Blower sometimes needs the defrost button to start

[Eric Eliel]

I've read plenty of posts about blower motor issues.

My blower motor sometimes (not always) requires the pushing of the defrost button to start turning. Once the defrost button is pushed, I can immediately push the normal vent button and the fan will continue to run.

One post suggested this was a blower motor brush failure. I can't understand why that would be the motor brushes.....if the brushes are bad, the motor won't run period.

I suspect the CCU or the blower motor controller, maybe a ground issue.

Any words of wisdom?

(yes I used the search function)

[toomany MBZ]

Classic symptom of a failing CCU. Check for cold solders. A rebuilt unit runs around $300, and that was a couple years ago.

[Cr from Texas]

I still think it could be failing brushes - defrost puts it into a higher speed with more current. Once it's started, maybe it is easier to keep it running over a bad spot in the brush to armature connection.

Wouldn't hurt to pull the blower motor, clean, and inspect.
Easy job - do a search.

[cdplayer]

I had a similar problem in my SEC. I changed the strip fuse (was severed under one of the screws). And cleaned the contacts. No more problem.

[Brian Carlton]

Originally Posted by Eric Eliel

I've read plenty of posts about blower motor issues.

My blower motor sometimes (not always) requires the pushing of the defrost button to start turning. Once the defrost button is pushed, I can immediately push the normal vent button and the fan will continue to run.

Will the blower run if you use the economy button and select the high fan speed pushbutton?

[tangofox007]

Originally Posted by Eric Eliel

My blower motor sometimes (not always) requires the pushing of the defrost button to start turning. Once the defrost button is pushed, I can immediately push the normal vent button and the fan will continue to run.

There are a couple of relays in the push button unit which power the fan regulator. Which relay is a function the selected button. The contacts on the relay that operates with the EC button could be marginal, so that it won't pass enough current to start the motor, but is adequate once the motor is running.

The K2 relay is actuated through the cold engine lock-out switch. The K3 relay bypasses the K2 relay so that the blower will operate when Bi-level or Defrost is selected and the cold engine lockout switch is open. If the cold engine lockout switch is closed, both relays will be actuated simultaneously when Bi-level or Defrost is selected. So...a "weak" K2 relay could cause the described condition.

[Eric Eliel]

Originally Posted by Brian Carlton

Will the blower run if you use the economy button and select the high fan speed pushbutton?

Once the blower is running, it will run in all speeds. Occasionally the blower will stop all by itself (bad ground or cold solder joint). Sometimes the blower doesn't need any help and starts right up. The blower problem happens in the EC mode and the A/C "on" mode (forgot the technical term). Defrost ALWAYS works, no issue there.

Is that cold engine lock out switch normally open or normally closed when cold?
Its not apparent from your description if the K2 and K3 relays are inside the CCU or on the fender under the hood (and I'm too tired to go out to the garage to look at my schematic.........but I will in a day or so).

Thanks!

[tangofox007]

Originally Posted by Eric Eliel

Is that cold engine lock out switch normally open or normally closed when cold?
Its not apparent from your description if the K2 and K3 relays are inside the CCU or on the fender under the hood (and I'm too tired to go out to the garage to look at my schematic.........but I will in a day or so).

Thanks!

The lockout switch (located on the engine thermostat housing) is open when cold; when closed, it provides a ground path which actutates the K2 relay.

The relays are in the push button unit. Sorry that I failed to make that clear.

If you blower starts reliably when Defrost (or Bilevel) is selected, and it doesn't start when Normal or EC is selected, the problem isn't the blower motor or its brushes. The only difference is with which relay is being used, which would point to a problem with the relay and/or its associated circuitry.

[GRIESL]

Staying tune to this thread--mine does almost the exact thing.

[RML]

Originally Posted by GRIESL

Staying tune to this thread--mine does almost the exact thing.

Mine as well. Exact same symptoms. I am planning on taking the CCU from my 85, which works perfectly, and putting it in my 84. I'll see if the problems go away. If so, I'll conclude it is all in the CCU.

[Brian Carlton]

Originally Posted by tangofox007

If you blower starts reliably when Defrost (or Bilevel) is selected, and it doesn't start when Normal or EC is selected, the problem isn't the blower motor ot its brushes. The only difference is with which relay is being used, which would point to a problem with the relay and/or its associated circuitry.

I agree.

[Eric Eliel]

When I get around to fixing this, I'll take some pics. Thankfully the car still drives and the fan will work. Its just annoying.

Notice I did my best to list the key words in the title which will help the next poor sap to find an answer.

[Lycoming-8]

Tango,

Can you describe specifically where on the CCU board these two relays, K2 & K3, reside? I am trying to resolve a problem with one of the relays on that board and have not been able to determine just what the function of each one is. Thanks for your help.