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TR7 Heater Rheostat - repair or modern alternatives?

Colin Lindsay

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I've just stripped out the entire dashboard and heater unit from the TR7 as the heater is only working on one speed instead of three.This rheostat lives on top of the heater box and is inaccessible to hands, but not to water ingress hence this one is rusted solid. I'm assuming this is what controls the speed, as the slider controls all work but only at the one speed, if I swap the spade connectors about. I can only get second-hand versions which are probably hit and miss. There must be a modern version or alternative that would replace this?

It's an OTTER G6D unit and there seem to be replacements, going by a Net search http://www.ottercontrols.co.uk/index.html but what would be the best one? Could anyone advise on replacement with something modern, and secondly a good accurate test to confirm, that a replacement is working (eg using a multimeter?)

I've managed to get a photograph of the replacement units Otter supplies; but the ones that look the same are much bigger; the original is 14mm wide. Any thoughts?

All advice welcome as always.








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as the coils are sound the problem must surely be the corrosion around the rivets why not have a bash at drilling them off and clean up use small bolts to refit ???

car builder solutions show a 3 spd switch with similar looking coils , but its rotary ... i would mend what you have to destruction and see if you give it a new life 


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I'm not completely clear on how the wiring in those photos fits in to the wider loom. The diagram posted elsewhere doesn't show the otter switch and I'm not really sure what purpose it serves. If you still have enough of the wiring together for a test, you can safely bypass the otter (connect a wire across it) to confirm whether that fixes your problem.

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Ok. With absolutely no formal qualifications in vehicle electrics, but a garage full of broken things that I now understand how they work AFTER they've been broken.. try this for a theory.

The heater blower works on a three speed slider control. At present it only works on one setting, which was position 3. It seems to work like Herald wipers ie power is always on and the slider control earths it to slider and then to black earth cable.

One end of the large coil is a green and purple wire which runs to the slider control, position 1.

The green and yellow cable, the thickest, goes to the other end of the large coil and was position 2 on the slider.

The end of the small coil is attached to a green and white cable, which then goes to the Otter and out the far side to a bullet connector, from which one cable goes to the heater blower and one to the slider control, position 3.

I can't get my head around how the coils work, especially when both ends of the same coil are connected to different sliders.

I'm assuming that the Otter bi-metallic control is there to prevent overheating of one circuit, as only one cable passes through it - the one to the smaller coil - and so the fastest speed? In which case the Otter is permitting current to pass, and is not the fault?

OR: does the Otter open when current passes through, thereby diverting power to a different circuit and so increasing or decreasing speed?

There is also a similar kind of circuit-breaker (called a self-resetting circuit breaker) in the headlamp circuit.

Consequently as I'm averse to vehicle fires, I want to insert a similar device into the loom at this point to operate as intended, so am exploring the options.


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Have you looked at the wiring diagram? The coils (actually air-cored wire-wound resistors) are wired in series.

In position 3, the fastest speed, the switch connects the motor directly to ground, giving the full 12V and not using the "rheostat" at all.

In position 2, the switch grounds the fat green/yellow cable, which provides ground to the motor through the green/white wire, the otter switch, and the small resistor. Some resistance, reduced speed.

In position 1, the slowest, the switch grounds the green/purple wire, which provides ground to the motor through the green/white, the otter switch, and both resistors in series. Loads of resistance, very low speed.

My best guess as to what the otter switch is for... is to protect against the resistors overheating if the motor stalls / gets jammed / has no air, although I can't see it working terribly well. It needs to be closed (conducting) in all non-fault conditions. I don't see it working like the headlamp pod breaker, which detects persistent over-current, because the current it's protecting is much lower than the full-speed current that it's not involved in. It must be trying to detect the heat from the small resistor.

Anyway, like I said above, you should be able to test the otter either with a multimeter or by temporarily bypassing it.

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22 minutes ago, NonMember said:

Have you looked at the wiring diagram?

Yes, repeatedly, but they show a static system, not what does what at different settings.

Now that you've explained the coil system - NOT as I thought they switched from one to the other, but that they go from one to BOTH, I can grasp how it works, and from that, work out why it doesn't. Much obliged!

I've cut the Otter out from the circuit at present, a replacement is en route but it's second-hand so I'll have to test it all anyway to see if there's any change to the original setup. 

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I've gone directly to the company and spoke to Otter Controls' Stephen Hollick, who informs me that they don't have any in stock, but sold a batch to the Rover SD1 Owners Club recently.

Kev Clark of that club informed me:

"This is a thermal cutout switch. I'm not familiar with the TR7 but on the Rover SD1 you have two fan speeds, and this switch is designed to cut power to the fan motor if the unit overheats. They tend to fail so that the fan only works on the fast speed, not the slow one. If you want your fan to work as originally intended, you need this switch working."

So rather than rebuild the system with the switch left out, which is feasible, I've managed to source a new one, and will solder that in as the manufacturer originally intended. I might as well, someone thought it was necessary at one time, and I'll not take any chances by leaving it out.


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Pete, you are spot on! I've just done a quick search and lo and behold, up comes a lengthy article including this paragraph:

What is causing the Vauxhall Zafira fires? 

The DVSA agreed with Vauxhall’s root cause investigations into the Zafira fires issue, which identified the cause of fires to have been faulty repairs of a thermal fuse in a resistor that forms part of the heating and ventilation system of cars with air conditioning or with no air conditioning – cars with climate control are not affected. 

In the first recall Vauxhall replaced the thermal fuse in affected cars, while also replacing the cabin pollen filter and checking for a hole in the windscreen surround that might have been caused by the refitting of a windscreen and could let water into the system, causing corrosion to the fan. Corrosion of the blower unit, or wear and tear through use, is the most likely fault, which should trigger the safe activation of the thermal fuse.

Vauxhall’s inspections involved random checks of 1,000 cars, of which 2.6% had been found to have badly repaired fuses. The fuse is designed to deactivate the system in event of a fault, to prevent overheating. However, Auto Express was given exclusive access to some of the unauthorised repairs where the fuse has been bypassed through highly dangerous, yet ingenious means – including using copper wire to reconnect the fuse terminals, holding it together with a crocodile clip or even screwing it back in place.

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14 hours ago, Pete Lewis said:

So you will feel, good when its done  then ?


Smug as a big smug thing, Pete.

Doug - the unit is encased inside the heater box and hangs well away from anything else, even if it melts and drops off it'll still land in the plastic case, which is quite solid and will take a lot of heat to melt through. Part of the challenge of older cars!

(Solid state? isn't that a modern Japanese thing, like MX5s?)


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