Electrical help with heater blower on '78 SA FJ (1 Viewer)

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Guys:
Good afternoon. I have installed the factory heater in my '78 SA FJ that didn't have one, all went well. The issue I have is that the blower works on both low and high speed with the engine off but when I turn the engine on I lose power to the blower. I have the wiring diagram (fig. 13.47 for FJ40 1978) but nothing jumps out at me. Any thoughts , ideas, or suggestions would be great. Also there doesn't seem to be much difference between low and high speed.

Thanks, Craig
 

TheHardWay

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That wiring diagram shows a light blue/red wire coming from the 15 amp heater fuse on the fuse box. The heater blower switch would plug into a plug branching off of the harness running behind the dash. Since your SA rig had no factory heater, how did you get power to the switch? I suspect that's the source of your problem.
 

3_puppies

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you are saying you have power with the key in the off position? or is the key in acc? or is the key in the run position?
 
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THW: I had to build a jumper in the engine compartment because the plug from the blower and the plug with the same wire color configuration were the same male connectors. I did find a four wire plug up under the dash.

3-Ps: I have power to the blower until I move the key from ACC to ON..

20201025_172557.jpg


20201025_172455.jpg
 

TheHardWay

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Just verified the blower should only work when ignition is in the On position. Did you plug the blower switch into the 4 prong plug you found under the dash?
 

John McVicker

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Could this have anything to do with Toyota’s negative ground switching to complete a circuit?


EDIT: that is, completes the circuit by connecting the ground as opposed to connecting the positive.
 
Last edited:
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By backwards I mean the blower should only work when the ignition is turned on, not lose power when I turn the key on.
Correct me but isn't the only reason why the blower has power when the ignition is turned on so you don't accidently leave the fan on running the battery down with the ignition off?
Couldn't I pull the 15A fuse, get a stand alone fuse holder, get power off the fuse bus, and call it good? Knowing I have the risk of the above situation?
It was 12 degrees here this morning!

20201026_143553.jpg
 

TheHardWay

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The fact that you have power to the blower when ignition is on Accessory, but none when in the On position makes your jumper setup suspect. IF the blower fan control knob is plugged into the harness plug behind the dash, and IF the jumper from the blower to one of the two plugs under the blower is connected to the correct plug, you may have your jumper wires connected to the wrong spades. You should test the connections with a multimeter to determine where the juice is coming from when the ignition is on Accessory position and On position.
 
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Guys:
Good afternoon. I put my multimeter to work.
Conditions:
- Disconnected my jumper from the heater blower to the 3 prong plug in the engine compartment
- Pull Fan knob pulled out to high
- Engine OFF
- Probing the 3 prong plug under the hood
- Black probe to ground
- Red probe (white/black wire 12v), (blue/white wire 0v), (blue/black wire 0v)

- Key in the ON position
- Red probe (white/black wire 12v), (blue/white wire 0v), (blue/black wire 12v)

Pull Fan knob in the off position
- Engine OFF
- All three wires read 0v

- Key in the ON position
- (white/black wire 12v), other two wires 0v
 

TheHardWay

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From the wiring schematic diagram, the white/black is ground. The light blue/black is power from the switch when on HIGH. The light blue/white is power when the switch is on LOW. When ignition is OFF your multimeter showed no power to either the light blue/black or light blue/white with the fan knob on HIGH, so that's good.

When ignition is turned to the ON position your multimeter shows no power to the light blue/white, and power to the light blue/black with the fan knob on high, which is good, so it appears your jumper is not wired correctly to the plug coming from the blower motor.

Time to run a jumper connection test.
 
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Guys:
Good afternoon.
I checked my jumper and color coded it to make sure. Tried it again with the same results, fan works with the engine off but not with the key in the ON position.
I swapped the blue/white and blue/black wires and it just switched the fan speed.

Craig

20201104_153012.jpg
 

Cromagnon

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Key in the ON position
- Red probe (white/black wire 12v), (blue/white wire 0v), (blue/black wire 12v)

Pull Fan knob in the off position
- Engine OFF
- All three wires read 0v

- Key in the ON position
- (white/black wire 12v), other two wires 0v
First off let's clear up a bit of confusion. You have two different conditions with the key in the "ON" position. Which one is correct?
Second, why is there 12VDC on the WB wire. WB is ground, ground should read 0VDC. If you or a PO have "fixed" the wiring and changed the wiring schema then you need to let everyone know up front so we can advise you intelligently. Otherwise WB is ALWAYS ground on an FJ40. You need to fix this first before anything is going to work on the blower. Let us know.
 
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I haven't done anything to the wiring and can't speak for the PO or POs.
Are you saying the white/black is ground and should be 0 volts?

My FJ40 didn't have a heater so when installing this heater I have found plugs that have matched with both the blower though too short to mate and the Pull Fan switch did have a matching plug up under the dash.

Could I have a wiring problem with the Pull Fan switch?
 

TheHardWay

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Humor me - try the same test, but with the pull fan switch on LOW instead of HIGH. What happens?
 

Cromagnon

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First, you have two separate conditions with the key in the "ON" position. That can't happen, so which one is correct.
Second, WB is GROUND, ground is 0VDC, unless you have a negative ground system. Trust me on this one...you do not have a negative ground system. Get the voltage off of the WB wire. You may have a wiring problem at the switch, the harness, the homemade jumper or some PO induced wizardry. You are the only one that will be able to determine that.
 

Cromagnon

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Use this, or some variation of it, to do a complete test of all possible conditions:

Engine not running.
Key in OFF position.
Fan Switich in OFF position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in OFF position.
Fan Switich in LOW position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in OFF position.
Fan Switich in HI position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in ACC position.
Fan Switich in OFF position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in ACC position.
Fan Switich in LOW position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in ACC position.
Fan Switich in HI position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in ON position.
Fan Switich in OFF position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in ON position.
Fan Switich in LOW position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Engine not running.
Key in ON position.
Fan Switich in HI position.
Black probe to Ground
Red probe to WB =
Red probe to LW =
Red probe to LB =

Let us know what you come up with.
 
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Guys:
I checked all the conditions above. This is with the blower out of the circuit and testing the jumper.
The 1st six conditions were WB 12V, LW and LB 0v
The last condition with the key ON
Fan OFF - WB 12v, LW and LB 0v
Fan LOW - WB 12v, LW 0v, LB 12v
Fan HIGH - WB 12v, LW 12v, LB 0v

When I plug the blower back into the circuit that is when it will run with the engine OFF but as I turn the key to ON I lose power.
 

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