The mysterious spedometer connection

[Peter Bull]

Does anyone know what the connection between the speedometer and the heater control unit relay on a Carrera 3.2 does?

Before you continue to read, please know that I am not an electronics engineere. Not by far.

I tried to find out what the connection was a couple of years ago, but it resulted in me getting stranded in the middel of nowhere with a disconnected drive shaft and an engine idling at 3000 rpm and refusing to stop. It kind of stopped my interest in the connection.

Anyway, that was a long time ago and the nightmares have stopped so I thought I would give it a new try.

What I found reason to assume then was that the connection gave a signal to the heater control unit allowing the footwell blowers to start. I have three quite good reasons for that assumption.

First, the blowers did only start when the engine revved above 2500 - 3000 rpm. Why rpms and not speed I do not know, but I checked that a lot of times in different gears.

Second, when I hotwired the plug where the lead from the speedometer connects to the heater control unit relay the blowers would start regardless of engine revs. They worked like a charm, almost. The only problem was that the engine would not stop sometimes. A loose earth connection was the source of that, but the mishap in the middle of nowhere put a stop to further experimenting with the electrical system.

Three, during my experiments I managed to earth the connection from the speedometer resulting in the circuit not working anymore. After that the blowers did not start at all.

This made it quite reasonable to assume that the connection to the speedometer interfers with the control circuit to the blowers. At least on my car. And probably on other Euro spec cars.

Since I managed to fry the circuit in the speedometer, I need a little help from a brave test pilot equipped with a multimeter and a Carrera 3.2. Preferably a Euro spec one.

To confirm, or reject, my assumption I need the brave test pilot to do the following:

Pull out the speedometer and locate the lead to the heater control unit relay. It should be marked with an "A" according to the wiring diagram.

Connect the multimeter to the connection for the lead in the speedometer and measure the voltage between the connection and earth. With a good multimeter this should not result in any damage to the speedometer. I hope.

Measure the voltage in the following situations:

Ignition off
Ignition on, engine off
Engine idling
At revs between idle and 5000 rpm
At speeds between 0 and 100 km/h (60 mph)

What I would like to get out of this is what kind of signal comes from the speedometer. If it is a step fom 0 to 12 V at a certain rev or speed, if it is a voltage that increases with rev or speed. Or if it is just a practical joke by a group of German engineers.

Depending on the results from this, hopefully I can hotwire the heater unit control relay on my car in a way that does not leave me stranded in the middle of nowhere. And hopefully that will answer the question to what the connection does.

Sorry for the long post.

/Peter

EDIT:
Forgot to say that my car has autoheat, there might be a difference between autoheat and manual. And maybe it would be better with one brave test pilot driving and one brave test engineer looking at the multimeter.

[ischmitz]

Terminal A is ususally a square wave with 8 pulses per wheel rotation that is derived from the transmission speedo sensor. The speedo does not generate this signal. It's what makes the speedo "tick". This signal represents wheel speed. On newer cars relays such as for the retractable spoiler, cruise control, etc listen to it. A mutilmeter will not show any sensible answer since we are talking about a square wave with variable frequency.

I don't know why the heater relay would care about vehicle speed. I would assume it cares about engine speed. For that purpose I assume it listens to the tach signal if anything.

I think you need a schematic and a descent scope to troubleshoot your issue. Sorry if this is not a "simple" answer.

ingo

[Peter Bull]

Thanks Ingo,

that was the answer I was looking for, even if it was not the answer I was hoping for.

Thanks again.

My thought when I discovered that the blowers started above a certain rev was that it might reduce the danger of pulling exhaust fumes into the pasenger compartment.

Does anyone have a detailed schematic of the circuits in the heater unit control relay. I found one schematic here but I am afraid it is not detailed enough.

/Peter

[Mysterytrain]

The carrera did have an unusual 'feature'. The engine compartment controller has a logic circuit that monitors a temp sensor located on the engine case vent cover and the car speed [speedometer]. If the sensor has reached the high temperature set point and the car is not moving then the rear blower will come on and push some air thru the heat exchangers to help move heat away from the engine.

[Peter Bull]

After having invested in a rather fancy Fluke multimeter I found the fault in the heat system. The famous reed switch in the heater unit relay had checked out.

After short circuiting that the heater system worked as it should producing a mild gale in the passenger compartment. I am still curious to why the symptoms were consistantly dependant on engine revs, but I guess that just proves that electronics is just black magic

/Peter

[scarceller]

Originally posted by Mysterytrain
The carrera did have an unusual 'feature'. The engine compartment controller has a logic circuit that monitors a temp sensor located on the engine case vent cover and the car speed [speedometer]. If the sensor has reached the high temperature set point and the car is not moving then the rear blower will come on and push some air thru the heat exchangers to help move heat away from the engine.

So does this imply that if the Heater Control Unit sees the speedometer signal ticks (car is moving) and the temp sensor is active (engine is very hot) that it will NOT turn the Blower on?

This means that the temp sensor only can turn the fan on if the car is not moving?

I have been trying to understand this speedo connection to Heat Controller. I have verified what you wrote: Turned key to RUN and then just grounded terminal #10 on the controller (Temp Sensor Terminal) and the blower turns on even with car not running. So this confirms the operation with NO ticks from the Speedo.

I suspect that if the speedo sends ticks and I try the same thing it will not turn the fan on?

No let me ask, why not turn the blower on regardless of speed if the temp sensor says the engine is hot? I would think that pumping extra air through the exchanger even if car is not stopped would be a good thing? If this is so, why not just disconnect this speedo connection to the Heat Controller?

Thanks for the tips here, I think I may after all figure out this speedo connection to the Heat Controller.

[scarceller]

Originally posted by ischmitz
Terminal A is ususally a square wave with 8 pulses per wheel rotation that is derived from the transmission speedo sensor. The speedo does not generate this signal. It's what makes the speedo "tick". This signal represents wheel speed. On newer cars relays such as for the retractable spoiler, cruise control, etc listen to it. A mutilmeter will not show any sensible answer since we are talking about a square wave with variable frequency.

I don't know why the heater relay would care about vehicle speed. I would assume it cares about engine speed. For that purpose I assume it listens to the tach signal if anything.

I think you need a schematic and a descent scope to troubleshoot your issue. Sorry if this is not a "simple" answer.

ingo

Ingo, I am looking into this speedo connection to the Heat Control Relay on the Carrera cars. I plan to try to figure out how this connection affects the heat relay. I would like to simulate the speedo pulses with a square wave generator would you know what the 'ON' voltage of this pulse is? I will also scope this line to confirm if you are not certain.

I spoke to Ron Orlando on this speedo connection and it's the collective thinking of several members on this board that this speedo signal is combine with the high Temp Sensor (on the oil breather tube). If this temp sender sees a engine hot condition it grounds it's output this grounds pin 10 on the heat controller relay and if the car is stopped the rear heat blower comes on, this I have confirmed by grounding pin 10. Now, here is what is believed to happen if the controller sees speedo ticks via pin 1: If the controller sees the speedo ticks it turns the blower off even if pin 10 is grounded. I wish to confirm this so we all know excatly how this works. So to do this I wish to simulate the speedo ticks and test. If someone has already tried this test then please let me know. I have searched for confirmation on this speedo signal but can't find it.

Thanks.

[rick-l]

Originally posted by Peter Bull
After having invested in a rather fancy Fluke multimeter I found the fault in the heat system. The famous reed switch in the heater unit relay had checked out.

/Peter

Has anyone ever figured out what this reed switch does?

It is not just a relay, it is designed to turn on with a specific magnetic field based on the reeds and Ampere –Turns.

The only thing I can think of that it does is preventing the foot well blowers from coming on unless the main fan is operating and drawing a certain minimum current. This might insure that there is always positive pressure in the heat exchanger exhaust interface. If the foot well blowers ran without the main blower maybe this would produce a slight vacuum?

[scarceller]

Originally posted by rick-l
Has anyone ever figured out what this reed switch does?

It is not just a relay, it is designed to turn on with a specific magnetic field based on the reeds and Ampere –Turns.

The only thing I can think of that it does is preventing the foot well blowers from coming on unless the main fan is operating and drawing a certain minimum current. This might insure that there is always positive pressure in the heat exchanger exhaust interface. If the foot well blowers ran without the main blower maybe this would produce a slight vacuum?

You are correct on the operation of the reed switch, it is activated by the current to the rear blower and it will only activate if the rear blower is running. Once activated it picks a larger relay (internal to the Heat Control Module) that feeds power to the front footwell blowers. I have confirmed this functionality on my 84 Carrera. Not sure why the engineers designed it this way? But it is for sure they did not want the front blowers to run unless the rear blower ran. I guess if you had a small exhaust leak in the rear exchangers or if the car was parked you could run the risk of drawing exhaust into the heat system if only the front blowers ran, because they would form a negative pressure in the exchangers, I think this is a good theory. The other design possibilty is that they wanted the owner to know the rear blower had failed, cutting out the front blowers would certainly be a clear indication that the rear failed.

Note: if you wish to run the front blowers without the rear blower you simply open the Blower Control Unit/relay and jumper the reed switch. You should see this thread:
Help! Does anyone understand the footwell blower circuit?
The folks who contributed to this thread deserve a lot of credit! This is a great thread to learn all you need to know about the electrical system for the Carrera heat. You will find very usefull wireing diagrams here as well, Including a diagram for the internals of the Heat Controller.

[rick-l]

Originally posted by scarceller
Including a diagram for the internals of the Heat Controller.

A very high level diagram.

No one ever reverse engineered the controler and drew a schematic. I guess that is because no one wanted to screw with their $250 relay.

[dfink]

If you really want to blow your mind try to figure out what the potentiometer inside the control unit does..... I think it controls the timing circuit that lets the gate controller function.

[wwest]

Originally Posted by Peter Bull

After having invested in a rather fancy Fluke multimeter I found the fault in the heat system. The famous reed switch in the heater unit relay had checked out.

After short circuiting that the heater system worked as it should producing a mild gale in the passenger compartment.

I am still curious to why the symptoms were consistantly dependant on engine revs,

Late to the "game", but for the benefit of currency....

A somewhat low battery voltage will result in low blower motor current to trip the reed switch, rising engine RPM, higher alternator output.



but I guess that just proves that electronics is just black magic

/Peter

[jimmcc]

Originally Posted by dfink

If you really want to blow your mind try to figure out what the potentiometer inside the control unit does..... I think it controls the timing circuit that lets the gate controller function.

Are you referring to the pot on the side of the control unit and not the top temp pot?

If so, it serves as a position sensor for the bellcrank and provides feedback to the controller board that seeks a balance between the temp setting, engine temp and cabin temp. The controller itself is analog.