Testing the Heater Control Unit

[MikeH]

Hi All

I've done a search but can't find what I'm looking for, so here goes...

I've got a 1977 Carrera 3.0, which has the same electronic heater control [] as the SC - temp. sensor behind the mirror, and a numbered dial in the centre, to give the illusion of analogue control.

Surprisingly, in the 4 years I've owned the car, it's been fine, but now that the warm weather's here it's decided to stick full on, and the car's completely unusable. However, with the info I've already found, I can now bypass the electronic servo and use the old fasioned cable method. However, currently the 'automatic' red lever is in the fully up, full heat position.

I've read the other posts about checking the heater valves, and I'll do all that, but I don't think that's the problem. When I close the dial (position 0) behind the handbrake, I can hear a click from a microswitch, but the servo motor doesn't run (I used to hear a gentle whine from it). So, does anyone know what electrical currents / voltages / resistance I should be able to measure and where, to test if the central control unit is ok? I'm hoping someone will have a Bentley Manual (I'm looking to buy one, if you're selling!) which might list some tests for the control unit.

Thanks for any help

 

[dugsud]

This sounds like the little joint situated inside the control unit has popped off it's ball! This is on the end of a rod which, in turn actuates the cables that open & close the flapper boxes under the car. There is a little inspection cover on the drivers side (RHD) of the unit that's really only accessable with the seat out. If the seat is in situ, you can pop it off and just get your fingers in to have a feel for anything loose!

 

[Geoff_Fields]

I do not have any experience with this older version but here are a few thoughts. The actuating rod and its prone to fail ball joints pull against the return springs on the heater control valves. So if the rod has failed the heater valves should open to the vent fully cold position. As I understand it, the manual red lever is for use if the electronic system has failed. It should be possible (with the ignition off for a test) to gently push the lever down and select the cold position. Since the red lever is attached to the same mechanism as the actuating rod and that it is permantly in the up full hot position I would first inspect the heater control valves, which may be siezed or their return springs. The issue with the servo motor being quiet is probably due to the fact that the machanism is at the limit of its travel and has activated a micro switch.

 

[Geoff_Fields]

Excellent info bones, that should help Mike out. I have saved attachements for future reference (heater issues always cropping up in conversation)

 

[Guest]

Mike

Well I have fuly removed my control box as the mother board in it appears to have damage. It now is fully manual. There is a bracket which connects the servo to the heater control arm which I had to remove to allow the manual operation. Additionally I have added a switch to controll the fan when the car is running less than 200 rev. The idiot who previously owned the car wired the fan so it was always on.

Regards

 

[MikeH]

Bones / All

Cheers for the images... I've now downloaded them. I'll read through and do some testing this weekend. It's already stripped for inspection, so I've ruled out the simpler stuff like the control rod joints, and the connections look clean and solid.

Just ordered a Bentley Manual from the States... assume that's where the info was from?


Thanks!

 

[MikeH]

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