So, when my electric fan kicks on I want to have my HOT light come on in the dash. Silly I know but I want to know when my fan is actually running. It appears the dash is hot (+) and the temp sensor will provide a ground when it gets up to temp. I think I can use a relay but I can only provide a positive trigger and not a negative one. I read a few articles so far and no solid answer yet. I found the wires on the firewall side of the harness and my bad eyes can't tell for sure if the wire is brown, green or gray?
Any help appreciated as I do things "my way".
I bet John D has an answer on this one!?!
Thanks Guys!
John D said
May 5, 2016
Lemme think on this one...
67ss said
May 5, 2016
What are you using to make the fans come on? Is it a ground controlled temp sender screwed into engine block to activate the relay? If so hook your wire to the light to that same circuit.
John D said
May 5, 2016
Ok... yup, you'll need a relay.
(Using "Bosch Ice-cube" terms here)
Terminal #30 connected to ground
Terminal #87 T-tapped into the negative-feed wire to the "HOT" lamp in the cluster.
Terminal #85 connected to ground
Terminal #86 tied to the same +12V circuit/wiring that trips your cooling fans to turn on.
What will happen is:
You're tooling along and the existing wiring/circuit trips your electric fans to turn "on".
- the tap into the +12V (feeding the fans) trips your new "HOT" lamp relay
- this relay triggers, and shorts the feed wire to the "HOT" lamp directly to ground (emulating your sending unit in the head/block).
The "HOT" lamp lights up (regardless of/overriding what the sending unit on the engine is doing)
- when the fans do their job and shut off, the normal circuit (sending unit) can now trip the lamp if need be.
4-door Chevelle said
May 5, 2016
The sensor is + in and completing the circuit activates the relay to turn on the fan.
67ss said
May 5, 2016
So your temp sensor has 2 terminals and you are feeding 12 volts to one of them and then the other terminal goes to the fan relay?
You may not need to add another relay But what John posted will also work.
4-door Chevelle said
May 5, 2016
So the relay really just completes the circuit from terminal 30 to terminal 87, simple enough.
4-door Chevelle said
May 5, 2016
67ss wrote:
So your temp sensor has 2 terminals and you are feeding 12 volts to one of them and then the other terminal goes to the fan relay?
You may not need to add another relay But what John posted will also work.
Yes, that is my current setup.
Chris S said
May 5, 2016
Not a bad way to know when the fans kick on Darren. I like to know what's going on under the hood as well.
Good luck and keep us posted!
67ss said
May 5, 2016
If you switched your temp sender to controlling the ground to the relay instead of power you can then hook your light to the sender without needing a second relay.
4-door Chevelle said
May 5, 2016
67ss wrote:
If you switched your temp sender to controlling the ground to the relay instead of power you can then hook your light to the sender without needing a second relay.
In theory that seems correct but would I want to bump that ground wire up to a heavier gauge? Maybe it wouldn't really matter as the electric fan has it's own ground.
Would you give this your blessing John?
John D said
May 6, 2016
Yes, Chris' setup will work just fine, and no extra parts needed. The wire would not need to be upsized, it's only carrying current to trigger a relay and a lamp (18ga is fine).
Alter your existing fan relay trigger wiring (#85 & #86), and your cooling fan temp sensor.
Instead of feeding +12v in and out of the temp sensor, convert it to switching -12v/ground.
Alter your fan relay so #86 is constant hot +12v (should be from a fused source)
Connect your (now) "switching ground" temp sensor to #85 and T-tap this same feed into the factory "HOT" light wire.
When the engine hits whatever temp your sensor is rated to, it will close, complete a circuit to ground, then fire the relay & turn on the lamp.
4-door Chevelle said
May 6, 2016
I will get this all printed out as I am going to tear in to it tomorrow. One last question from above:
I found the wires on the firewall side of the harness and my bad eyes can't tell for sure if the wire is brown, green or gray?
I seem to have two wires labeled coolant light, maybe brown and brown with a white stripe. There should only be a single wire though right?
4-door Chevelle said
May 8, 2016
So I found the wire that when applying a ground lights up the HOT lamp in my dash. The curious thing is there is some juice on that wire, enough to dimly light my test light?
67ss said
May 8, 2016
That is normal don't worry about it. Depending on the bulb in the test light it may light even with the voltage drop across the bulb in the dash.
67ss said
May 11, 2016
Darren how did the new engine turn out, got any dyno numbers on it?
4-door Chevelle said
May 11, 2016
I am going to go talk to him tonight and see how it is coming. I know it isn't done and it may be a few weeks until it is so I am getting a little antsy!
4-door Chevelle said
Jul 13, 2016
I am back to print this out as I am ready to finish up all the connections. It sure is a lot of work when you convert from all mechanical gauges to all electronic.
The dyno numbers came in at 472HP and 476TQ on the stand. I will get it on a chassis dyno once I get some miles on it.
It took a looooong time but it will be worth the wait!
4-door Chevelle said
Jul 26, 2016
Imagine my surprise when I went to wire this up and I was already using the sensor as a ground so all I had to do was the T John mentioned. I bet John suggested this 6 years ago when I put this car together?!?
So, when my electric fan kicks on I want to have my HOT light come on in the dash. Silly I know but I want to know when my fan is actually running. It appears the dash is hot (+) and the temp sensor will provide a ground when it gets up to temp. I think I can use a relay but I can only provide a positive trigger and not a negative one. I read a few articles so far and no solid answer yet. I found the wires on the firewall side of the harness and my bad eyes can't tell for sure if the wire is brown, green or gray?
Any help appreciated as I do things "my way".
I bet John D has an answer on this one!?!
Thanks Guys!
What are you using to make the fans come on? Is it a ground controlled temp sender screwed into engine block to activate the relay? If so hook your wire to the light to that same circuit.
(Using "Bosch Ice-cube" terms here)
Terminal #30 connected to ground
Terminal #87 T-tapped into the negative-feed wire to the "HOT" lamp in the cluster.
Terminal #85 connected to ground
Terminal #86 tied to the same +12V circuit/wiring that trips your cooling fans to turn on.
What will happen is:
You're tooling along and the existing wiring/circuit trips your electric fans to turn "on".
- the tap into the +12V (feeding the fans) trips your new "HOT" lamp relay
- this relay triggers, and shorts the feed wire to the "HOT" lamp directly to ground (emulating your sending unit in the head/block).
The "HOT" lamp lights up (regardless of/overriding what the sending unit on the engine is doing)
- when the fans do their job and shut off, the normal circuit (sending unit) can now trip the lamp if need be.
So your temp sensor has 2 terminals and you are feeding 12 volts to one of them and then the other terminal goes to the fan relay?
You may not need to add another relay But what John posted will also work.
Yes, that is my current setup.
Not a bad way to know when the fans kick on Darren. I like to know what's going on under the hood as well.
Good luck and keep us posted!
If you switched your temp sender to controlling the ground to the relay instead of power you can then hook your light to the sender without needing a second relay.
In theory that seems correct but would I want to bump that ground wire up to a heavier gauge? Maybe it wouldn't really matter as the electric fan has it's own ground.
Would you give this your blessing John?
Alter your existing fan relay trigger wiring (#85 & #86), and your cooling fan temp sensor.
Instead of feeding +12v in and out of the temp sensor, convert it to switching -12v/ground.
Alter your fan relay so #86 is constant hot +12v (should be from a fused source)
Connect your (now) "switching ground" temp sensor to #85 and T-tap this same feed into the factory "HOT" light wire.
When the engine hits whatever temp your sensor is rated to, it will close, complete a circuit to ground, then fire the relay & turn on the lamp.
I found the wires on the firewall side of the harness and my bad eyes can't tell for sure if the wire is brown, green or gray?
I seem to have two wires labeled coolant light, maybe brown and brown with a white stripe. There should only be a single wire though right?
That is normal don't worry about it. Depending on the bulb in the test light it may light even with the voltage drop across the bulb in the dash.
Darren how did the new engine turn out, got any dyno numbers on it?
The dyno numbers came in at 472HP and 476TQ on the stand. I will get it on a chassis dyno once I get some miles on it.
It took a looooong time but it will be worth the wait!