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Post Info TOPIC: How to wire HOT lamp in dash to come on
4-door Chevelle


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How to wire HOT lamp in dash to come on
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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!



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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
John D


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Lemme think on this one...


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John D. - St. Louis Park, MN.

1965 El Camino - LT-1, 4L60e, 4wh discs, SC&C susp.
2013 F-150 Platinum - Twin Turbo 3.5

2018 Factory Five MkIV Roadster build thread
67ss


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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.



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Chris P
East Central, Mn

66 Chevelle 300 deluxe
John D


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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.


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John D. - St. Louis Park, MN.

1965 El Camino - LT-1, 4L60e, 4wh discs, SC&C susp.
2013 F-150 Platinum - Twin Turbo 3.5

2018 Factory Five MkIV Roadster build thread
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The sensor is + in and completing the circuit activates the relay to turn on the fan.

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
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67ss


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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.



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Chris P
East Central, Mn

66 Chevelle 300 deluxe
4-door Chevelle


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So the relay really just completes the circuit from terminal 30 to terminal 87, simple enough.

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Darren - Crystal, MN
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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.



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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
Chris S


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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!



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Chris S.

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67ss


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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.



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Chris P
East Central, Mn

66 Chevelle 300 deluxe
4-door Chevelle


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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?



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Darren - Crystal, MN
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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.



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John D. - St. Louis Park, MN.

1965 El Camino - LT-1, 4L60e, 4wh discs, SC&C susp.
2013 F-150 Platinum - Twin Turbo 3.5

2018 Factory Five MkIV Roadster build thread
4-door Chevelle


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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?

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
4-door Chevelle


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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?

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
67ss


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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. 



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Chris P
East Central, Mn

66 Chevelle 300 deluxe
67ss


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Darren how did the new engine turn out, got any dyno numbers on it?



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Chris P
East Central, Mn

66 Chevelle 300 deluxe
4-door Chevelle


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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!

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
4-door Chevelle


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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!

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
4-door Chevelle


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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?!?

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
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