Aircon, FD3S, AC Request signal diode weirdness

[Neil Clayton]

Hi,

Trying to get aircon working on an FD3S with a v1.2 board G4.

AirCon Req (DI1) is set to LOW, with pull up OFF.

If I connect the signal directly, I get:

Off: 13.9
1: 1.6v
2: 1.7 - 1.9v
3: 2.9v
4: 3.6v

and, as others have seen, AC works on fan1 and fan2 only.

So. Diodes.

If I put two diodes (1N4148) in series... nothing works.

Diagram is:   ECU <- |--- <- |--- <- DI/ACReq from loom.

where |--- is the diode. (black band towards ECU)

The signal current is 8.4ma. I read the same 1.6v (etc) on the non-ecu side of the diode pair, but the ECU side just reads 12v. I've tested the diodes and there's an expected 1.2v drop across both in series.

So I don't understand ... it's like the diodes arent working.  Or the signal isnt getting through.

I've tried pulling the ECU side to low via a 22k resistor (only cos I saw that on another diagram, not because I know what I'm doing). no change. 

Any help / tips?

 

[Adamw]

Is it a s6 or s7/8 plug-in?

[Neil Clayton]

s7/8 

[Adamw]

Yeah that S7/8 plug-in has quite an unusual circuit on the AC request input that I dont fully understand.  Can you try a 1K resistor connected between the AC request wire and ground.  

[Neil Clayton]

after or before the diodes?

 

I've tried after, (1.5k, 10k and 20k) and this results in similar behaviour. The voltage is pulled a little lower with less resistence in circuit, but still ~8 -> 10v across all fan settings.

I'll remeasure today; but when off the resistence between (before diodes) from ACReq -> ground is about 2kOhm.  I'll try with car on to see if this changes as well.

 

[Adamw]

No diodes, just the pull down 

[Neil Clayton]

With 1.5k fitted to ground, and car running:

 

Fan speeds, v reading:

1. 1.65

2. 1.90-2.12

3. 3.01

4. 2.8

 

same as before, fans 1/2 works, fan 3/4 not (guessing it has to be ~below 2v)

[Adamw]

Hmm ok.  Any chance youve got a smaller resistor to try - something like 1000 or 500ohm?  If 500ohm it would need to be at least 1/2w rated. 

 

22 hours ago, Neil Clayton said:

guessing it has to be ~below 2v

Im not sure since its hard to work out how much current is passing through the other circuit that this adapter board already has on the adaptor board - that is meant to reduce the incoming signal voltage.  But from your tests it sure looks like needs to go below somewhere between 2.0 & 2.8V

 

Actually I've also just noticed in our drawings the early boards had a different circuit on that AC request.  Can you also check what PCB version you have.  V1.1 has a more basic circuit than V1.2 & V1.3.  You mentioned V1.2 in your original post but I suspect that is from the topboard rather than bottom board.

 

[Neil Clayton]

I will try lower ohms later, got a bit on at the mo. Appreciate the continued replies.

v1.2 for both boards as it turns out.

[Neil Clayton]

OK.

With 500ohm resistor:

 

Off, 12.54v

1. 1.55v, then 3.54 when AC clutch is OFF

2. 1.8-1.9v.  3.57 with AC clutch OFF

3. 2.85-2.9 (AC clutch never switches on)

4. 2.57-2.62 (AC clutch never switches on)

 

[Neil Clayton]

@Adamw any further ideas?

[Adamw]

Sorry, I kept forgetting to come back to this.  Im going to have to talk to some of the smarter engineers to see if they can suggest something.  Its got a very odd circuit on this input and I dont really understand how it is meant to work.

[Neil Clayton]

thanks - if it all gets "too hard" I'm quite happy to write code and slap an arduino on it. it's just that that seems, a little ... extreeme

 

[Adamw]

Sorry for the delay, we discussed this the other day and I forgot to reply.  So this ecu has quite an odd circuit on that AC request input, it looks like it was originally designed to achieve a very specific volt drop with very little influence from external factors.  This was done many years ago so we are not sure of the reason it was done this way - the original engineer possibly found some odd behavior from the AC switch and needed this to get around it?  But the problem is this circuit has been made quite "fool proof" - changing the voltage entering the header pin with resistors or diodes etc makes very little difference to what the actual ECU input sees on the other side. 

There is a pull-up resistor on the bottom board (tiny smd), we think removing this (or possibly could scratch through the track) would be the easiest way to make it work (will still likely need a couple of series diodes added in the wiring).  Let me know if you are prepared to do this and I can give you a bit more info.  

[Neil Clayton]

Yep, can give that a go.

[Neil Clayton]

@Adamwcan you fling me more details please?

[Neil Clayton]

Taken me ages to get back to this, but now I have room to work.

Before I cut the track on the ECU or attempt to remove the resistor, wanted to check I'm modifying the right thing.

So far, I've been working off this diagram.

 

According to pinouts as per wiring info in Link help, I map DI1 -> 4V = AC Request, and DI5 -> 4AG = AC Switch

Before I go further:

1) do I even need to do anything with DI5? I think when I was measuring (the first posts) I hadn't done anything to it.

2) I didn't have DI5 setup as per diagram in initial measures. Might this have thrown off results when measuring DI1?

3) I've seen other diagrams using no pull up/down resistors to the left/right of the diodes (but they might be for g4+ boards). I presume those resistors are still required (especially if I'm about to remove existing pull up from the ECU board)? Or can I leave them off and just go with the diodes?

 

 

[Neil Clayton]

So, I got impatient

Started by carefully cutting the trace on the board.  Verified no connection.

Then put 3 diodes in line.  This worked, mostly, except when AC was supposed to be off, 4V measured ~0v ish. Guessing it might be floating?

I pulled it to 12v with a 2k resistor before the diodes.  Everything now works!

All fan speeds work, AC compressor is engaged/disengaged, and all turns off when you either turn off AC or switch off fan.

Yay.

Thanks for the help.

[Adamw]

On 9/9/2023 at 1:55 PM, Neil Clayton said:

I pulled it to 12v with a 2k resistor before the diodes.  Everything now works!

Huh?  Isnt that what you just removed (well 1.5K?)  can you confirm what the circuit you have looks like now?

[Neil Clayton]

I don't know enough to say for sure. The 2k pull up is before the diodes, not after at it would have bee at the board.

Other than that I can't say.

Before I went ahead, I re-did all the tests I'd done before, and for sure, it didn't work. 

So I can only assume that the on board resistor (guessing it was a pull up) was somehow different (I have no idea how).

All I know is it now works.