What Water Temperature for Warning Lamp?

Hey all,

I've always said I'd do this on my cars, but I've never actually gotten around to it. My biggest fear while driving is cooling system failure and the subsequent havoc that it wreaks. This is especially true of long trips at highway speeds in decrepit old cars like mine.

So, I'd like to wire up a simple warning lamp circuit. When the coolant temp gets to a certain level, a warning light will alert me to a potential problem, at which point, I can monitor the temperature gauge and assess the situation further. A gauge by itself is not going to help me catch the problem before I really start cooking stuff.

So, my question is, what should that temp be? I have a feeling that this might depend on the engine/vehicle in question. Throw out whatever you think.

In particular, I want to do this to my '84 F350 Dually tow vehicle that has a 460. (back story: The factory gauge reads alarmingly high...I'm curious if it's just the gauge that's old an wonky, or if my truck is really running too hot and needing attention to the cooling system.)

I was thinking a temp switch at 240 F would probalby be good. I don't imagine any damage has been done at that point, but I don't think it would be in the normal operating range of most engines under load.

Maybe I'm wrong though... (?)

Sensors aren't that expensive, I can always swap one out later if I find the engine likes running at or about 240 on a regular basis.

Any insight appreciated, Clem

I'd set it for about 25F over the thermostat temperature.

I have a cooling fan trigger that is adjustable. Wire one of those in, then set it so the light is just off when the car's at normal operating temperature.

Tons of American cars have fan switches which are essentially a digit sensor (open/close) and activate the fan to come on between 210-225. GM ones are my favorite, single pin connection, 3/8" male pipe thread, near free at your local U-pull from a ton of different GMs (look for mid-late 80s FWD cars for the most common ones, don't forget the connector with some wire to splice to!). Run ignition power to your idiot light, then from the idiot light to the sensor. When the engine gets to ~220 the switch will go to ground through the sensor and your idiot light will come on. You should have a pipe thread port open on your cooling system somewhere (look for plugs) but if not you can easily T the sensor in to somewhere else with brass parts from the local hardware store. They look something like this:

Edit: Oh yeah, and if you want to buy one new, call your local parts store and get a radiator fan switch for an 88 Fiero V6...should be about $10-15. They'll likely have a connector, if not you can get them from the junkyard from tons of cars, they used them on other sensors like knock sensors too.

Bryce

I think there is a similar one in the 944's that could be used to trigger a fan/lights as well. I will have to look at that as well.

My local NAPA has a Big Book of Sensors that gives on/off temperatures as well as thread pitch and connection type. Pretty handy. Going GM makes sense, as they literally make millions and they should interchange.

Keith wrote: I'd set it for about 25F over the thermostat temperature. I have a cooling fan trigger that is adjustable. Wire one of those in, then set it so the light is just off when the car's at normal operating temperature.

I'd set it WAY over that.

Older cars have the thermostat opening at 175, more modern cars have them opening at 190. So that would trigger the light at 200-215. I suppose if you didn't have a pressureized cooling system, that would be about right. But since the boiling point is much, much higher (closer to 250+), that seems pretty early.

Oddly enough, the 460, and any cast iron block/head engines are pretty robust to overheating. They would be able to drive w/o coolant for some time without causing permanent damage.

If you are looking for a temp to stop all driving, I would shoot for 250-260F.

E-

Thanks all,

Nashco's plan is pretty much what I had in mind. Good that you put it down in print and photo for anyone else who might be interested...thanks!

For me, I'm not looking for a "Shut her down NOW" light, but one to tell me when things are getting toasty and I should start watching the gauge.

In the past, I've noticed some other symptom (Like steam from the tailpipe!) before I looked at the gauge. By then, it's likely too late to save engine integrity. A light can catch my eye and then there's a gauge, to tell me what's going on. They work well together.

And again, I appreciate the discussion!

Clem

alfadriver wrote:

Keith wrote: I'd set it for about 25F over the thermostat temperature. I have a cooling fan trigger that is adjustable. Wire one of those in, then set it so the light is just off when the car's at normal operating temperature.

I'd set it WAY over that. Older cars have the thermostat opening at 175, more modern cars have them opening at 190. So that would trigger the light at 200-215. I suppose if you didn't have a pressureized cooling system, that would be about right. But since the boiling point is much, much higher (closer to 250+), that seems pretty early. Oddly enough, the 460, and any cast iron block/head engines are pretty robust to overheating. They would be able to drive w/o coolant for some time without causing permanent damage. If you are looking for a temp to stop all driving, I would shoot for 250-260F. E-

We try to run Miatas at no more than 110C (230F). Once they get above that, they tend to keep climbing because they are way beyond their ability to keep cool. That's why the Link ECU had an intervention setting at 110C. That's with a 82C (180F) thermostat. On a Miata, I'd want my light at about 105C (221F). So yeah, I was a bit on the conservative side there. Thanks for making me check my math :)

At 250F, I would definitely be looking for a place to park.

Since most fans don't turn on until 220, I think 240-250 would be about right.

ClemSparks - what did you end up doing?

I recently had the same thought - the dash temp gauge is often not noticed until it's too late. My thought was to use a buzzer instead of or in addition to a light. I plan on wiring in an on/off switch so that I can turn off the buzzer once I've been alerted.

My first thought was to add a sensor in a coolant hose, but if the hose fails and dumps the coolant, the sensor is not going to register a high temp, so I need to figure out where to install the sensor.

I would potentially do this on 3 cars - 2005 Subaru Legacy GT, 99 Miata, and 91 Miata.

Anyone have anything to add?

Thanks, Evan

Almost forgot, through my very brief search, I discovered the Fiat 500 has a buzzer for high coolant temp, so at least 1 oem has thought of this, I wish the rest of them would too.

At the pick-a-part, look at longitudinal-engined SAABs.

They have an inline temp sender in the upper radiator hose that cuts off the a/c if the car exceeds a certain temp.

Splice that into your upper hose, add a flipflop relay, and you're done.

Nothing really to add, just thought I would chime in to get all the Evans in a row.

I put one in an old Pontiac 389 that grounded itself at 250. The water pump was on its last legs. It kicked on a few times around town and I could usually get it to shut off by revving it for a bit at lights.

Never did any damage, but another large part is the rest of the cooling system, the condition of the water jackets, the age of the head gaskets, etc. At 250-260 you certainly won't do any damage to the iron, but you do run the risk of blowing a hose, boiling coolant, blowing a radiator tank, or other smaller issues.

You could do like the Nascars do. Just run the over flow hose in front of the windshield.

Evan wrote: ClemSparks - what did you end up doing? Evan

Well...oddly enough, I haven't done it to any of my vehicles yet. The tow vehicle in question was traded several trades ago (that trade chain is still going...might trade one of its descendents today!).

I HAVE been thinking about this still/recently, though. This resurrection of the thread may just be exactly what I need to get off my butt and finally do an idiot light. (so...thanks!)

Clem

EvanR wrote: At the pick-a-part, look at longitudinal-engined SAABs. They have an inline temp sender in the upper radiator hose that cuts off the a/c if the car exceeds a certain temp. Splice that into your upper hose, add a flipflop relay, and you're done.

My electronics knowledge is very limited, what's the flipflop relay for?

Evan wrote:

EvanR wrote: At the pick-a-part, look at longitudinal-engined SAABs. They have an inline temp sender in the upper radiator hose that cuts off the a/c if the car exceeds a certain temp. Splice that into your upper hose, add a flipflop relay, and you're done.

My electronics knowledge is very limited, what's the flipflop relay for?

My memory is limited, but I think the SAAB temperature sender opens at high temp. The exact opposite of what you want. A flipflop relay can be used to reverse that. Look for a standard Bosch "ice cube" relay, one that has a terminal 87a - not all of them do. If you decide to go this route, PM me and I will send you a wiring diagram.