Never buy another HVAC resistor

[JimBee]

Did you know that a heater fan can take out your blower resistor but still be operable?
Here's why and you can protect that resistor in 5 minutes.
That circuit is over-fused with 30 amps.

Okay, honestly, I don't know what level of fan amp draw can take out your resistor. I didn't have an ammeter when I did the tests, so I'm just guessing.

My test: My resistor fried. I hot wired the fan, it started and seemed to run okay at full speed, but judging by the hot spark when I touched the contact to activate it, starting it pulled a lot of amps (notably, the 30 amp fuse didn't blow). I didn't have an ammeter to determine the amp draw but I do have a new fan and when I hot wired that one, there was hardly any spark on fan startup.

I decided not to buy another resistor. When I have a little more time, I'll write up my workaround with some pics.

But, for now, I'll suggest toss the 30 amp fuse on that circuit and replace it with a 10 amp. If your fan motor is dragging on startup and fries the 10 amp, your resistor is in its cross hairs. I'll even suggest doing this crude test with a new fan motor—on startup there should be hardly any spark. If the new one gives a notable spark, I wouldn't use it. That's a return. (To pin things down a bit more, if you use the OEM circuit but bypass the resistor to test the fan, as noted below, there should be hardly any spark when you hot wire the new fan. You could do this with the old fan, too. Protected at 30 amps and still runs? Okay. What about at 20 amps, does the fuse blow? What about 10 amps, does the fuse blow?) I'll experiment more when. have time to try to bracket the amp level that takes out fuses of different amps.

Afterthought: I'm not electronics savvy, but even on the 10 amp mod, for those who are, you might consider inlining a shunt to bleed off a hot pulse on contact while allowing the full amps to pass through.

Comments welcome, of course.

[Vova585]

I think majority of members would recommend to replace both items at the same time and to keep original wiring intact. At least I would..

[scot850]

As the brushes on the fan wear down and you get a build up of dust and crud on the contact faces the motors will need more current to kick into motion. Also the bearings dry out with time. Cleaning the contacts (can't now recall how easy they are to access) and adding a couple of drops of light oil onto the bearings can kick the motor back into life. One of the issues is the bearing wear and eventually the motor can stick. I've had that situation when you accelerate the motor stuck and on slowing down it freed off. At that point replacing the motor is a no-brainer.

Good luck!

Neil.

[JimBee]

True, there's a right way to make the repair, just replace both parts, at around $250.00—which for my '93 850 as a 3rd car backup, seemed a bit pricey.

The problem that I'm pointing out is that you only know when the fan motor is dragging enough to fry the resistor when the fan takes out the resistor. I've been through that twice with two of my 850s. I wonder if the electrical techs who designed that circuit were chuckling about the expected sales of those resistors.

This happens because the circuit is over-protected at 30 amps—for the resistor. It's also overprotected for a failing motor, which is why I suggest down-regulating the circuit to 10 amps as a quick safety feature. If your fan runs at 10 amps, leave it protected at 10 amps. If it runs at 15 amps but not 10, maybe you can get away with a 15 amp fuse. Going higher than that, you're rolling the dice for the resistor. ( I also bought an ammeter that one of these days when I find some tinker time, I'll test my fan motors—the old and the new.)

If the resistor has failed, and you install a new fan motor, the easy workaround that will set you back less than $35.00, and that comes with a guarantee that you won't be buying another resistor, is a DC motor speed controller, which is what I'm using.
Link below.
If you look closely at the picture, you can see the on-off switch at the far left side of the terminal bus. The controller comes with a 30 amp inline fused lead. I'll replace that fuse with a 15 amp.

In my case, the dash control module isn't working, either. For that, I got partial heat by hot wiring the servo motors to close the ambient air box vane, and move a couple of heat vanes. I haven't pulled the servo motors to test them, but I'm pretty sure they're not all working. What to expect on a 32 year old car? Otherwise, it starts and runs well and has lots of upgrades with not many miles on it, for a 93 (my first Volvo bought in 2008). I seldom drive it now, but probably will in the winter to keep my other 850's away from road salt.

I used 10 gauge stranded wire to connect everything (equivalent to OEM). Home Depot sells different color coded wire by the foot. I'll probably rewire it with 12 ga. There's a 10 mm chassis bolt to the right of the fan motor, perfect for your ground lug. The spade connectors you'll hook up to are 3/8", so you need several 3/8 female connectors and small ones for the terminals on the controller. Harbor Freight sells a kit of assorted male and female crimp connectors for around $15.00. Autozone sells a 6 pk of 3/8 female spade connectors.

A permanent location for mine will be in the glove box. A small hole to pass the wires through is all that's needed.

Shade tree fun!

https://www.amazon.com/RioRand-7-80V-Mo ... rp_crs_1_t