Need some help with cooling system / temp light coming on

[Michael Pace]

Hello all. I am fairly new to classic cars and definitely Mustangs so take it easy. I have recently inherited a 73 Mustang Convertible with a 302. The care runs great, but recently has been giving me an idiot light on the dash for temperature. It has not boiled over or anything like that, but the temperature light comes on after about 20 minutes of running. It is very hot in Southeast NC. However, the temp light is not super bright if that makes sense. When I start the car from cold, the temp light flashes on for a second the light flashes and it very bright, then goes off as it should. After running for10-15 minutes I get a faint temp light, not like the startup on. Can someone help me understand that one? I have attached a video from when the car was already warm.

I replaced the radiator cap, checked the hoses already. They look fine. I used an IR temp gun and found about 15 degrees of difference between the in / out radiator hoses. So I assume water pump is working / thermostat is opening. Fan is kicking on. It is presently very hot outside, but it was also hot in June when I was driving this car and I did not have these issues.

Where would you start? Is the way the temp lights on (super bright at startup, not so much when it comes on) normal operation for this car?

Thanks in advance.

 

[Steve McMahan]

Is this the only light that seems to be acting up? What about the oil pressure idiot light? You could replace the sending unit. It’s not difficult to access. Here are the pages from the Ford service manual to diagnose the problem. If you don’t have a set of these, you should get them:

 

[Cleveland Crush]

Michael, what was the actual temperature read on both radiator hoses when you shot them?

 

[Hemikiller]

The temperature warning light is controlled by the sender in the intake manifold. The sender is simply a switch that provides a path to ground for the light at a pre-set temperature. It's possible that the sender was replaced with the one intended for the gauge package cars, which provides a variable resistance reading.

A simple test is to remove the wire from the sender and ground it to the intake. That should give you a bright warning light.

Use a multimeter and check the sender post to intake. You should have no continuity at all, infinite resistance. If you get an Ohm reading, then it's probably the wrong sender.

 

[giantpune]

You say the fan is kicking on. Is this some sort of aftermarket electric fan? From the factory, the car should have a mechanical fan attached to the front of the water pump that is always spinning when the engine is running.

What does your radiator, fan, and shroud look like?

 

[71fast]

The ignition switch in start will test temp light bulb every time car is started.
I don't know if you read wiring diagrams, but you can see it on 1973_full.pdf page 3. Free diagram available on this forum.
Red with white stripe wire 39.
If you unplug the red with white stripe wire from the temp switch and ground it with the key in run it's the same thing as start test.
Light should be the same brightness as the start brightness.
That will test the wiring. Probably a bad temp switch.
When you replace the defective temp switch be sure and use sensor safe sealant as the switch needs a good ground.
Be sure to get a temp switch for temp light and not a sensor for a gauge.

Your first post. New car smell. Welcome to the forum.

 

[mrgmhale]

Not meaning to parrot prior posts, but as a matter of tossing my thoughts in for reiterative purposes if nothing else, I offer the following comments:

My first thought, when I read about how your temperature idiot light is behaving, is exactly as HemiKiller stated. I suspect the incorrect style sending unit has been installed previously. Removing it and testing it per HemiKiller's suggestions is the exact approach I would be taking and suggesting (Well done, HemiKiller, again).

Using a thermal temperature gun reading device and seeing the difference in coolant temperature between the upper and lower hoses was an excellent move. It helps tell the larger part of the story. I avoided doing needless repairs due to an oversensitive temperature gauge on our 1969 Shelby GT500 by using an infrared temperature gun to verify the temperature being as high as reported by the temperature gauge (it was at 180 degrees, whereas the oem gauge showed the temp at the upper end of the "normal level. The new sending unit let the gauge show a temp reading at the slightly lower 12/3 of the normal range on the same oem gauge. I have been a raving fan of verifying a problem prior to beginning repairs from my earliest days as a repair technician, but the infrared temp gun bolstered that approach.

There have been some other excellent thoughts and suggestions also proved by other members of the forum for whom I have tremendous respect. In order to help you acquire the needed manuals and schematics suggested, I offer the following link to the Forel Publications site where you can find some excellent manuals in PDF format that of excellent quality.

This link is specifically for 1973 Mustangs and Cougars. Going up one level shows additional years of manual coverage.

https://www.forelpublishing.com/clickbank/index.html#1973


It is your good fortune you have a 1973 Mustang Convertible for a few reasons. First of all, Lynda and I also have a 1973 Mustang Convertible with a 302, C4 Auto Tranny, and 2.79:1 rear axle ratio open differential gear set. It was literally a "Barn Find," and when we purchased it a few years ago it had less than 20,000 original miles on it. Despite our adding Classic Air A/C, LED taillight panels (VintageLEDs.com), and installing a Dual Electric Cooling Fan system, the Mustang is largely unmolested, where the upgrades performed were done with an eye on excellent workmanship. So, in addition to having excellent documentation from Forel, we have what is likely a close twin to your vert for comparative system purposes.

But wait! There's more! Although in 1972 the Forel electrical schematic manual began to provide far improved and complete information as compared to the preceding years of electrical schematics, I have found the 1973 Forel Color Electrical Schematic manual to be as close to perfect as possible. More important, in 1973 the Ford Shop Manual, Volume VI, provides truly excellent and complete emission control system and engine emission control related vacuum hose "Calibrations" (aka schematics), whereas in the prior years the Shop Manuals were devoid of that level of information. There are, however, two vacuum calibrations missing from the 1973 vacuum routing schematics, but that affects only the 351C 4v engine configurations, so you are not adversely impacted by the absence of those calibrations. None-the-less, I will include a file attachments for the missing calibrations so you have it on hand in case you ever need that info.

I have some snippets of the Shop Manual and/or Electrical Schematics Manual I want to share with you. Some of the snippets are annotated by me to bring out some clarification I felt might be helpful to myself or others. Despite providing these snippets I encourage you to license and download a full set of manuals, to include the Shop Manual and Color Electrical Schematics, so you have properly licensed and complete documentation on hand for your Mustang. The pricing is very reasonable, actually on the very inexpensive side, especially considering how much information is packed in those PDF files.

If you run into a pinch or need to bounce things off the wall, I am sure my fellow enthusiasts would join me in welcoming your questions or concerns. That said, I know that you can count on me providing information, photos, videos, etc., as needed to help you out. And speaking of videos, for what it is worth Lynda and I have provided a lot of files covering various subjects on the diagnosis, maintenance, and repair of these First Generation Mustangs. They are found under my YouTube Channel ("Gilbert Hale"). If you need clarification that is best served by using a video to show the answer or direction, it will likely end up being added to our channel. If not, just post the question or concern.

https://www.youtube.com/@gilberthale7777/videos


Following are some links for the documentation snippets I referred to above. They are usually too large to attach to posts or email, so I put them on my Google Drive to share as needed. I hope you find them useful:

For the entire folder:

https://drive.google.com/drive/folders/1TdZ-HG5Y2dpOFR1iNdDFOkvE9DiANibl?usp=drive_link


For individual files:

https://drive.google.com/file/d/1B7km6-h8YTZPpc1_aMLqaov_qAK5dPnV/view?usp=drive_link

https://drive.google.com/file/d/1KbgNEsC3bZ7t1GWSjURODFda1IgTDhtq/view?usp=drive_link

https://drive.google.com/file/d/1x2SdykXlUfQTPMQdzhG8X1SxXYVpMfQO/view?usp=drive_link

https://drive.google.com/file/d/1FdPOb1qkf9E-9GlBpPtFxOJITMK0DN1n/view?usp=drive_link

https://drive.google.com/file/d/13z5Lrpv3PaWy-uoaKoV2xZzuw8EFms4d/view?usp=drive_link

https://drive.google.com/file/d/1gO2kbjEmqHJj4ITwACIqo1TUxZOWXNAi/view?usp=drive_link

https://drive.google.com/file/d/18P9oiebm2nUcYjQ-RFJA9XWTcB8VhOod/view?usp=drive_link

https://drive.google.com/file/d/1-foyzcIRVLOT3ZmdroMukADeWhG-B-CY/view?usp=drive_link

https://drive.google.com/file/d/1qzTAmUkLPVvzHzCpeh61NskeW-oLN11C/view?usp=drive_link

https://drive.google.com/file/d/1fA7YK2vV5D-FAh_uHKynktF-lx1xOIzC/view?usp=drive_link

https://drive.google.com/file/d/1D2hBFNmQOMtGQByZg6KJbzwCc93bTo3M/view?usp=drive_link


Finally, with respect to your instrument panel bring laden with idiot lights instead of analog gauges, both of our 1973 Mustangs (Convertible and Mach 1), were also equipped with the idiot light based instrument panels. Rather than try to track down oem analog gauges for the Instrument Panel and the 3 gauges to be mounted in the center vertical bezel, I opted to purchase and install the Dakota Digital VHX Analog Gauge panel. We got ours with red back-lighting and Carbon Fiber trim. Following it their web site for the 1973 VHX kit. You can navigate around to look at their digital display panel if you prefer that look. I have also attached some photos pf you the Dakota system looks installed on out Mustangs.

https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=922/category_id=434/mode=prod/prd922.htm