ISO a device to kill engine in overtemp or under (oil)pressure conditions

[justthisguyyaknow]

Having completed spending lots of money on a rebuild bill, and now getting a 2.5L 120 Mercruiser re-installed after a quick overheat caused by a bit of sand in the outdrive, it occurs to me that... it's 2015, so why can't I find a simple device to shut down an (old, 1972) engine when the temperature is too high.

Does anyone know of any such device? I don't even want to water test the engine without such a device (even if it means designing it myself, though I'd rather find one pre-built).

It seems like the simplest idea... Anyone?

 

[thumpar]

I don't know of a device to shut if off automatically but you can get an alarm.

 

[justthisguyyaknow]

Yeah, I have an alarm. Trouble is, that in addition to owning the boat, I also ski behind it. This means tgat I have to trust my wife/daughter to have the presence of mind to hear and react to the alarm in a timely fashion.

 

[JustJason]

Wire in a temperature switch to a relay that when activated it shorts ignition coil negative to ground. All you need is a basic GM temp switch, a standard 30 amp bosch relay along with a relay socket and a little it of wiring. And maybe a couple of brass fittings to plumb in a temp switch. If the engine overheats, the temp switch will close, grounding out the switch, which will trip the relay and in turn that will short out the coil, which will stop the engine. That is all you need to do.

 

[thumpar]

Are you sure you have an alarm? They didn't come with them. They are loud and I wouldn't trust someone to drive my boat that couldn't hear it.

 

[justthisguyyaknow]

Right now, there's a single-wire temperature sender which feeds to the temp gauge on the dash. It's my understanding that as this type of sender gets hotter, it decreases the resistance between its single terminal and ground.

I presume that some voltage is passed through a resistor in the gauge and then on to the wire going to the sender. Thus, the gauge is essentially a voltmeter (calibrated to sender to match the correct temperature scale). If this is correct, it seems like I'd need a voltage comparator circuit to compare the voltage of the sender to a fixed (or adjustable via potentiometer) voltage, and then to simply, as you say, short the coil when the voltage gets pulled too low on the wire going to the sender (because the sender is getting hot and grounding its terminal).

Someone in the past had actually jumpered straight from this sender wire to the coil in the past, presumably to cause this shorting effect. Trouble is, since it's not an all or nothing switching action, running the engine too hot would simply start to decrease spark, not eliminate it.

 

[justthisguyyaknow]

Thumper: There's an aftermarket "buzzer" on it. To be honest, with the engine at WOT running across the lake, music playing, etc., I can't even hear it. I bench tested the buzzer before I installed it, and I KNOW it's no where near loud enough.

 

[thumpar]

Thumper: There's an aftermarket "buzzer" on it. To be honest, with the engine at WOT running across the lake, music playing, etc., I can't even hear it. I bench tested the buzzer before I installed it, and I KNOW it's no where near loud enough.

Get a new "buzzer". I have a large sound system and a V8 and the alarm is deafening over it.

 

[stonyloam]

If it has an alarm, then you can use the alarm circuit to activate a relay to kill the engine. When the buzzer goes off it can activate a normally closed relay to cut power to the coil or a normally open relay to short it to ground.

 

[justthisguyyaknow]

Yeah, the "buzzer" isn't really a circuit. It simply relies on the grounding of the temp sender. Once the temp sender gets close to grounding, sufficient current goes across to "move" the little buzzer "motor". I suppose if one picks a relay with the correct coil resistance, one could certainly have some control over the temp at which it would close, thus eliminating the need to build a complete comparator circuit. In fact, now that I think about it, I could put a potentiometer inline to the relay to control the amount of the voltage reaching the relay at any one point. The relay would only engage when there was sufficient voltage, and it could then short the coil. In fact, one could even have two relays tuned in this way to different temps, and have a warning at temp t1, and an alarm at temp T2. I'm thinking of using a compact car's horn. Seriously.

 

[thumpar]

The only problem with it killing the engine is the rush of water at the rear that may cause water ingestion if at a good speed.

 

[Bondo]

It's my understanding that as this type of sender gets hotter, it decreases the resistance between its single terminal and ground.

Ayuh,.... Welcome Aboard,..... That's a Sender, not a Switch,.....

You can get an alarm Switch here at iboats, that'll screw into a tapped hole near or in the t-stat housin',....

The switch goes to Ground, no variations,......

In the most basic circuit, a wire from the coil's (-) terminal, to an over-heat Switch, will kill the ignition, killin' the motor, at the switches set-temp,....

 

[justthisguyyaknow]

I just found the "Aquaalarm" engine overheat detector, which seems far simpler. Is has two terminals and closes these at 200 degrees fahrenheit. Presumably, this could drive whatever, as it appears to a device which is either open or closed, rather than a temp sender which is a varying voltage. Yeah, sounds more fullproof (or foolproof).

 

[Fun Times]

JustJason's idea is a good one.

The temp alarm switch comes in two heat ranges before it activate. One is designed to sound around 196 degrees and the other is in the 215 - 225 range.

If the OEM switch has Red Mylar Sleeve then it should sound somewhere between 190-200?F. Mercruiser part number 48952
If the OEM switch has a Black Mylar Sleeve then it should be somewhere between 215-225?F range.

Since you bought an aftermarket alarm kit, I'd be concerned with what temp it's set to go off at especially knowing you had one overheat problem already.

If you know your switch is the correct value, then you should be able to utilize that switch already installed...Plus it'd be much easier to find an on/off type switch then to try to find the correct value of an adjustable switch like mentioned above.

Either way, good luck.

 

[justthisguyyaknow]

Ayuh,.... Welcome Aboard,..... That's a Sender, not a Switch,.....

You can get an alarm Switch here at iboats, that'll screw into a tapped hole near or in the t-stat housin',....

The switch goes to Ground, no variations,......

In the most basic circuit, a wire from the coil's (-) terminal, to an over-heat Switch, will kill the ignition, killin' the motor, at the switches set-temp,....

That's EXACTLY what I'm looking for: an alarm "switch". Do you have a link/number for that?

 

[justthisguyyaknow]

I found it, right here at iBoats!

thumpar: I think to address the issue you pointed out of an instant, unexpected shutdown causing a backrush of water (or issues with insane PWC drivers who don't expect you to just "stop"), I may install a simple timer circuit to give a warning immediately, and shutdown like 10 seconds later.

 

[thumpar]

I found it, right here at iBoats!

thumpar: I think to address the issue you pointed out of an instant, unexpected shutdown causing a backrush of water (or issues with insane PWC drivers who don't expect you to just "stop"), I may install a simple timer circuit to give a warning immediately, and shutdown like 10 seconds later.

That sounds like a good plan.

 

[UncleWillie]

I...I think to address the issue you pointed out of an instant, unexpected shutdown causing a backrush of water (or issues with insane PWC drivers who don't expect you to just "stop"), I may install a simple timer circuit to give a warning immediately, and shutdown like 10 seconds later.

You are over thinking this!
This is the same situation as when the safety lanyard get pulled.
It is what Manifolds and Risers are designed for.

What you are looking for is called a Thermostat, A switch that actuates with changes in temperature.