Guessing the value of heater speed control resistors

Seems fine to me.

What fuse feeds the motor? If it's a 10A fuse, then P(watts) = V x A, so the maximum power would be 14 x 10 (14volts whilst alternator is at full output) = 140 watts. Personally, I'd add at least 25% to allow for the amazing regularity with which these things seem to blow on cars. Badger.

Good!

I can picture a car battery (my lab supply is only rated to 5A), my

My intention is to mount the resistors outside the heater assembly to allow for accessibility in case of future failure.

Cheers

Richard

The present resistors are where they are to allow for cooling!

Badger is probably not far wrong on his estimate, and you'll need to mount the resistors in the airflow otherwise they'll get hot enough to cause damage.

In article , Richard Savage writes

Coupla things:

The resistance rises with temperature, substantially so. In this context, that's used to advantage - the motor gets a good voltage kick to start it, but the volts drop off as it gets going.

Without knowing which wire has been used (nichrome, constantan, etc.), you can only guess at the intended _operating_ resistance. For values in that range, I wouldn't attempt to measure it that way. V and I measurements would work, in operating conditions. Y

Is there a chance you can get that from HBOL or a known-good heater?

I have these joys to come shortly - Marge's power steering pump gave up recently, so I've taken advantage of the worst foulness to get her mended (recon pump + steering box - ouch!), but when she's done it's the heater matrix and that annoying wiper self-parking switch, etc.

Will let you know if I can get a good measurement, but she's a 110, so it may not be the same value resistor mat.

I was (am) concerned about cooking the resistor and wondered about mounting it (a) on a sod-off big heat sink and (b) fan cooling the enclosure. Alternatively I had pondered building a pair of fixed current sources. However, that's probably too complex.

Richard

Understood.

In this

Wouldn't my crude current measuring technique work? From that I can work out each resistance. Unfortunately there are no markings on the resistor.

HBOL just about admits to the existence of the heater ;-)

or a known-good heater?

Possibly, but as far as I can tell my pair of resistors are intact. I simply want to replace them with new to fend off failure. Measuring those from a good heater presents the same problem doesn't it?

Where will you source the replacement matrix? Mine came from Famous Four and is 'almost-but-not-quiet-identical' to the original. It's a very tight fit where the end pieces sit in the case and the flow/ return pipes are not as elegantly swept into their end piece. This forced a bit of plastic surgery on the heater case.

Thanks

Cheers

Richard

If you're going that far Richard, you may as well just use a PWM circuit.

Steve

When i had problems with my heater (only one speed full on) on a 1990 RRC, I had taken the heater unit apart before and knew there was a resistor in there so I removed it and checked it out and there was nothing wrong with it, I checked the multi plug to the heater and found that only the two motor wires were used. The resistor that was used for the motor speed was situated under the decker panel. This seemed a lot of work to get access so I changed the wiring to use the one in the heater unit (three years ago still working). I don't know if yours is the same, mine is a G reg but it is the

The speed resistors for the auxillary A/C fans are mounted on the wiper motor blanking plate under the scuttle, which is not really in the air flow. However it is a much larger wire-wound ceramic resistor. I see no reason why the main blower shouldn't use a similar resistor mounted likewise.

Alex

Just been through this excercise with my Toy*** Surf, your resistor values are in the right ball-park. I used a 10 watt 1R5 to replace one o/c element in mine, then checked the current when the fan was on. Discovered that fan speed 2 (out of 4) dropped too much voltage across the resistor for its power rating. So now I just don't use speed 2. By the time you've messed around with resistor packs, heatsinks etc you may as well just replace the original thing, its quicker if not cheaper and certainly neater.

Gordon ex RR Classics but still with an 80 and an 86.

Multiple replies to various answers: I know that the fan control resistor is mounted where it is to assist in the cooling. I just don't want to reinstate it in such an impossible to get at location. There are no resistors mounted under the decker panel in my 1990 3.9 RRC. It seems that mine was built (slung together between tea breaks) just before the resistor pack was fitted under the decker panel

Hi Steve

Yep, which means dragging out my _very old_ college notes. I can't help hearing someone saying 'keep it simple, Stupid', every time I think of an overly-clever solution to what is a simple problem. At least I am pleased to have found that RS stock 50W resistors.

Cheers

Richard

Hi Gordon

You're right about the long term cost - both in terms of money and time - of a clever solution to a simple problem, but that's part of the fun (apparently!) of owning a RR, isn't it?

Thanks for resistor value corroboration.

Richard

I just happen to have shipped this very evening a very dirty little circuit that can do it - built for a fellow AFL member's projects. Go on, you know you want to....

How many amps again ?

Steve

Evening Steve

Until I test the unit I can't tell you. SWMBO has cancelled all her normal evening activities during the week (squash, Red Cross etc) on account of the snow. Thus I am obliged to postpone useful tasks, such as RR heater assembly reinstatement, to keep her company.

Your circuit sounds inviting! Is it a direct substitute for the two WW resistors, i.e. it retains the existing switch and loom whilst providing the two reduced fan speeds?

Happy to continue this conversation at my real address as used previously if you would prefer it.

Cheers

Richard