesn,t show a TS-7 at all. If you have a Carquest store there you might see if they will get you a copy. I could copy a couple of pages & send them if you want by snail mail. My copy machine is not into my computer.

Thanks WB. Let me see if a local FLAPS has some catalogs that I can look at first. Will let you know.

Mike

Lots of good info, but I still am not sure which sending unit to use. How do I find out which sending unit is compatible with my old gauge?
Willie

Still researching and working on this as well Willie.

Is there any detail from OEM that tells where or what the resistance should be for gauge/sending unit? My manuals, etc show that the sending units for my old truck were capillary type. Both my original and later parts truck purchase had electrical temp gauges. I have not found much in the maintenance manual that I am using. Will let you know if I find better info. Thanks in advance.
Willie

49 Geezer
Did you get at chance to try the new sending unit? Curious about results. Thanks
Willie

Bought an original NOS gauge from a guy in Canada. Part number 1511960 which matches the number in my parts catalog. Found out that the original sender number is a 1511734. Been looking for that all over but no luck so far. Have just purchased a 40's and 50's parts catalog which lists various vendors and substitute part numbers. Haven't gotten it yet so will check it out when I receive and advise what I find.

Hey 49 Geezer
Any new info on the temp gauge/ sending unit issue? I have been working on other stuff with my truck, so thought I would check.
Thanks
Willie

Negative. Have the gauge and it matches all the part numbers as OEM. Catalog I bought didn't cover the truck. Been looking for a NOS temp sensor all over the internet. Finally found a guy in Canada that will sell me a NORS sensor for a Pontiac that also used a 100 to 220 degrees gauge. Great, I say! How much? At $90, I'll keep on looking. Am thinking of going to a couple of junkyards in spring to see if I can find a GMC with the engine still in with an electric gauge. Probably get the sender for $5. Until then, the search continues......

I will keep an eye out while touring the local junk yards. If I find a couple of sending units for a reasonable price, I will pick them up in case you still need one by then. Please keep me in mind if you find more than one also.
Thanks
Willie

Thanks would appreciate it.
Mike

run check to see if dash unit works,,you might have to use a pot to check ohm,s of dash and match reading with an engine unit...doc

Update on this item. Have purchased a sender from a company in MA. The guy there said it will match the 100 to 220 degrees range. Once I get it and install it, I'll respond on what happens. Hopefully, this one will be right.

49 geezer what was the out come? i temporary disconnected my electric temp. guage when i went to 12 volts and i need to put a voltage reducer on it, what kind did you use and what senser did you use and did you problem get fixed

Hey, I installed the new sender and the weather was still cold to really get the engine up to temp. I did move off of 100 to about 130 or so but I could still hold my hand on the upper radiator hose with no discomfort so I knew that the engine was running cool and never got to 180. Supposed to be warmer this weekend so will give it a try again. Also have a lead on another sender so may try that out as well. Will advise.

Hey 49 Geezer
Did you ever get a chance to give the new sending unit/temp gauges a good test?
Thanks

No unfortunately. Was going to take the truck out for a Memorial Day run but had to mow 4 acres and do other miscellaneous honey do stuff. Am going to try for a good long run this upcoming weekend so I'll get back to you. Seems like the gauge will probably still read low but we'll see. If it does, I'm going to experiment and bench test with moving the set screws on the rear of the spare gauge that I have to see if I can match it with the range of a TS-6 sender. If I can, I'll have it nailed!

Thanks for the update. Like you, I have been working on everything except my old truck. I will be looking forward to the results of your test!