dskippy888

I recently had the A/C in my '86.5 928S fixed at the dealer. However, when I operate in any setting, the blower will randomly and intermittently start going at full speed for 5-10seconds and then return to the original setting.

I was wondering why it is doing that, and what is the remedy? Is it a big fix?

Perhaps this information may be useful in diagnosing the problem.

The service note says they performed the following:
"100496 A/C in OP 10.00 Have to retrofit system, R12 to 134. in order to check system and see any leaks. Instealled rec, drier, and removed comp to drain and refill oil in to system. No major leaks at this time. Dye has been added. Temp is good. 5 C."

LT Texan

search "magic blower syndrome"

dskippy888

Thanks! I found it.

But I had a few questions. Rennlist says on [http://members.rennlist.com/pirtle/svc_blo.html] that one needs only to replace a resistor or retrofit a modification, but the service note said they did some kind of retrofit to it. Is that the same as what the website was talking about? Terribly sorry to be a bother, I'm quite helpless when it comes to the mechanics of cars... Much appreciate any help you can give me though!

SeanR

You found the correct site.

I'd just replace it with a new one.

j.kenzie@sbcglobal.net

Yep! Not that hard to do it yourself. Check to see what exactly the service guys replaced.
Basically the fan speed is controlled by a selection of resistors. These tend to get hot, so they are placed in the air duct of the HVAC system. When the resistors fail they are designed to turn the fan to full speed as a safety measured to cool the resistor pack maximally. Later they modified the design to default to a full shutdown if any resistor failed.
Good luck,

WallyP

Well, sorta...

If a resistor fails, you lose the low speeds.

The "magic blower syndrome" is slightly different. There is a thermally-operated switch that bypasses the resistors when it gets hot (earlier cars). This is intended to cool the resistors to a safe level. On later cars, the switch cuts the blower off until it cools.

There are several possible reasons for the switch operating:
- The blower bearings may be getting dry, and the blower is pulling more amperage than designed. The bearings can be lubed, but getting the blower out can be a hassle.
- The thermally-operated switch sometimes just gets tired, and operates at too-low a temperature. You can tweak it by some judicious bending.
- Sometimes, crud builds up on the resistors and switch, increasing the operaing temperature.
- Very high ambient temps can cause the resistor temps to get high enough to trigger the switch.
- If the rubber seal at the back of the engine compartment is out of position or missing, very hot under-hood air will enter the HVAC plenum entrance, causing the switch to operate.

You can try just running on the next higher speed - that will often prevent the switch from triggering.

Go to the Nichols site for some good info on the situation...

dskippy888

Wow!! You guys are so knowledgable!! Thank you so much for the in depth information. I will try to replace the resister, but certainly check out the other possibilities that Wally has named. Suppose I will also have to first check with the dealership techs to see what it was they actually did to the system... it's always a pain because the service techs change all the time!

Once again, many thanks!

j.kenzie@sbcglobal.net

Wally is one of the most knowledgeable people on the forum. I hadnt seen him posting for a while.
Good to see you, Wally!