how to relay q

after expensive overheating problems with my 4x4 i purchased a radiator cap that has a built in coolant level sensor having problems hooking it up to my landrover discovery

radiator sensor cap has 2 spade terminals when fused 12volt power applied to sensor in normal position (ie: coolant at proper level 12volt output)

when fused 12volt power applied to coolant sensor in low position no power (ie: coolant dangerously low)

need a negative output to warning light and piezo buzzer on instrument panel

so i have a trigger that has 12volt output in normal position, with no power output when coolant magnetic reed float drops due to low coolant and i need a negative output when coolant level drops

how is a relay wired for this

if you check your sensor, it SHOULD actually read as Grounded when the coolant level gets low - so it should read:

12v + when coolant level normal

12v - when coolant level low

to use this correctly, you need only to install a 12v light wherever you want it, then connect one wire from that bulb to the sensor from your cap, the other end of the bulb to your ignition on + wire.   you turn the ignition on, and there will be a + on one wire of the bulb from the radiator cap sensor, and a + on the other wire from the ignition on wire ..... the bulb will be off.  should the coolant level drop to low, the sensor wire will drop from 12v + to 12v - and the light will come on. 

if you check your sensor, it SHOULD actually read as Grounded when the coolant level gets low - so it should read:

12v + when coolant level normal

12v - when coolant level low

This is how i intended to install this sensor,  problem i have is when coolant sensor low i don't have 12v- multimeter reads 001 when coolant level normal multimeter reads 12.54 but will not power a small bulb but will power a piezo buzzer with specs of 3-30 volts DC (current 2.5-19mA)

i tried using a relay hoping to convert polarity with no luck seems sensor will not output enough power

So it buzzes at normal  level? Was the cap a kit or a factory part? It might need the factory electronics.

I just finished using a transistor to boost a piezo alarm to turn on a bosch relay to flash the lights. You could use a small relay and hook the light to '87a'. When the level is low, relay turns off, the light comes on.

It might be enough to trigger a relay... but the polarity doesent need to be converted... you simply need to use the relay like this:

87 12V+

86 -Ground

85 - Wire comming from sensor

30 12V+ Output for bulb and buzzer

Now, if i understand your post correctly this will work.

Hooking your meter ground lead to ground, and testing the sensor reads .001v when the sensor sees coolant (coolant lvl normal) and reads 12v+ when there is no coolant (or the coolant is low).

Hooking it up just as i showed above will help you out.... is that correct though?

mikeshonda750 wrote:

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It might be enough to trigger a relay... but the polarity doesent need to be converted... you simply need to use the relay like this:

87 12V+

86 -Ground

85 - Wire comming from sensor

30 12V+ Output for bulb and buzzer

Now, if i understand your post correctly this will work.

>>Hooking your meter ground lead to ground, and testing the sensor reads .001v when the sensor sees coolant (coolant lvl normal) and reads 12v+ when there is no coolant (or the coolant is low).<<

The exact reverse is what i get, when sensor reads low coolant multimeter shows .001v when coolant is normal multimeter reads 12v+ which means i would need a led and buzzer working when coolant is full not practical

the sensor is a genuine landrover but made for a different model i am guessing it is normally connected to cars elect control module via 12v+ with low level mil amps and when this signal is terminated via low coolant the elect control module powers a warning light on cars instrument panel i actually do not need to reverse polarity, all i need is a switch relay that will trigger from a current 2.5-19mA at 12v+ and be able to power a warning light and perhaps a buzzer

i have tried a few relays wired as you state but signal is not enough to trigger relay switch, if i use power from a 5amp 12v+fuse the relays do trigger and supply enough current to power warning light and buzzer i am just assuming i need a relay that will trigger from a current of  2.5 -19mA at 12v+

thanks for any idea

Hooking it up just as i showed above will help you out.... is that correct though?

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