Evaporator blower motor voltage

[dw1]

There have been several threads on the subject of the evaporator fan motor, for example:
https://forums.pelicanparts.com/porsche-911-technical-forum/744948-911-sc-ac-blower-over-condenser-smugglers-box-dead-not.html

I mention this because I diagnosed the "chirping" at speed settings I and II (but not at III) as the squirrel cage fan rubbing on the plastic plate inside the evaporator housing. (The car is an '87 3.2, so the fan motor points down.)

I was able to fix this, but while "I was there" I decided to check the fan output, but first I measured the voltage applied to this fan at various settings. I am VERY glad I did, because applying 12v directly to the fan to test it may cause more harm then good.

I measured:
fan setting I: approximately 3.1 V
fan setting II: approximately 5.4 V
fan setting III: approximately 9.1 V

Does anyone else have similar experience/data?

[dw1]

Btw, the above fan voltage measurements were made at a battery voltage of approximately 12Vdc , measured across the battery posts.

Remeasuring this at a "normal" engine idle battery voltage of approximately 13.7Vdc results in measurements that are a bit higher than those above (the new measurements being 3.5v, 6.2v and 10.5v respectively) but still well below 12v.

It still appears that testing the evaporator blower motor with 12v applied directly to the motor is a bad idea, at least for the '84-'89 911's.

[86 911 Targa]

^^^^^
Hope this helps.

Gerry

[87maniac]

re: 12v directly to motor is a Bad Idea

Why would a 12V motor not operate well with 12V?

In my VW Westfalia camper, I boosted my alt voltage to 14.7V and
replaced stock wiring to the AC panel with fatter wires.

Now the evap motors spin much faster and move more air.
The stock AC system in a Westy is similar to 911= barely adequate.

If your motors are only getting 10.5V with 13.7 at the battery posts,
I would think you would have better AC performance with a full 12V
at the fans (and better with 13.5V or so).

Now I am not electrically savy or experienced, so proceed at your own risk.
Hope a Pelican that has electrical skills will comment.

[87maniac]

86 911 Targa -
Is 1st photo battery voltage -11.69v ?
AC is operated with engine on, with ~13.6v

[dw1]

Originally Posted by 87maniac

re: 12v directly to motor is a Bad Idea

Why would a 12V motor not operate well with 12V?

In my VW Westfalia camper, I boosted my alt voltage to 14.7V and
replaced stock wiring to the AC panel with fatter wires.

Now the evap motors spin much faster and move more air.
The stock AC system in a Westy is similar to 911= barely adequate.

If your motors are only getting 10.5V with 13.7 at the battery posts,
I would think you would have better AC performance with a full 12V
at the fans (and better with 13.5V or so).

Now I am not electrically savy or experienced, so proceed at your own risk.
Hope a Pelican that has electrical skills will comment.

It can be an issue with the motor coils overheating if the motor is operated with higher than rated voltage. The question is what voltage is this fan motor really rated for? Since it is provided with less than 12v "by design" is it really a 12v motor, or is it rated for 10v max?

[dw1]

Originally Posted by 86 911 Targa

^^^^^
Hope this helps.

Gerry

Gerry - Thanks. The measurements you provided were quite informative.

Btw, nice Fluke 77. Even though I have an 87 (and a 105 scopemeter) my trusty old 77 is still my go-to DMM. And yes, as an R&D engineer (semi-ret.), I have a go-to DMM.

[wazzz]

At setting III, the blower is pushing enough air to take off and stay away from the plate, while at lower settings, the wheel is rubbing against the plate due to motor end play. Same happened to mine on my 88 Carrera and I too was able to fix it with shims under the plate.

Voltage at setting III should be the same as battery voltage, as there is no resistor in the way. I wonder why you get only 9 volts. Also, this Bosch motor is made to handle 12V. It is rated for 12V. It can draw as much as 15 amps.

[dw1]

Originally Posted by wazzz

At setting III, the blower is pushing enough air to take off and stay away from the plate, while at lower settings, the wheel is rubbing against the plate due to motor end play. Same happened to mine on my 88 Carrera and I too was able to fix it with shims under the plate.

Voltage at setting III should be the same as battery voltage, as there is no resistor in the way. I wonder why you get only 9 volts. Also, this Bosch motor is made to handle 12V. It is rated for 12V. It can draw as much as 15 amps.

Rather than shims, in my car I reformed the plastic plate a small amount with the help of a heat gun.

About the voltages - I strongly suspect it is the difference between the "load" (motor connected) and "no load" (motor not connected) voltages. I'll check this and post the results.

[targa80]

I might be wrong, but then again, I might be right!

The evaporator fan circuit has a two resistor pack installed to change the speed of the evaporator fan.

1. At low speed setting both resistors are in series with the fan motor limiting the voltage drop across the motor and limits motor speed current.
2. At medium speed setting only one resistor is in series with fan motor, this allows a higher voltage drop across the fan motor and allows a higher fan speed current.
3. At high speed setting both resistors are eliminated from the circuit and the fan motor receives the full 12 volts or battery voltage and maximizes the fan motor speed current.

Based on ohms law the current draw is voltage/resistance= amperage. Each speed position changes the total resistance of the series circuit and thusly changes the current draw of the circuit. Higher resistance lower current draw, lower resistance higher current draw.

The resistor pack is circled in green in the below schematic. Please excuse my scribble on the schematic since I have removed most of the wiring in the schematic from my car.