Pet peeve - part store temp sensors

For some reason people feel compelled to replace everything they can on our Cougars with cheap crap from the auto part store or shoddy repop parts that cost a lot but don’t work right.

I commonly find that old Ford products have temperature gauges that read too hot when in fact the car is not actually overheating. The replacement sending unit is simply not calibrated correctly. I just removed probably the 25th one that I have found with this problem and decided to document the differences.

The one on the left is made someplace overseas. It is sold by many companies and many part numbers as a direct replacement for our cars. A 11/16" socket will not fit. It measures 18MM across the flats of the hex.

The one on the right is Fomoco P/N C6DZ 10884-A. It was used in 1968 on 302 and 289 engines installed in Cougar and many other Ford products. Notice it has a white plastic insulator around the stud.

Ford calls for a C6DZ 10884-B for 390 and 428CJ but those are very hard to find. The -B version has a red plastic insulator. I find it interesting that the same gauge would use two different sending units. In any case the functional differences between the two Ford products is minimal compared to the totally screwed up aftermarket sender.

I did tests at room temperature (75 degrees in my shop) and at 115 degrees. Both temperatures were checked using a digital thermometer.

The OEM Ford unit was 300 ohms cold and 100 ohms at 115 degrees.

The repop part measured totally different. At room temperature it reads 190 ohms. At 115 degrees it reads 50 ohms. So it reads warmer on the gauge at any temperature and is not even close to what an OEM sender reads.

Fascinating and damning stuff. Thanks!

I have a Motorcraft in my 302. It’s later, but not current production. Might be a Mexican-made one.

Great info royce and this info can apply to other senders like the fuel sender as well. I have wanted to test mine since I first saw one of your posts about these being way off. PO replaced the one on my car at some point in the past with aftermarket. I assume at the same time the intake was replaced. Thanks for posting the OEM base line readings.

Its odd that the first thing people will do when they think they have an over heating engine is replace the sender, in reality that is typically the less likely issue. I blame the kid at the autoparts store, you know the one…He always says the check engine light is caused by the O2 sender! :laughing:

The other issue that will plague these older cars is parts availability. If that sender is discontinued by ford your stuck trying to find a good used one or NOS on ebay so people just go with the repro and move on to ignore the gauge or wonder why it never reads right.

Thank you. I’m digging that vintage analog meter you have. Neato! I have the same story with my radator fan. Pulled the aftermarket plastic junk fan off and replaced it with the proper 390 fan and clutch. Works better in every regard. You get what you pay for.

Thanks for taking the time to do this, it has been needed. Maybe post it also on ConcoursMustang there was recently someone trying to solve ‘hot’ running with buying a new sensor.

But you really need to get a digital meter. :stuck_out_tongue:

I bought that meter new from a MATCO truck around 1980. It is actually a Simpson made product. It is a very handy tool since it also has an AC ammeter shunt that can measure battery starting current. It measures ignition dwell and can act as a tachometer too. Many things that you don’t see available in one meter in the Fluke product line for example.

Heathen. Next you’ll be suggesting home amplifiers shouldn’t have tubes.

@Royce - great info and timely as I just went through this issue on mine.

Posted over there yesterday. So far as I know there is not a new sensor that works properly available anywhere.

Totally. Is that meter battery powered or needs to be plugged in to a wall socket? Digital meters are focused more on general microelectronics and they would rather sell you an external shunt and an oscilloscope. :laughing: I have seen a few modern automotive meters but they are basically an ohm meter with a duty cycle feature for testing fuel injectors etc… It is what it is.

Adding a data point, WCCC gives a rating of 15.2-19 Ohms Resistance At 220 Deg F for the version they sell.

Back when I needed a new one for my CJ, I found one at NAPA with that same rating. I noticed when looking today that NAPA no longer gives the rating, so perhaps that part is no longer available. In any event, it seemed to be accurate as my needle went into the red zone when I got caught in stopped highway traffic after cruising at 70mph in 90 degree weather. Ahhhh… memories (was a great trip notwithstanding the heat issues).

The problem with the sending units is that the part inside that measures the temp is a thermistor, a resistor that changes value depending on temperature. As it warms up the resistance drops. Most of these are sold based on the resistance at about room temperature. This is obviously way out of the expected operating range of the device in your car. The next issue is that the required change in resistance is not linear. (the value for cold is not double the middle reading, and the value for hot is not half the middle reading. So you need a thermistor that has the correct curve to match the gauge.

I have to believe that the correct thermistor is no longer made so they use something “close enough”. Or they are just not interested in doing the work to sample and test thermistors. There is a specification that is used to describe the curve but I can say from experience that it is inadequate for this purpose.

I wonder what it would cost to get the right thermistor produced again and then have them built up into accurate senders? I am guessing that a finished sender would end up being expensive as the custom thermistor would probably be $$ and you would have to buy a ton of them to get them made. I think that $40 would the very top dollar.

That is actually a huge range. The gauge is really very sensitive in the mid to high end of readings

One other complication is that the boiling point of water is a function of elevation. For ever 1 pond of pressure the boiling point goes up by three degrees. Mixing coolant with the water will raise it even more. In general it seems that with a properly working radiator cap (13 psi) boil over at sea level was above 250 degrees F with water and about 260 with a water coolant mix.

With the cost of some reproduction parts that are only “close enough”, I would have to say $40 would be a bargain for a sending that works properly.

I paid $90 for one. $40 would be a great deal and I’m sure lost of guys need them.

It would be very helpful if we could get a NOS sender hooked up to an ohm meter while being heated up in a pan of water with a candy thermometer so we can plot the resistance and temperature readings. If any one has one that they would be willing to loan for testing I would appreciate it. I would like to get several examples to see how consistent they were originally.

How practicable would it be to modify the gauge to match the sender …
Thus having a matched accurate set ?

You could possibly get the gauge to read correctly at one temp, but the non linearity of the thermistor would make it incorrect every where else.