Temp sending unit that is accurate

Murf_old · June 30, 2009 at 2:45 PM

I am looking for a replacement temp sending unit. The thread size is 3/8 pipe and normally this would take an 11/16 wrench to fit the brass hex. Seems that most units now are "generic" and put the needle on the temp gauge about 2/3 toward HOT when in reality the engine is operating between 180 - 195. Has anyone found a sending unit that has specs that are in line with those of the original Ford units that kept the needle in the center of the doal when the engine was at normal operating temps? Adjusting the CVR in the dash unit is out of the question since the gas gauge and the oil pressure gauge seem to be very accurate. I have a replacement unit in the car now and hope that someone can offer suggestions as to an aftermarket unit that is calibrated as the originals. I appreciate any ideas, or solutions any of you may offer.

289kford · June 30, 2009 at 3:56 PM

If you never find a sender that works you could try putting a resistor in line to tell the gauge the engine is colder. I don't know the ohms range off hand but doesn't the gauge go full high when the wire goes to ground? I thought maybe the ohms range would be in the book and you could get an idea of what to add to the circuit. Ford dealers used to have a gauge sending unit simulator, you could adjust the ohms on the tool and watch the gauge move around. Maybe your local dealer still has the tool.

Mu5tanggt_old · June 30, 2009 at 4:07 PM

I've noticed my OEM sending unit has a 260 stamped in it and tends to read low (1st white line in the normal band) at normal temp, I replaced it with one stamped 240 and gauge reads about 3/4 of the way to H at normal temp. I wonder if the number is related to when the gauge pegs IE: 260 degrees or 240 degrees

Murf_old · June 30, 2009 at 6:02 PM

The operating range is determined by the resistance at 220 degreesF. on the senders that NAPA sells. For instance, the sender for a 68 302 on the NAPA website says "15.2 - 19 ohms at 220 F.

Seems that present day parts catalogs have grouped a lot of applications under one listing in order to manufacture and stock fewer parts thus although they are somewhat functional, they are not accurate enough for those of us who are sily enough to restore Mustangs. I will look for a sender with the temp range stamped on it and see what is offered. Few manufacturers publish the specs on their replacement parts, or at least I have been unable to find the specs. Also it is difficult and painful to bring our engines up to the 220F. temp where the NAPA is specified to have between 15.2 - 19 ohms resistance. Has anyone found a table of specs for other brands?

Richard P. · June 30, 2009 at 6:28 PM

Murf I've had that same problem. I went to salavage yards and removed the senders out of any 1965-1968 Mustangs and other Fords with 289, 302 and 390 engines. The senders usually have Autolite and 280 stamped on the bodies. You are correct the ones sold over the counter are not normally calibrated correctly.

Murf_old · June 30, 2009 at 7:40 PM

Most of the original senders that use the 11/16 wrench to change and were used on the Mustangs seem to have "260" stamped on them. I wish that it were that simple now to locate the correctly calibrated sender. And yes, it may be that I have to incorporate a resistor in series with the sending unit to get the gauge to read as it should. Most of us here understand the quest for the correct part, and hate to use a "crutch" to make something work as it should, but it may be the only way. Will be checking out the various brands of sending units, searching for one that will make things period correct.

Murf_old · June 30, 2009 at 8:03 PM

Looked in the service manual and could find no specs on the original units. Anyone have anything such as the temperture vs resistance relationship for the Ford units?

Murf_old · July 2, 2009 at 2:44 PM

OK, today a temperture sending unit was found that allowed the gauge in the dash to read as I feel it should. This one is available from NAPA by purchasing a # "TS6628". The speds show a higher resistance at 220 degrees F. than the unit that NAPA lists for a 66 and later Mustang with 3/8 pipe thread in the intake. This unit made my gauge read just slightly less than in the middle of the scale when the water temp was 180 F. The unit that is listed as correct in the NAPA sales catalog will give a reading ABOVE the middle of the gauge, in fact about one third of the way between midrange and full scale. Talked to the guy at Echlin where the senders are manufactured about getting a unit that was accurate but he was less than interested in helping. More or less indicated that I should not be so particular and as long as it was not reading IN the hot range that they were OK with their listing. ODD, since they also have in one that WILL BE CORRECT... Anyway now my gauge reads as I feel it should and all ended will in the quest for something that is correct, not just "functional" as the guy on the NAPA tech line indicated was their goal. So if you should encounter a higher than normal temp gauge reading AND YOU ARE SURE that the water temp in your engine is what it should be you may want to try this sending unit. It cured my problem and made this a happy day. OH, and the cost of a happy day? - less then $13.00 !

6hypo5conv_old · July 2, 2009 at 6:09 PM

I have a LATE '65 (July) and it has the correct intake on it with the bushing to use the smaller 3/8" threaded sender-right now my guage is not working-my restorer said the culpret most likely was the aftermarket sender I got from NPD that is the problem, should I get anouther new sender unit from NAPA and try it? should I expect if it works then for it to read correctly?

Murf_old · July 2, 2009 at 6:27 PM

If you have a bushing in the intake than it will not use the 3/8 pipe thread sending unit. The intake with the bushing use a 1/4 inch pipe thread unit. Pipe threads are unusual in that when we say 3/8 pipe it means that the hole in the center of the pipe is 3/8". The thread size would be about 5/8 of an inch if it were measured as if it were a bolt. It is also tapered. That being said, if you drill out the bushing and do not destroy the threads in the intake you will find that the 3/8 sender fits your intake. And yes, I would expect the gauge to read correctly. You need an infa- red thermometer to see what your water temp really is BEFORE you monkey with what you have. Fixing a real problem is difficult enough - fixing a problem that does not exist is impossible.

RalphJr · July 2, 2009 at 10:00 PM

You don't have to drill out the brass threaded insert - you can use a broken bolt remover. Take out the sending unit and put in the bolt remover - one that fits down inside the bushing all the way and fits snugly - bushing should come right out.

A late '65 intake will have the bushing, but earlier intakes will not have the larger 3/8" pipe thread hole - Ford switched sometime in late '64 or early '65 from the cast small 1/4" NPT hole to the larger 3/8" NPT hole - GT cars use the larger temp sending units, but the non-GT cars used the smaller 1/4" sending unit with the bushing.

It doesn't matter which size you use - it is all resistance and you just need to use one that works correctly - AND like Murf sadi check your temps really well before during and after to make sure the guade is reading correctly.

I don't have an infrared thermo - I have a Fluke meter with a thermal couple to check the water temp - does require leaving the system open or making a special fitting - but it works. Mine is nice because it is calibrated to a 1/2 degree.

I've known old timers who used large meat themos in the radiator too.

Just find something accurate to calibrate your guage....you don't want to over heat and not know it.

bigal_old · July 3, 2009 at 6:20 AM

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>

You don't have to drill out the brass threaded insert - you can use a broken bolt remover. Take out the sending unit and put in the bolt remover - one that fits down inside the bushing all the way and fits snugly - bushing should come right out.

A late '65 intake will have the bushing, but earlier intakes will not have the larger 3/8" pipe thread hole - Ford switched sometime in late '64 or early '65 from the cast small 1/4" NPT hole to the larger 3/8" NPT hole - GT cars use the larger temp sending units, but the non-GT cars used the smaller 1/4" sending unit with the bushing.

It doesn't matter which size you use - it is all resistance and you just need to use one that works correctly - AND like Murf sadi check your temps really well before during and after to make sure the guade is reading correctly.

I don't have an infrared thermo - I have a Fluke meter with a thermal couple to check the water temp - does require leaving the system open or making a special fitting - but it works. Mine is nice because it is calibrated to a 1/2 degree.

I've known old timers who used large meat themos in the radiator too.

Just find something accurate to calibrate your guage....you don't want to over heat and not know it.

I second that. About the GT cars having the Larger Temp Sending Unit

6hypo5conv_old · July 3, 2009 at 6:21 AM

I have read and re-read the information in BOB Mannels book about the sending unit size and I have determined that my engine IS correct in having the bushing installed so it can use the smaller 1964 sending unit, the intakes for 1965s were prepped for the larger sending unit so the hot/cold sending unit for Galaxies could be used.

I hate to remove the bushing which in my case is correct as my intake has a correct date code for my car/engine, again LATE 1965 (JULY)just to be able to use the larger sending unit-is anyone having a problem with the smaller unit making the guage read correctly or is it just the larger one?

RalphJr · July 3, 2009 at 9:43 AM

You might try what Murf did - go to your local Napa - a full service one if you have it - and see if you can find a small size sender with the same specs as the one Murf found.

cobraboy · July 5, 2009 at 6:04 AM

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>

OK, today a temperture sending unit was found that allowed the gauge in the dash to read as I feel it should. This one is available from NAPA by purchasing a # "TS6628". The speds show a higher resistance at 220 degrees F. than the unit that NAPA lists for a 66 and later Mustang with 3/8 pipe thread in the intake. This unit made my gauge read just slightly less than in the middle of the scale when the water temp was 180 F. The unit that is listed as correct in the NAPA sales catalog will give a reading ABOVE the middle of the gauge, in fact about one third of the way between midrange and full scale. Talked to the guy at Echlin where the senders are manufactured about getting a unit that was accurate but he was less than interested in helping. More or less indicated that I should not be so particular and as long as it was not reading IN the hot range that they were OK with their listing. ODD, since they also have in one that WILL BE CORRECT... Anyway now my gauge reads as I feel it should and all ended will in the quest for something that is correct, not just "functional" as the guy on the NAPA tech line indicated was their goal. So if you should encounter a higher than normal temp gauge reading AND YOU ARE SURE that the water temp in your engine is what it should be you may want to try this sending unit. It cured my problem and made this a happy day. OH, and the cost of a happy day? - less then $13.00 !

Hi John

I checked my gauge today with my infa red temp reader and with the gauge needle right in the centre of the dial the top tank of the rad gave a reading of 90 deg c or 194 deg f at both ends of the tank, From your calcuations is this about right?

Also the question of whether or not to have a sender bushing, why were the GT cars singled out for a large sender unit? Thanks Mark

Edited by - cobraboy on 07/05/2009 06:33:41

Murf_old · July 5, 2009 at 6:51 AM

I feel that Ford intended the needle to be straight up when the temp at the sending unit (just behind the thermostat) was 195 F. This is the thermostat Ford used at that time. Also I think that the correct sending unit had 37 ohms resistance at 192 - 195 F. The temp sending units with the "260" stamped on their base seem to be those itended for Mustangs from the 66 on up era. Why the parts listings of today show the correct unit as something other than this I am not sure, probably just so they can consolidate applications. As to the reason for the different size sending units I have no idea. I have a 65 K GT that has always used the small unit and had a brass bushing, contrary to what others have found. It was scheduled to be built Apr 22, 65 if that is of interest. Ford did things in that era that made sense to them at that time but in retrospect leave us scratching our head. Anyway, I always measure the water temp just behind (toward the carb)the thermostat housing since that is where the most meaningful temps are found and is also where the Ford engineers intended. The temp of the water in the radiator taken with a probe under most circumstances also reflects the same information so it seems your gauge information is as it should be.

cobraboy · July 5, 2009 at 10:09 AM

That is good to know as I thought the gauge might be reading low as it normaly sits on the cool side of centre. But I feel I can trust it now. Also good to hear about your small sender. Thank you for the help.

Mark

kar-nut_old · July 17, 2009 at 6:34 AM

260* F sender appears to be logical. 50/50 mix of antifreeze & water boils at approx. 223*F combined with the 13# radiator cap which adds pressure to the cooling system(every pound of pressure raises the boiling point of the coolant 3*F)boiling point: 223*plus13x3*=262*

Murf_old · July 17, 2009 at 9:33 AM

Thanks for the explanation of the number stamped on the Ford sending units. That is the first meaningful relationship of that stamped number to temperture anyone has offered.

Murf_old · July 20, 2009 at 6:27 PM

Today I had time to fool around with checking the actual temp at the thermostat housing, taking readings behind the thermostat and at the top radiator neck with an infa red device. This happened to be ona 68 302 that I had been working on, but using the same sending unit as mentioned in this thread. At 180, the needle reads just to the left of center in the gauge. Was charging the AC so had an opportunity to run the car in the shop for a while and when the needle was straight up and down the water temp was 195, just as it was on the 65 hipo. After turning off the engine I watched until the needle reached the little vertical mark at the end of what appears to be the "normal" range. NOT all of the way to the H, just even with the high end of the operating range on the gauge. The temp at this needle position was 225 F. That was as high as it got, so really have no idea what temps/needle relationships are after that point. Just wanted to throw this info out in case anyone wants to have an idea of the actual temps using the NAPA sender. I feel it reflects an accurate indication of the engine temp. I will use that particular part from now on no matter what the parts books say.

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