Heater switch/wiring

[Wike]

When I purchased my car, ('74 TR6) back a few years ago, the heater unit was in a box in the trunk. The hose connections on the fire wall were plugged. I disassembled the heating unit, pressure tested the core by plugging one end and hooking a garden hose to the other end and then submerging in water to check for leaks/loss of pressure. Everything ok, cleaned the heater box, fan motor, etc., painted. Tested fan motor operation by applying power to green/yellow wire first, low speed ok. Second, applied power to grn/brn wire, high speed ok.
This past weekend, after having installed the heater unit in the car this past winter, I hooked up the new heater fan switch, turned the ignition to on and the green wire that provides power to the heater began to melt and smoke. After removing this wire from the new harness, I am trying to find the cause. If I connect power directly from the battery to the # 4 spade on the heater switch with grn/yellow and grn/brn wires on #6, and #8, I see that the power feed wire begins to heat up very fast with the switch on, no fan movement. If I run power to grn/brn or grn/yellow wire fan operates. Could it be the switch?

 

[NutmegCT]

"After removing this wire from the new harness ..."

might be a clue.

When you tested the motor before installation, were you using the new harness?

Just wondering if your new harness may have a problem itself.

Tom

 

[Wike]

By isolating the harness, not having it in the test, directly getting power from battery to switch, feed wire from battery to #4 spade on switch heats up. Power directly from battery to fan motor wire(s) fan operates.

 

[NutmegCT]

Wike - just a thought. There are a couple old posts here referring to problems when the heater (blower) switch on a TR6 is mounted upside down. The un-used terminals can ground out.

Can you remove the switch from the dash, then try the system?

If it still gets hot, then I'd say it's the switch.

Tom

 

[DNK]

That switch is a real pain with the metal around it
Do as Tom said and remove the switch.
On the Masters panel Dan supplied a bigger wire to carry the load of that switch

 

[TR3driver]

Just a thought, are you sure it's the right switch? Stags with AC use a switch that looks nearly identical, but has different internal logic.

It's possible the switch is actually defective, but not likely IMO.

One other thought, what are you using for a fuse? The original Lucas fuses used a different rating system than US automotive fuses do. The difference is nearly 2:1, so if you are using an AGC or SFE type fuse, it should only be rated for 20 amps, not 35.

Good idea, IMO to put some heat-shrink tubing on those unused terminals.

 

[hondo402000]

sounds like you are grounding the hot lead, you should also have a digital multimeter, then you can check the switch in the off, low and High positions to make sure it works

Hondo

 

[NutmegCT]

Wike - what's up? What did you find?

Tom

 

[Wike]

Here's the findings....apparently when installing the heater switch in the new switch plinth the bottom spade terminal, (#1)bent downward and wedged or grounded to the metal dash facia. I was totally unaware of this until just this morning when I went to pull the switch out to test. I pulled the switch, tested it with great results. Before re-installing I will be heat shrink tubing all spades on the switch not needed. Thanks to one and all for all the knowledge, suggestions and advise. Now I will be looking for a new windshield wiper switch, the shaft that holds the knob broke off! The big three no longer stock or sell these anymore.

 

[TR3driver]

That's great, Wike, but I encourage you to look into why the fuse did not blow. Fixing it now may save you grief the next time there is a short.

Also FWIW, I believe adding one relay would let you use almost any 3-position switch for the wipers, instead of requiring the rather unusual logic of the original switch.