VDO Speedo and auto heater

[Peter Bat]

Very detailed question. Trouble shooting my auto heater on my 84 Carrera targ. The Speedo pin A is connected to the heater fan control relay and keeps the fan off until the car is moving. This alleged to prevent sucking fumes into the cabin. Does any one know what this pin A looks like electrically. Relay closure, voltage level, pulse? This pin A also goes to the turbo controller if turbo equipped.
Thanks in advance.

[spuggy]

Originally Posted by Peter Bat

Very detailed question. Trouble shooting my auto heater on my 84 Carrera targ. The Speedo pin A is connected to the heater fan control relay and keeps the fan off until the car is moving. This alleged to prevent sucking fumes into the cabin. Does any one know what this pin A looks like electrically. Relay closure, voltage level, pulse? This pin A also goes to the turbo controller if turbo equipped.
Thanks in advance.

The short answer is that terminal A on the speedometer outputs a pulse



Probably a 12V square wave pulse.

On my '77, this terminal originally drove a pulse counter behind the dash that lit up the EGR light as a "servicing required" indicator. Hit the button on the black box to reset the count. Or just don't connect it...



Factory fitted a lot of extra "stuff" to CIS 930s which aren't generally regarded as necessary and certainly doesn't improve reliability as the parts/wiring age. And the relays are NLA to boot. My transplant never had it, I was grateful

On the Carrera, apparently there is some interaction with the speedo pulse from terminal A and the CASIS light in the tach.

[wazzz]

Originally Posted by Peter Bat

Very detailed question. Trouble shooting my auto heater on my 84 Carrera targ. The Speedo pin A is connected to the heater fan control relay and keeps the fan off until the car is moving. This alleged to prevent sucking fumes into the cabin. Does any one know what this pin A looks like electrically. Relay closure, voltage level, pulse? This pin A also goes to the turbo controller if turbo equipped.
Thanks in advance.

I have already answered the same question recently. Though I am not sure, I think you were the one who asked it in another topic.
In this answer I wrongly stated the Motronic as being the source for the speed signal while in fact it is the speed sender on the transmission.

Anyway the short answer is indeed a pulse signal (ranging between ground and 8 volts), same frequency as the speed signal output from the speed sender, but with a fixed width of 2.5 ms. Signal goes from high (8V) to low (0V) for 2.5ms, then back to high for rest cycle. The 2.5ms low states are pulses that are the same frequency as the speed signal from sender, which has a different duty cycle (longer pulses at low speed).

And no, it is not meant to keep the fan OFF until the car is moving, but the other way around, it forces the fan ON when car is not moving or moving slow, up to a certain speed (which I don't know), and only when an overheating condition is reported by the thermo switch connected to pin 10. I have posted a link to the detailed diagram of the "analog" part of this relay, which I traced a while ago. If you have a look at it you should be able to understand the logic function of this somewhat complicated relay.