[Ivy66GT]

Previously I posted technical details of the oil pressure sender units. I see repeated questions about the other instruments as well so I think I need to post details about them also. Today its the temperature senders, specifically for my '66 Mustangs, but it is applicable to all Fords up until the gauges changed sometime in the 80s.

Attached is a plot of the sender resistance versus temperature in degrees F. The horizontal temperature scale is linear but the vertical scale is logarithmic (Google it if you don't understand) which makes the plot nearly a straight line. A great many temperature sensing resistors have a similar straight line characteristic. To help relate this to a Ford temperature gauge I have added bold horizontal lines that indicate the resistance at which the gauge needle should point to the lowest mark (Cold = dark blue). The very bottom of the plot is 10 Ohms which is the very highest mark on the gauge (Hot). In between these extremes is a marked operating range marked on the gauge. I have indicated the bottom, center and top of that operating range with the bold Yellow, Orange and Red horizontal lines.

The data is for 3 different temperature senders measured in water heated and measured both with immersion thermometers as well as my calibrated IR temperature gun. Since I live on a mountainside at 6150' elevation, water boils just below 200F for me on most days. That explains why the plots all end just below 200F since none of us can have unpressurized water in a pan that is above its boiling point. I could have added antifreeze to raise the boiling point but this was difficult enough as it was.

The first sender I measured (dark blue line - on top) was a small, pre-66 Ford unit (1/4" pipe thread) marked with 250. These were intended to peg the temperature gauge at 250F and you can see that the 3 points I added at the bottom of the plot do indeed extrapolate that senders straight line to the point where the gauge would be pegged at 250F. Those 3 points were not measured but added to demonstrate what the 250 on the side of the sender should mean. Later units were marked 260 which would have shifted the lines to the right so that hotter engines would still have a temp gauge reading near center scale.

 

[Ivy66GT]

The yellow plotted points are the first sender I received from one of the Mustang parts houses. Since a randomly selected part from any of the other sources for these senders would likely not have been any better or worse I won't mention where it came from but it cost $6. When I contacted them and said that it wasn't quite what I thought it should be they sent me a replacement which is the maroon data points. It was somewhat better and is the sender I am now using in my engine.

Note that all the senders start out around 300 Ohms when at room temperature. (The highest resistance point shown is at 315 Ohms. The lowest resistance for the yellow plot is 19.8 Ohms.) Although 75 Ohms, or so, is supposedly the low end of the range for a Ford gauge, the needle actually starts moving at about 100 Ohms; that movement is veeeeery small but it does move. The Cold line I drew on the plot is actually at 78 Ohms which is a water temperature somewhere around 120-130F. Only above that will the temp needle seem to be telling you anything.

Both of these recent senders were consistently to the left of the Ford sender. They work nearly identically to the Ford unit but will cause the needle to indicate a hotter temperature than was intended. The first sender (yellow) shows about 25F hotter and the 2nd one (maroon) about 10F hotter in the range around 190F where the engines would normally operate. I wanted my temp gauge to indicate center scale (Orange) when at operating temperature. The second sender was pretty close to that so I added 2.5 Ohms in series with the sender which is indicated by the light blue plot between the dark blue and maroon lines. You can see that with the resistor the 2nd sender is almost identically what the Ford unit would have given me. I couldn't use the Ford unit since its the small pipe thread and wouldn't fit the '66 manifold I am using.

When operating as Ford intended you can see that the 5 temperatures I indicated on the plot are fairly evenly spread out over the range. The resistances for those indicated lines are 78, 50, 26, 16 and 10 Ohms (if you count the bottom of the plot as the last line). For the Ford sender those were roughly 130, 160, 200, 230 and 250F. Since Ford thermostats were 192F in those days that made the needle point pretty much to the center of the gauge but also allow the needle to tell you when you were approaching the boiling point of the pressurized antifreeze in the engine, i.e. 250F.

From what I read nearly all the new senders seem to read high. Provided they aren't too far off in left field you can shift them down to read like an original by adding a small resistor in the line from the sender to the gauge head. If you have to add as much as 10 Ohms, such as I would have had to do for the first sender, then you may limit how far to the right the gauge can move. The less resistance you have to add the more likely it is you won't run into such problems.

 

[Ivy66GT]

With a Ford gauge needle pointing to the right of its gauge there is not quite 1/4 Amp of current flowing through the sender and gauge. Using the resistor power formula, P = I*I*R, will tell how hefty a resistor you might need. For a 2.5 Ohm resistor that works out to less than a 1/4 Watt resistor so a half Watt or larger should work just fine. The larger the resistance value used, the higher its wattage needs to be.

 

[buzzboy]

So if I'm sitting at a light and it climbs to 3/4 that means it's hitting 200°? Ish?

 

[Ivy66GT]

Unless its a screwy sender I would say yes, maybe even a little hotter depending upon several things. Not a problem. With both pressure and antifreeze in it, its a long way from 200 to boiling. Its perfectly normal for any engine to heat up when sitting at idle since there is little air going through the radiator. On my newer cars with calibrated temp gauges its not uncommon for me to see 230F under those conditions even though it runs at about 195 otherwise.

An IR reading on the top of the thermostat housing at the front of the intake manifold is a fairly accurate way to measure the actual temp of the coolant. That will tell you if your gauge is anywhere close to accurate. My 'new' sender said the engine was overheating. My IR gun correctly said the sender was screwy.

 

[B Smith]

For an older (1966) engine, what is too hot? At what point should I be concerned?

 

[Ivy66GT]

By older do you mean Jan '66 as opposed to Dec '66?

I don't know that 'older' makes any difference on temperatures. They originally ran with a 192F thermostat which doesn't fully open until 205F or so. All '66 Fords had factory 50/50 antifreeze and with the Mustang 13 psi cap that means the coolant wouldn't boil until nearly 250F which was at the very top end of the gauge.

I have never had a problem with any 289 I ever owned overheating whether it was a new one in 1968 or my current ones today. A SBF is not all that temperature sensitive like it would be if had aluminum heads, etc. Its a cast iron boat anchor that can take quite a bit of abuse. My wife once blew a radiator hose on the freeway in AZ (summertime) and to catch me (I was driving ahead of her) to tell me about it she drove ~5 miles at 80 with the temp gauge pegged. Didn't bother the engine a bit. Let it cool, put on a new hose, filled it up and continued with our move to California.

An occasional low 200s is not a problem for these engines. They were designed to run that way. Since the gauges are not super accurate I don't know how high they actually get at times but 210-200 wouldn't surprise me. As I have said before, my 'new' Al-head Mercedes normally runs around 190F but when idling in the summer with A/C, etc., it easily gets up to 230F+ on a regular basis. Has done that since it was new, 20 years ago. Its not a problem as long as it cools down when you aren't sitting at idle.

 

[waid302]

Resurrecting very old post

Ivy66GT,

I am almost done with my Explorer 5.0 swap in my 1964 Ford Falcon.

What is the correct temp sender should I be using and make it work correctly with my instrument cluster ?

I have some senders marked Motorcraft 270 and measures about 300 ohms at room temperate. This one screws right in to my aftermarket Weiand Intake.
Can I use this one? What about the correct Thermostat ?

I am also converting to electric radiator fan.

Thanks

Waid

 

[Ivy66GT]

I have some senders marked Motorcraft 270 and measures about 300 ohms at room temperate.

The Ford temp senders were electrically the same for a 1964 Falcon as for an early Mustang although some of them had different sized mechanical threads. What are you defining as 'room temperature'? The best sender I have is an original one from Dearborn in March 1966. It measures 315 Ohms at 66F and 280 Ohms at 73F. The newer, and more inaccurate ones I have bought are considerably less Ohms than that. What you have sounds like it may be what you need.

As for a thermostat, all of the Fords of that era used a 192F thermostat as standard equipment. The standard thermostat sold today is more likely going to be a 180F. 180F will work fairly well and is what I now use. I wouldn't go any colder than that since cold temps mean shorter engine life.

I am not sure what benefit you think an electric fan will give you. The standard factory fans work fine for me without any concerns about all the electrical bits working correctly. If the fan belt isn't broken there are no cooling problems with a factory fan unless your radiator has problems. If so, new brass radiators very similar to the originals are today only about $130 at AutoZone.

 

[waid302]

The Ford temp senders were electrically the same for a 1964 Falcon as for an early Mustang although some of them had different sized mechanical threads. What are you defining as 'room temperature'? The best sender I have is an original one from Dearborn in March 1966. It measures 315 Ohms at 66F and 280 Ohms at 73F. The newer, and more inaccurate ones I have bought are considerably less Ohms than that. What you have sounds like it may be what you need.

As for a thermostat, all of the Fords of that era used a 192F thermostat as standard equipment. The standard thermostat sold today is more likely going to be a 180F. 180F will work fairly well and is what I now use. I wouldn't go any colder than that since cold temps mean shorter engine life.

I am not sure what benefit you think an electric fan will give you. The standard factory fans work fine for me without any concerns about all the electrical bits working correctly. If the fan belt isn't broken there are no cooling problems with a factory fan unless your radiator has problems. If so, new brass radiators very similar to the originals are today only about $130 at AutoZone.

At 73 Deg F, the resistance I have is 318 ohms for both Motorcraft 270. I sold my old 170 I-6 long time ago and I don't know what temp sender it had so I don't know the resistance values. I just want to make sure that I get an decent coolant temp reading on my dash gauge with the the Motorcraft 270 sender.

I am not sure how well the Explorer fan will work with my setup so I am going electric route. I will be using a BMW 91 / 99* C (180/195*F) dual temp switch and 94-97 850 Volvo heavy duty relay dual fan controller to control a Hyundai Sonata 3-Speed fan. This fan seems to fit just right on my original radiator. I am going to try to use the six cylinder radiator since it came with the car and its new. I have less than $25 in it.

Waid

 

[zray]

For an older (1966) engine, what is too hot? At what point should I be concerned?

A stock Mustang received the 192 F thermostat from the factory. It isn't even fully open until 212 F according to the Ford shop manual.

210 +/- 10 degrees is COMPLETELY normal. A vintage Ford engine NEEDS to be at least 195 degrees for maximum efficiency. The farther you go under 190, the worse off your engine will be. If the temp doesn't hit 200 then the moisture in the oil will not be burned off. It's impossible to overstate how critical this is for engine life. Water from blow by gasses is very damaging to the engine if allowed to circulate continuously. Plus, the power of the engine is degraded if it doesn't reach optimal operating temperature.

If an engine hits 220-225 F for short periods , like traffic jams, or stop lights, that's perfectly normal. The real danger to your vintage engine is not too much heat, it's the trend toward using 160 or 180 thermostats, that don't let the engine warn up fast enough. The stock 192/195 F thermostat should always be the first choice.

The stock gauge, and / or sending unit, in these old cars should never be trusted. A modern temp gauge should always be installed, if only temporarily, so one knows what temperatures the engine is really running, and to calibrate the stock gauge. Replacement stock sending units are not a very high quality item. If one needs to replace the original sending unit, then a search for a NOS Autolite or MotorCraft unit is the best strategy.

Z

PS. the stock clutch fan moves a lot of air, more than most electric fans by far. When used with a shroud, they are very very effective in cooling an engine. The primary reason manufacturers have moved toward electric fans is to eliminate the maintenance associated with the cooling system fan belts. Installing an electric fan often creates problems rather than solves them.