BJ70 Low Cab Heat - Seeking Ideas

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Jan 3, 2022
Dartmouth, NS
Hello 70rs:
On to the next challenge: very little heat in the cab: BJ70 420,000kms.

The engine temp is in the mid range after warming up, the heater sliding controls are put in red/warm, and the direction settings are set for the floor vents. Fan is on mid range and there is lots of air blowing. (great smell of a dead mouse too, but that is for another story). To complicate things, the previous owner installed a dash toggle switch to turn on the heater relay (i think that is what they told me) which is in the "on" position. I was going to start by pulling out the dash and looking at the controls to see if all is moving in the right fashion (Famous last words we know so well: "Can't be that complicated can it?") .

If this were your challenge, where would you start to look for a solution?
Thanks in advance. Paul
Check the hoses and see if they were disconnected. Then make sure the heater control cable actually opens the valve in the engine bay. Mine was broken and the switches did nothing. After that start digging into the dash. I would start with the controller and make sure it's working, then move on to making it back to factory if you can (helps with troubleshooting).
Thanks Irish11. I am unsure what role the heater relay plays in the providing heat. My thought was the sliding thermostat temperature selector on the dash would engage heat or cooling depending on what is needed.

Any thoughts on how the function of the "heater relay" works?


I believe the heater relay is just to proved power to the blower straight from the battery source and a relay to trigger it.

Now why they used a switch, I don't understand. Possibly the relay went bad so they decided to put it on a switch to override it? Either way there should be no actual temp control with electricity.

It's possible like said above, the heater control valve at the firewall isn't opening. You can verify that by sliding dash controller to cold and looking at the valve, then sliding to hot and making sure that valve has changed.

Here is where the valve is on the firewall.


My gut feeling tells me the heater core is plugged and fluid isn't going through it, hence no heat. It's easy enough to disconnect hoses on the heater core to flush it out. They are both high and easy to reach on the firewall.
Thanks FJBen. Will start with checking for pinched or disconnected hoses, then the heater core valve, then flush the core, then pull the dash.....then pour a fine stout and set the whole thing on fire...
Relays are used when a device requires a good bit of amperage. A low-amp dash switch makes the relay engage the high-amp circuit.
This way you won’t get fried by flipping the switch is something is wrong.

I have a toggle switch in my dash that sends power right to the ac compressor clutch… no relay. It’s a bypass.
well, the plot usual.
I expected to find a heater control valve on the firewall hose that is controlled by the temperature sliding switch on the dash. Instead, there appears to a vacuum operated valve. See pics.

Anyone seen this type before?



Agreed that is definitely not factory. There should be a mount like FJBen posted. Where a cable connecting your controls turns the manual valve open and closed. I suspect that is your problem. Mine was that cable was broken. You should have the cables coming from your controller out through the firewall instead of that diaphragm looking thing. Maybe search around and see if you can find a factory firewall mount. The cable is still available on Megazip.


Now I am after one of the little white plugs that you cannot buy separately. (so if anyone has one I am all ears)

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In the British car world, some 50’s cars had a heater valve like a little faucet under the hood: no cable control. I have seen homemade cable operated heater valves made from brass ball valves with modified handles. In your case I’d get some bent tubing of the right diameter and run without any valve at all for a while.
Yes, agreed. I think I will disconnect the "no factory thing" and put in a 90 degree 3/4 inch elbow. If I get heat then I know the heater core is fine and will drive the way it is for the winter. If no change to cab heat, then will pull the dash to see where in the heater control cables *actually* go....
I jury rigged my ac about 4 years ago when the ac amplifier stopped working. I may not ever fix it properly.

MGB’s have the heater valve bolted just above the distributer. A leaky valve means dead car. Her’s a pic of a home repair.
Yep, the plot gets weirder.

The original heater valve was replaced by a vacuum valve. The vacuum valve was plumbed to an electric vacuum switch and which was plumbed to the brake booster line. The steel heater selector cables from the dash have been removed. I suspect the vacuum switch is operated by the dash heater relay switch that i previously mentioned. Kind of baffled why someone would want to rig it this way but they must have had their reasons.

Going to reverse engineer the system back to original. Will remove the v switch, and patch the hole in the brake booster line. Put in a temporary 90 degree elbow to let coolant easily flow to the heater core. Pulling the dash tomorrow to see what craziness was employed in the heater controls....



Hang on to that electric vacuum switch. Could be used as engine shut off valve/switch.

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