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Ady1971

Autoheat fault

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Hi,

I am trying to get the auto heat working on my '87 Coupe.

The issue is that the servo extends to max heat position on selecting position 1. According to the Bentley Manual it's an open circuit on the cabin heat sensor, or the setting of the potentiometer.

On checking the wiring, sure enough I have an open circuit on the cabin sensor.

However, when I check for continuity along either wire with the connector plug at the sensor - it has continuity.

So, my question is whether the fault is with the wiring to the sensor, or a fault within the sensor itself (I measured resistance across the 2 terminals and it was very high at 2.28k ohms)?

Any help welcome... 

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Hi Ady,

Sorry to maybe dissapoint you by not having a solution to your auto heat issue.

I am just curious what the culprit may be cause I have the same thing going on in my 84 Coupe...

As a matter of fact I cant even find the sensor in the cabin.  It is supposed to be somewhere along the upper windscreen edge or on the right of the steering

wheel, I understand.  I only see a solid closed, circle-shaped plug in the dash, next right of the wheel,   is that it?

I hope some one comes up with a solution.    I'll try to gather some info too.

Good luck, Willem.

Amsterdam.

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Autoheat, it's problems and workings, have been very well documented on here in the past, have a search for "autoheat".

 

Mark

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3 hours ago, SilverWT said:

Autoheat, it's problems and workings, have been very well documented on here in the past, have a search for "autoheat".

 

Mark

Mark,

Thanks for taking the time to reply.

I have searched through the forum, hence tracing the fault to the cabin sensor being an open circuit.

I am not very knowledgeable on car electrics so perhaps I should have asked different questions:

What is an open circuit?

Can you have an open circuit in the wiring to the sensor, or is it likely to be the sensor unit itself causing the problem?

Can I check the sensor unit to see if its within tolerances etc?

 

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Open circuit =. A break in the wire. 

Short circuit =. Wires connected where they shouldn’t be. 

Check the resistance across the two wires going to the sensor with a meter.*

The sensor should give a reading   My guess would be  about 10k ohms.  It should not be open or zero.

* Google how to do a resistance measurement with a meter. 

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14 hours ago, Jonny Hart said:

Open circuit =. A break in the wire. 

Short circuit =. Wires connected where they shouldn’t be. 

Check the resistance across the two wires going to the sensor with a meter.*

The sensor should give a reading   My guess would be  about 10k ohms.  It should not be open or zero.

* Google how to do a resistance measurement with a meter. 

Jonny,

Thanks for this.

I found the reason for the open circuit - the wires in the multi-plug were in the wrong positions!

However, having rectified this, the resistance values are still too high.

The values are wires from auto heat module to fan assembly = just 0.3 ohms and with the fan connected its 2.57 / 2.62k ohms (Bentley manual says 1.9k ohms).

Presumably the sensor in the fan is shot?

Can this be replaced, or is it a new assembly needed?

 

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6 hours ago, Ady1971 said:

The values are wires from auto heat module to fan assembly = just 0.3 ohms and with the fan connected its 2.57 / 2.62k ohms (Bentley manual says 1.9k ohms).

Presumably the sensor in the fan is shot?

Can this be replaced, or is it a new assembly needed?

 

The output wires from the autoheat are not directly connected to any fan. The output goes to the heater relay in the engine compartment. The relay powers the engine bay fan, and the footwell blowers on a 3.2 (providing the engine blower fan is running and it is a bit more complicated as the autoheat box also selects one of three speeds for the footwell blowers via three resistors depending on the setting selected on the autoheat), when it is energised by input from the autoheat box.

No fans have any sensors.

Are you checking the correct connections? There is a heat sensor in the nearside heater pipe just before the flapper box. This gives the autoheat input on the temperature of the incoming hot air from the heat exchanger. From memory, Bentley gives resistance values for this sensor.

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Mark,

I am referring to the cabin temperature sensor (said fan - should be 'inside temp sensor blower' according to Bentley wiring diagram).

This appears to be connected to the auto heat via connector B and should have a resistance of 1.9K Ohms.

Mine is reading between 2.5 and 2.6k ohms.

From looking at the part (911.624.065.00), there is a resistor and thermistor (?) at the end (which I assume changes resistance as the cabin temperature changes) which is connected to the auto heat via blue / blue / white wires at point B.

My question is therefore, can you just resolver new resistors in place and if so, anyone know the values (a thread on PelicanParts suggests 2.2k Ohm thermistor with an 11k ohm resistor in parallel)?

 

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Re reading your original post, have you checked that the internal servo rod length is adjusted to 118 + or -2 mm ? 

This is not mentioned in Bentley, only the Porsche manual.

 

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9 minutes ago, angusb said:

Re reading your original post, have you checked that the internal servo rod length is adjusted to 118 + or -2 mm ? 

This is not mentioned in Bentley, only the Porsche manual.

 

Angus,

Thanks. Yes - I have, and it is.

I have also checked the value of the potentiometer when set at position 5: that's almost spot on too.

I am convinced its the interior cabin sensor being too far out of range: hence my recent question regarding replacing the resistors in this unit.

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...so I finally got the auto heat working.

I traced the fault to the cabin sensor having too much resistance: the servo fully opening as a result (the same effect came be obtained by disconnecting the cabin sensor).

The resistance at the connector for the cabin sensor was over 3k ohm - target is 1.9k ohm (at 20 degrees)

I removed the old resistors, and found a 2.2k Thermistor combined with a 6.8k ohm resistor in parallel gave 1.86k ohms.

Removing the old resistors, and soldering in the new, was quite straight forward.

Refitted to the car today and appears to be working - it was harding getting to 20 degrees today, but using a hair dryer to warm up the cabin, the servo would adjust back and forth depending on the setting.

IMG_3560.JPG

IMG_3571.JPG

IMG_3575.JPG

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6 hours ago, Dr Rock said:

Excellent - you can come and do mine now then :signs85:

Join the queue 😊

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On 3/10/2019 at 10:08 PM, Wingnonut said:

Join the queue 😊

Richard,

Swing by anytime - its the least I can do given all the help you've given me 😉

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1 hour ago, Ady1971 said:

Richard,

Swing by anytime - its the least I can do given all the help you've given me 😉

Will I then owe you a breakfast 😜

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17 hours ago, Wingnonut said:

Will I then owe you a breakfast 😜

if you're offering 🤩

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