martindevoy
New Member
I've derived much help from this forum and I hope someone finds this contribution helpful.
My heater first misbehaved intermittently, then permanently, blowing maximum hot air on all settings of temperature except Lo, particularly on Auto. Relief in part could be obtained by manually turning the fan down but even then a window would have to be opened periodically and it's winter.
Suspecting blend motor faults, I connected my Nanocom to find the blend motors all operating as they should.
On viewing the sensor values the problem became apparent. The Aspirator (the in car temperature sensor) was showing a value of minus 15 degrees. The Aspirator was checked, cleaned out and found to be ok. The sensor should read about 10 Kohms at about 25 degrees. Mine did. The fault persisted.
The culprit had to be the Hevac unit itself and with a digital testmeter on a 200 Kohm range, I found that the input resistance for the Aspirator was unusually low. (pins 9 and 18 on the 20 pin socket). It should be the same as the Ambient sensor input (pins 7 and 8) on the 12 pin socket.
After much hunting, I discovered that there is a compound diode in the input circuitry (on mine marked A4). This is a tiny three pin device containing 2 diodes with both cathodes connected to the middle pin. It can be identified by holding the board with the 20 pin socket up and to the right. Towards the left of the top edge of the 20 pin socket are two tiny three pin devices and the leftmost one is marked A4 with additional letters. One of the diodes in this device had broken down and although not reading a short, was reading around 2 Kohm. I believe the A4 to be a BAV70. The most expensive part of these is the postage!
I didn't have any of these but did have some discrete fast switching diodes, 1N4148. I soldered in two diodes with the cathodes together to the where the central point of the defunct compound diode had been. The two anodes go one two each of the other connecting points.
The Aspirator input resistance was now the same as Ambient input resistance.
Following installation, Nanocom confirmed that my internal temperature was reading correctly and as the car heated up, the unit settled down as it should and responded to the temperature control. The servo motors all work normally.
I suspect the function of these diodes is to protect against electrical spikes which might harm the internal processor but I'm not sure.
Replacing this compound diode requires a steady hand, a small soldering iron, very thin solder and good eyesight!
I should stress I was using a digital multimeter on a high resistance range. I would counsel against using an analogue meter for fear of damaging components in the unit. The usual precautions for static should also be observed.
I'm sorry if this is a bit long winded but it will have take far less time to read this than it took me to identify the problem!
Good luck!
My heater first misbehaved intermittently, then permanently, blowing maximum hot air on all settings of temperature except Lo, particularly on Auto. Relief in part could be obtained by manually turning the fan down but even then a window would have to be opened periodically and it's winter.
Suspecting blend motor faults, I connected my Nanocom to find the blend motors all operating as they should.
On viewing the sensor values the problem became apparent. The Aspirator (the in car temperature sensor) was showing a value of minus 15 degrees. The Aspirator was checked, cleaned out and found to be ok. The sensor should read about 10 Kohms at about 25 degrees. Mine did. The fault persisted.
The culprit had to be the Hevac unit itself and with a digital testmeter on a 200 Kohm range, I found that the input resistance for the Aspirator was unusually low. (pins 9 and 18 on the 20 pin socket). It should be the same as the Ambient sensor input (pins 7 and 8) on the 12 pin socket.
After much hunting, I discovered that there is a compound diode in the input circuitry (on mine marked A4). This is a tiny three pin device containing 2 diodes with both cathodes connected to the middle pin. It can be identified by holding the board with the 20 pin socket up and to the right. Towards the left of the top edge of the 20 pin socket are two tiny three pin devices and the leftmost one is marked A4 with additional letters. One of the diodes in this device had broken down and although not reading a short, was reading around 2 Kohm. I believe the A4 to be a BAV70. The most expensive part of these is the postage!
I didn't have any of these but did have some discrete fast switching diodes, 1N4148. I soldered in two diodes with the cathodes together to the where the central point of the defunct compound diode had been. The two anodes go one two each of the other connecting points.
The Aspirator input resistance was now the same as Ambient input resistance.
Following installation, Nanocom confirmed that my internal temperature was reading correctly and as the car heated up, the unit settled down as it should and responded to the temperature control. The servo motors all work normally.
I suspect the function of these diodes is to protect against electrical spikes which might harm the internal processor but I'm not sure.
Replacing this compound diode requires a steady hand, a small soldering iron, very thin solder and good eyesight!
I should stress I was using a digital multimeter on a high resistance range. I would counsel against using an analogue meter for fear of damaging components in the unit. The usual precautions for static should also be observed.
I'm sorry if this is a bit long winded but it will have take far less time to read this than it took me to identify the problem!
Good luck!
