rabbit low coolant/overheat warning light

[Fatmobile]

I learned how to design circuit boards and sent in my design for a PCB that senses the low coolant level and overheat idiot light. A dozen circuit boards were on my doorstep when I got home so I soldered the components on one and tried it out. I compared the operation to a stock '86 temp gauge with the idiot light built in and it worked just like it.
In tests, the coolant level sender triggers the light whenever it is disconnected from ground. The light flashes for awhile then stops until it's triggered again.
I'll need to get some flashing LEDS because the circuit doesn't cause the flashing.
Red? or Blue?
I'll still need to document how it's installed and how well it works when it's actually in an instrument cluster, but a low coolant warning light is something I've been wanting in my dash every since I lost all my coolant and killed the engine I'm putting in my GTI.... a little over 5 years ago.

[Vincent Waldon]

I vote for Red... loss of coolant is a "pull over right now" offense.

Sounds like a very useful project... 'course I'm a Jetta guy and the flashing light has saved my ass at least once !!

How are you figuring out "overheat"... an inquiring electrical engineering mind wants to know !



Vince

[Fatmobile]

I installed it in the Rabbit today, swapped in a coolant resevoir with sensor.
The low coolant warning light does the opposite of what it's supposed to... lights up when it's plugged into the sensor with plenty of coolant and turns off when unplugged... so there are some bugs to work out.
The overheat warning light uses an op amp, when the sensor resistance gets too low, it turns the output high.... it works great.
Yeah, the first time I saw one of these indicator lights and resevoir with the sensor was in an A1 Jetta.
The '85/86 Jetta/Golfs had the same kind of resevoir, that will fit in a Rabbit... later they went to a round resevoir that mounts on a different resevoir bracket than the Rabbit's.
I'm not sure when they did the switch or what other cars the resevoir can be salvaged from.

[Buzz]

I say leave it the way it is - you have an "everything is okay" light...

[Fatmobile]

My very first car was a beat up 65 mustang... the alternator light being on meant it was working,... right? Then why are the lights getting dim,... hmmmmm. Can you hear the laugh from the guy on the other side of the parts counter? I still hear it
It appears the coolant level sensor is not purely resistive. I put my multimeter on it to read the resistance in antifreeze and the resistance kept increasing. Put the leads on in reverse and it wasn't the same.
I turned the plug around and it works just like planned. Unplug the resevoir and the light turns on, plug it back in and it turns off.
Taa daa.
I'll have to post some pictures of it, but company just stopped by so it'll have to wait.

[CoolAirVw]

Neat! Nice job. Whats next? Veggie start up and shut down controller?


Ps. Maybe you could have some OPCU cards made up. (Probably not really needed)

[Fatmobile]

Ok, I jumped the gun.
The plug goes in either way.

There were a few bad solder joints.
I ordered a soldering station but it isn't here yet. The trigger pull Weller soldering gun is too big to get in close to the leads.

I think it is working like it's supposed to.
Turn the key and if the sensor is plugged in (coolant level is high enough) the light will turn on,... then fade out. I don't have a flashing LED yet so it fades instead of stopping blinking.
If I unplug the sensor when the car is running and still cold, the light doesn't come on but when plugged back in it lights up and fades, just like when the key is turned on.
If I unplug it when it has warmed up the light doesn't come on until it's plugged back in,... and then it stays on.
Unplugging the sensor for the engine temp gauge and plugging it back in will reset the light... until you unplug the level sender again.

Whats next? Veggie start up and shut down controller?

That's on the mental drawing board but since I switch to vegy before I turn the key... I don't want auto vegy.
I would like to have something to warn me when I shut it off in vegy mode... maybe time out if I shut the vegy off when it's still running...

Hey my brain is still smokin' from this coolant level sender and you've got me stewing on a new project already.

[Fatmobile]

This was plan A, with a wire and plug coming off it:


Plan B has it wired into the factory plug:


On my computer, it says the picture isn't there but if you click on it, a screen with a blown up picture will appear.

[Fatmobile]

I took it into town today and if the engine is warm and I shut the car off then back on,... the light latches on.... until I unplug the engine temp sensor,
I am going to have to change a resistor value.

[Fatmobile]

OK, I can shut the car off warm and when I start it back up the light fades out just like a cold start.
Where do you think the overheat warning light should come on?
I set it to the white mark just before going red.
I unplugged the rad fan and ran it up that high a couple times to test it.
The resevoir wasn't overflowing at that temp.
The light came on and latched until the engine temp was back to normal.


I'm not sure this works the way I'd like it to but it works just like the Golf.
If I unplug the resevoir sensor or lift the sensor out of the coolant the light doesn't come on until I plug it back in.
I'd like to see it turn on when it's unplugged and off when it's plugged back in. I'm not sure it would come on and stay on if I lost all my coolant.... I'll have to drain the coolant and see.

[Vincent Waldon]

I think you've set it right... just on the other side of what the fan can handle... the more warning one gets the better, but if the fan is going to look after it a false alarm would be annoying.

Interesting that it doesn't come on when the sensor is unplugged... must mean that it's sensitive to a range of resistances and opencircuit is out of range ?? Guess you'll find out when you drain the fluid.

Dunno if VW does this, but industrial fluid sensors often use AC to charge the probes 'cause with DC they become polarized over time and lose sensitivity or exhibit strange behaviors.... fluid sensors are surprisingly tricky beasts !

[rwest1]

Fatmobile

Some good work going on, sounds like your having lotsa fun. For a future enhancement think about a Sonalert (you can get one out of an old smoke detector to play with, need a driver though, or Radio Shack Piezo buzzer). You know Murphy is going to have you looking at the girls, or driving into the Sun when the Sh** hits the fan. Also might want to OR in the low oil pressure to have a total engine save package. Regards

[Vincent Waldon]

Good idea... if you drive a boat with a diesel motor there is almost always a low oil/coolant/overhead buzzer that disrupts the quiet morning as you're trying to sneak out of a harbour.

[Fatmobile]

I think you've set it right... just on the other side of what the fan can handle...

It's actually quite far away from where the rad fan turns on. I had to unplug the rad fan and wait awhile for it to heat up this far.
I went down to pick up my line bored head, went in and got it so I wasn't out of the car for long,... and when I went to start it the light latched on again so there is still some adjusting needed.

Interesting that it doesn't come on when the sensor is unplugged

That's what I think too but that's how my Golf works too.. I'd like it to turn on whenever the circuit is broken.
Problem is the stock op amp is using the resevoir for Vcc-,... so whenever it's unplugged the op amp can't go low and turn the light on.
I think that's so the light will come on when you first turn the key,... but only if the coolant level is high enough.
We'll see if it latches when I drain the coolant.

Dunno if VW does this, but industrial fluid sensors often use AC to charge the probes 'cause with DC they become polarized over time and lose sensitivity or exhibit strange behaviors.... fluid sensors are surprisingly tricky beasts !

So it might be a good idea to turn the plug around occasionally.

Some good work going on, sounds like your having lotsa fun. For a future enhancement think about a Sonalert (you can get one out of an old smoke detector to play with, need a driver though, or Radio Shack Piezo buzzer).

I hadn't thought about using it to trigger a buzzer, good idea. Probably save it for a future upgrade. I think there is a buzzer plugged into some of the fuse boxes, that I could salvage. I know my Golf plays La Cucharacha when I turn the key, that should get your attention .
It has been fun learning how to get printed circuit boards made online.... and putting my electronics degree to use again.
I got my soldering station now and it's a huge improvement over the fat, overheating tips on my weller soldering gun or soldering iron.

[Fatmobile]

I installed the circuit and light into my GTI dash and have been putting some test miles on it.
The light has been coming on during turns lately so I checked the coolant level and it's a little low.
The warning light is working just the way I wanted.
It doesn't latch on,... if the coolant quits splashing around and dropping below the sensor, the light goes back out.
It only comes during turns, hard braking or acceleration