Hi guys!
This is my very first post here.
I've been following you for ages, now decided to register and share with you my design, to hear your thoughts and ideas about it, and maybe improve it.

History:
So, I've allready had a couple of cars that went to scrapyard because a radiator hose blew and engine overheated. No light or alarm sounded, and temp gauge was normal, since it measures water temperature, and there was no water...

Idea:
I need an effective way to be warned if a coolant leak happens. I though this could be done in a couple of ways:
- My first Idea was to dip two probes in the water reservoir, and measure it's resistance. When no water would be present, resistance should be 0, and an alarm would sound. Decided not to go this way, because there would be corrosion due to electrolysis, maintenance would be required.
- Second idea was based on my first, but using a stainless steel float switch in the coolant reservoir, but I'm afraid that it malfunctions overtime due to rust and grease present in the water.
- So I decided to measure the coolant circuit pressure. When pressure drops below some level, means there's a leak somewhere, and an alarm will be turned on.

Design:
So this is what I've got so far:

WPS1 will be a 5v 15PSI water pressure sensor, VOut ranges from 0.5v (0 psi) to 5v (15 psi).
LD1 - Red alarm LED
LD2 - Green OK LED
I'm using a LM393 dual comparator, with same reference and threshold but inverted between comparators, meaning that when comparator A is 'on', comparator B will be 'off', and vice versa. So when LM393's pin 3 voltage (coming from pressure sensor) is below reference the red LD1 will light up, and LD2 will go off. If voltage on pin 3 goes above reference, then LD1 will go off and green LD2 will be on.
Reference voltage is set using VR1, wich will be set to normal pressure when car is running idle.

I've breadboarded this, and it seems to be working fine.
I still don't have the pressure sensor, so I've just been testing with random input voltages, and it seems to be working as intended.

Still, since I'm a newbie enthusiast, I think there's always room for improvement.
Will it work as intended once assembled on car? Should I add any kind of protection or noise filter?

Any comments, thoughts or suggestions on this will be appreciated.

Thank you all!

 

Welcome to AAC!

Have you given thought to how the circuit will function when coolant is cold? This will be indicated as low pressure. How will you distinguish between cold coolant and no coolant?

Two of my cars have low coolant sensors. The one that I've worked on is just a float in the bottom of the overflow reservoir. That car also has at least two temperature sensors and it can detect other issues like air in the cooling system. I can't remember what the message was...

Don't know what my other two vehicles have but, since they're both Ford, it wouldn't surprise me if they don't have anything.

 

Hi Dennis! Thank you for your feedback!
Honestly, I was just assuming that once the engine is running, pressure would build up and would be >0 after a few seconds, even when cold.
The behaviour I was looking for was a red alarm light when you turn the engine on, changing to green after a couple of seconds.

Since I haven't received the pressure sensor yet, I'm kind of working in theory only, that's why I posted my project.

 

The pressure likely won't build up until the engine is near its operating temperature.
It's the vapor pressure of the hot coolant that creates the pressure.
In cold weather it may never go much above atmospheric.
So I don't think measuring coolant pressure will do what you want.

 

The cooling system will still build pressure even if there's a significant amount of coolant loss.

The symptom I described for an old BMW gave me an over heat warning before the temperature gauge indicated abnormally high temperature.

 

Couldn't find it right away, but here it is:

I found this when I was searching what's the normal pressure on a running engine.
It seems that besides temperature, pressure is also built by engine's acceleration (RPM). So an idling engine might have a low pressure, but it'll allways be >0. I think!

 

That would make the pressure indicator just another idiot light.

You'd probably be better off just doing more maintenance. When I have a vehicle with a coolant leak, I monitor coolant level frequently until the source of the leak is identified and repaired.

 

hummmm
I guess I'll have to consider de floter in the water reservoir

 

Perhaps you could use one of these.
Not sure exactly how they work but, since they send a signal to the engine computer, you should be able to determine how they work by sensing their resistance with an ohmmeter both in and out of water, and go from there.

 

Ford here in Australia had a system that worked very well on AU model falcons. It was a temperature sensor on the rear of the cylinder head, as this would continue to heat up when the coolant was lost. It gave an over heat warning, if this was ignored it would disable individual cylinders in random to try & cool its self. If this was still ignored it shut the engine off. If there was an emergency like stoped across a railway line, you cycled the ignition & it would start & run long enough to get out of danger. So a temperature sensor on the rear of the cylinder head would work.

 

So a temperature sensor on the rear of the cylinder head would work.

Yes, I agree. It would be easier to retrofit than a float switch in the reservoir.

 

Perhaps you could use one of these.
Not sure exactly how they work but, since they send a signal to the engine computer, you should be able to determine how they work by sensing their resistance with an ohmmeter both in and out of water, and go from there.

Those seem to be brand/model specific, for vehicles that have built in coolant leak alarms. That would be the probe-in-water aproach, my first idea. I suppose these work using AC instead of DC, to avoid electrolysis.
A while back I did mount a DC bench working model, that's when I noticed the water electrolysis. Since I'm a newbie and can't quite understand "AC", I wasn't able to make or find an AC working circuit to suit my needs.

 

Ford here in Australia had a system that worked very well on AU model falcons. It was a temperature sensor on the rear of the cylinder head, as this would continue to heat up when the coolant was lost. It gave an over heat warning, if this was ignored it would disable individual cylinders in random to try & cool its self. If this was still ignored it shut the engine off. If there was an emergency like stoped across a railway line, you cycled the ignition & it would start & run long enough to get out of danger. So a temperature sensor on the rear of the cylinder head would work.

Hum, that actually is a good idea, although I was trying to avoid overheating, but I can consider to build a overheat alarm. I'll look into that!

 

Since I'm a newbie and can't quite understand "AC", I wasn't able to make or find an AC working circuit to suit my needs.

You can generate an AC signal using the common 555 timer connected as an astable multivibrator and run that signal through a resistor and capacitor to the sensor.
(The capacitor prevents any DC from going through the sensor and causing electrolysis.)
You can then monitor the amplitude of the signal to the sensor (after the resistor) which should give an indication of whether the sensor is immersed in liquid or not.

 

You may find this of some interest, its a course I did as a Ford mechanic in 1998 about the Cylinder head temperature sensing. I don't remember any of these engines being destroyed by loss of coolant.