6 replies to this topic

[Steve220]

Hi all,

 

I've got another twin speed heater unit here but the resistor has snapped. I've also got the original from the car and my plan is to replace the dated curly wire resistor with either a wirewound or metal film one.

 

My days or electronics and maths are behind me so thought i'd ask here. A few facts to lay down first.

 

Motor - 12v 1amp (as per wiring diagrams and research)

Resistor on working unit - fluctuates between 3.6-4 when set to 200 on multimeter.

 

Don't know what wattage resistor i will need - or in fact what resistor i will need. I've trawled the internet most of the afternoon for someone that may have done this before, but no one has got a definitive answer! So, in the name of science, here we go!!

Edited by Steve220, 27 December 2017 - 04:34 PM.

[Swift_General]

Using your figures, the resister would convert 2.1 watts to heat if wired in series with the motor with 12V across them. However, I would have thought that the blower motor would have drawn substantially more current than 1A, like 5 or 10 times more.

Edited by Swift_General, 27 December 2017 - 06:29 PM.

[Steve220]

Its 1amp

[Sprocket]

The fuel pump draws 6 or 7 amps, the blower must draw more than 1 amp

[Spider]

The Smiths Heater Blowers I've measured are just under the 5 Amp mark, but that's likely an earlier type that what's fitted in yours.

 

Regardless, just working from the figures you've given, the voltage drop needed across the resistor would be around 4 volts. That across a 3.6 ohm resistor is 1.11 Amps and that works out to a 4.8 Watt Resistor. None the less, I'd suggest fitting a 10 watt as a minimum and preferably a 20 watt type just to keep the spot temperature of it down.

[Steve220]

Unless all the paperwork for the motor is wrong as well as the wiring diagram, then Im not sure. It all says 1amp.

[alex-95]

EDIT: See the bottom of this thread for the wiring diagram ----> https://www.theminif...eater-resistor/

 

I was looking for that information the other day so I could make my SPI heater into twin speed. I ended up buying a rover one and modify it fit the heater (pictures and writing below), the resistor is in the air flow of the fan to keep it cool on the MPI's. I had to go through all the coils to get it to what I wanted which was 8 volts which matches make spiders figures above.

 

So one of the winter improvements was the heater, I modified it so that it would fit flush with no dash rail padding fitted (pics of it a few pages back) but when the fan was switched on it would push the mechanism down to the car position  . So with a bit more thinking, the mechanism was improved so now it stays in the position it should. Another thing with the single speed SPI heater it's fast and noisy so I wanted to change it to a 2 speed heater like the MPI (1 of only 3 good things about them, The others being the seats and the intermittent wiper switch ) So I had a look at how the MPI heaters were slowed which is just a resistor fitted into the heater so the fan cools the resistor when it's on. I had a look on ebay and found a rover one clicky  which when it turned up, was a bit big.

 

 

 

Not a problem though...

 

Just needs to be cut to these sizes, I cut a template from card then placed it on the heater where it needed to be and traced and cut a hole for the resistors to go through. notice the corner cut off for the clips to attach still. We found it had to be cut to one side as the resistors were close to the heater body.

 

 

 

Once the holes cut I drilled the mounting holes. ignore the black screw, bit of trial and error...  I powerfiled a curve on the side to curve with the heater body and cut a rubber seal out of a bicycle inner tube.

 

 

clearance to the fan.

 

 

 

 

 

We temporarily wired it up the H and L terminals which with full power gives the 12v and on the other speed is 8v which I think is roughly what the MPI speed is. Now I just need to wire it fully into the car.

Edited by alex-95, 25 October 2022 - 05:00 PM.