740 Help me identify this ECC dome light diode!

Please

If you know what its role in the circuit is, or have one that you don't mind popping out of your head liner to read the part number on, that'd be great! Thanks. Image of blown diode:







The trouble with unidentified diodes is that they can be several different types...

Can anyone explain what a circuit is doing in the dome light in the first place? I popped the light out of my caravan roof earlier, and it's very dull and empty, just bulbs! I'm guessing it's ECC related, though I have no idea what exactly. All info appreciated!


While we're on the topic of ECC, my snow button is flashing and pressing it doesn't change that. The fan was fully seized (now replaced), and the power stage cooked (not yet replaced, but any day now...), but now having noticed this diode I'm wondering exactly what is wrong. I guess I should look up the fault code finding method and do just that. Links welcome, instructions to search like a n00b also welcome

Cheers!

I can't tell you anything about that circuit as I'm a 240 guy and ECC is rather primitive in a 240 (a temp probe in the evaporator) but I can suggest -- if you think replacing the diode is going to somehow cure the reason it smoked -- just replace it with a 1-amp rectifier with at least a 50V PIV rating, e.g. a common 1N4004 or better. Should be easy to find.

Try it. If it was a Zener, which I doubt just looking at the surrounding circuit, it can't hurt anything else. My bet is it is a steering diode, and if you follow the traces on the board and wiring, you'll deduce what it's purpose is, if someone else doesn't post with a definitive answer.

The tricky part isn't replacing the diode, but finding what smoked it.

cleanflametrap said:

The tricky part isn't replacing the diode, but finding what smoked it.

Click to expand...

Agreed!

The fan smoked the power stage module thing. The fan is now fully tested and reinstalled. Just need to get the rebuild power stage in there, and test it out.

What killed this, I have no idea. To even guess, I'd have to have a clue what it does. There are a number of other transistors, diodes, caps and resistors on there, so although it's simple, it's not trivial.

It's got the body size of a zener, a fraction bigger than a 1N400X, of which there is two of on there, as well. But it might just be a chunkier diode. Like you say, I'll figure it out when I get it apart, but if anyone expert in 7/9 cars knows, I'd be grateful

Based on more careful looking... I retract my bet on the steering diode. It is for sure different from the other diodes in the module, and my bet was based on the majority of epoxy case diodes are used as steering diodes in 12VDC electronics. This looks like maybe a minority example. Sorry, I wish I had a 7/9 around to look at.

I'm another 240 owner so this is purely guessing.

Does your dome light still work, and does it have a fancy fade away feature instead of just shutting off immediately? If so, I'm guessing that your picture shows the fancy fade away circuit on the main board, and that the burnt D2 on the 2nd board is some sort of temperature sensor for the ECC. I'd find it strange if any of the other ECC electronics were mounted in the dome light since most of the rest of the ECC related parts are behind the dash.

Yes, quite right. I realised this last night while playing around with the ECC stuff and taking a tiny step in the right direction. That's almost certainly what it is! Which makes me much less worried about it being broken - I don't care.

Yes, the lights work, but the switches were not seating properly due to loose PCB mounting, which is why I took it out.

I'll try to fix it anyway and report back here with further info/updates just as a source of clues for the next guy who has this issue and bothers to sort it out.

Re the ECC issues, I guess I should check the trouble codes first, and put the light back in before I do that. However it's worth noting that the fan comes on briefly when you power it up, then goes off again. Briefly as in 1 second or so, and at a low speed. Indicating that our new power stage is at least partially working. Maybe the over current circuit is falsely reporting that to the CCU now. Not sure.

It's possible to use a standard 0.6v drop diode, or a transistor wired as a diode, as a temperature sensor but I'd think that it would have accuracy problems with a remote diode and the wires running through one or more connectors. Look up "Diode Temperature Sensor" on the net for more info. You could always try installing a generic 1N400x for D2 and see if it's happier.

Didn't pull the PCB out yet, but reached in and chopped it off

BZY
970 or 97O
22

Seems to be a ~22V Zener clamp! Possibly died on an over voltage then went short circuit, then burned up.

While cutting off the leads I noticed that the trace on the other side is totally delaminated from the substrate... extreme heat will do that

I wonder if anything it was supposed to be protecting died at the same time? Will pull it out and have a look sometime soon.

OK, so that diode is definitely an ultra simple clamp circuit that I guess tries to rely on the resistance in the wiring loom to not explode on the first over voltage, as it's directly across 12V and ground.

Here's how the guts look:

Switch moving contacts: https://twitter.com/FredCookeNZ/status/751348225999638528






Circuit boards: https://twitter.com/FredCookeNZ/status/751348410599350273






Connector labeled: https://twitter.com/FredCookeNZ/status/751353971026038784






Door circuit traced: https://twitter.com/FredCookeNZ/status/751354485524537345






So I polished the contacts on the board, will bend some more tension into the moving switch contacts [DONE], and reflow a bunch of dodgy dry joints that are waiting to happen or already have (hard to tell, sometimes...) [DONE].

And, after all of that, I think I'll leave the diode out. Unless I just happen to have one here somewhere. Which I might. Nah, stuff it.

The small transistors on top are:

T1 BC558C
T2 BC337 40
T3 is hidden under the bulb holder in a TO-126 like package, configured in open collector style, driven by the smaller ones, probably in some resonant fashion.

I guess I could test T3 on the bench... Nah, stuff it.

Out to see if the ECC works, and the lights are consistent Hey, it might even fade after the reflow

OK, went out, did a one wheel burnout into and through second gear (slush box, but bring on the G80...), and now the interior light switches function perfectly (no thanks to the cloud of smoke, mind you). Mission accomplished in that respect. While I was out there I thought I'd read the diagnostic codes as described on this page:

https://www.volvoclub.org.uk/faq/HeatingAirConditioning.html#ECCClimateUnitDiagnosticCodes

What is "vent"??? They're ALL vents. Fail. What it means (after some trial and error) is forward facing vents, not foot vents, not windscreen, not up/down (which I assume is windscreen and feet??) etc. And the results, fresh in from a chilly rainy night are:

1-6-1 because I didn't have two phones with me...
1-2-2 open/short to 12v on the ambient temp sensor
2-3-3 "Starting current too high, motor seizes or turns sluggishly" except it's lying! Because I tested it: https://www.youtube.com/watch?v=SgUWnIPi4hA

So I can only assume that the thing that does the sensing of current got blown up with the big transistor and is telling lies to the CCU box. Or it needs a reset, somehow. Though it seems that it should just reset when you turn off the key. You can hear the motor spin up briefly when you turn the key on, too. Sounds healthy for a moment, then stops.

The transistor in there could be dodgy with not enough HFE or something, too.

Anyway, this is good. I can sort out the issue with the ambient sensor, then figure out what's wrong with the power stage reporting excess current. Maybe just hard wire it to say all is well regardless and let the fuse take care of preventing fires.

There was great little write up by a guy on the power stage box... let me find it... here: http://www.matthewsvolvosite.com/forums/viewtopic.php?f=2&t=54553

The damn thing is too clever for its own good. It'd be nice if 5 meant 5 regardless I'd be OK with that. Who needs lower settings?

Bed time, I think. Progress, but not a sealed deal, yet.

FreeEMSFred said:

1-2-2 open/short to 12v on the ambient temp sensor

Click to expand...

ECC speaks the truth! Broken wire on the element - too hard to reattach it, broke right at the glass bead. Reading about 8.6k at 20C or thereabouts. Does anyone know what curve these thermistors have? Can I fix it by soldering in some IAT element? Or is it oddball?

I guess Volvo dealership is my only option for the real deal, right? Any hints on supply of this thing appreciated:




My toes are cold.

Fred, you've done a lot to document this system. Don't give up before finding out the answer to the curve question. Makes sense the thermistor would be one used elsewhere for air temp, so, if you don't have junk parts around to measure, hang a remote thermometer probe up there and simulate the thermistor with a pot you can grab easily. A few cycles and you'll have a few points on the curve. An IAT sensor seems like a good bet to me.

Heat wave here.

PS, I have a junker IAT somewhere if you need some measurements made.

Sorry. Just spotted this reply. I actually managed to fix it! So I could pull it out again for a measurement session at some point, but I'm going to focus on getting the fan to turn first, some aspect of the feedback to the ECC module isn't right. Pic of dirty fix:

Dear clean flame trap (oh how I wish I did the day I got each Volvo, lol) I have recently acquired two more of these modules that work and work well!

One of them is IN my 960 wagon working and working well - I can measure this in the car.

The other I got from a really clean 740 wagon that a guy crushed I got many many parts from it, including this module, so I can put it my 740 and measure that, too. Then I can look at what the modified/fixed one does and try to tune the non-working out and use it and keep the OEM one and this thread as a spare/backuup plan

Just a little update