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 Post subject: Modding the flasher / hazard switch for LED indicators
PostPosted: Mon Jul 30, 2018 6:47 am 
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Before we begin: this has largely been written as notes to myself. Some things may not be structured or flow well as a result; I just needed references to my thought process for when I come back and pick up this project again later. If anything doesn't make sense, please ask - I'd like others to know what my thoughts are, but realise that the way I've explained them may not be completely clear.

Some months ago, I replaced all of the external lighting in the KJ except for the fog and headlights with LED bulbs. I've done this on pretty much every vehicle we've had for the past decade or so, and in general, it's been a pretty simple affair - replace the bulbs, then replace the flasher relay with an electronic unit so that the indicators don't hyper-flash when you're signalling a turn.

The KJ threw me for a loop on that last part: I wasn't expecting the hazard switch to contain not only the switch mechanism, but also IC control of the turn signal / hazard flasher relays.

I'm not a fan of splicing resistors in at each indicator position to cure hyper-flash - one, it removes the power consumption advantage of going to LEDs in the first place; two, it introduces the possibility of another point of failure in the electrical system, which is something I'd rather not have to deal with in the future. Clean, simple solutions are what I generally prefer over 'just make it work', so started looking into options for doing exactly that.

First stop was to do a bunch of reading up on how others have approached this problem. It's not that I didn't find good ideas, just more that they were ones which left me wondering if it might not be possible to approach this in a slightly different way. Having finally made a long-delayed trip to the junkyard, I pulled a couple of switches and decided to start looking a bit more deeply into how they work.

So, for the rest of the discussion, here's a visual aid in the form of a (not real great) photo of the top of the motherboard inside the KJ hazard switch:

Image

After doing some digging, I was able to determine that the Atmel 6431B IC is part of the Atmel 643 family of automotive flasher ICs. The 6431B appears to be Chrysler-proprietary, but as the pinouts on the ICs in that family match across the board, I'm fairly certain that this one will too.

That shunt between the two relays and the header for the electrical connector (barely visible, but it's the thing inside the oval that's usually described as an 'M-shaped piece of metal' when people are talking about this modification) is interesting. From reading through the datasheets for the ICs, it appears to have two purposes: one, to provide a reference voltage to the IC for telling if bulbs are good or blown; two, to act as a safety valve for the IC in the event that a bulb or its wiring is shorted.

So, basically, the IC and the shunt are co-dependent on each other to know the state of the bulbs. It makes sense that when people shaved down the shunt, their indicators wouldn't hyper-flash - they were effectively lowering the amount of current that the IC was being told to expect the bulbs to draw, and, since LEDs use less current than incandescents, it follows that feeding the IC a value more in line with what the LEDs were drawing would solve that problem.

My concern with this approach: if, for some reason, you ever need to go back to incandescent bulbs (or just temporarily replace an LED with an incandescent), things could get weird. I'm not sure what form that would necessarily take, but given that the shunt is also designed to protect the IC, my guess would be failure to work from seeing too much current coming across the shunt.

However, I did come across something interesting in the datasheets: there are ICs in that family designed to take a 30mOhm shunt, and there are ones designed to take an 18mOhm shunt. There are also ICs in there that are labelled as, "Frequency Doubling Disabling" - which is what I suspect we're looking for.

From a quick read of the various datasheets, frequency doubling is exactly what it sounds like - the technical name for hyper-flash. These ICs are designed to work in such a way that below a certain reported current draw, they don't hyper-flash, which has me suspecting that they're designed to run both incandescent and LED bulbs at the correct flash rate. All of the ones with this capability use 18mOhm shunts, and I'm not sure what the value of the shunt in the KJ flasher is - my guess is 30mOhm, but without pulling and measuring, there's just no good way to know until I do.

There's also an added complication in that the ICs that disable frequency doubling use a different physical form factor to the one that's already in there. They're a much smaller package, which would mean having to fit an adapter onto the board to be able to install them. This wouldn't normally be an issue, but I'm having a hard time finding one that will fit where the existing IC is due to the close positioning of the relays. Given the amount of free space inside the case when the board is mounted, though, I think it may be possible to replace the existing IC with a socket, run standoffs from that to the adapter board to get it up above the level of the relays, and still be able to fit everything back in. Just ;)

Speaking of the relays: I don't think that just swapping them out for electronic ones will solve the problem. Their pulsing is controlled by the IC from what I've been able to tell so far (haven't done a full chasedown of the circuit, but enough to believe that this is the case), so they're probably just best left alone. What has me suspecting this is that the turn signals will hyper-blink with LEDs installed when worked from the stalk, but not when worked from the hazard switch.

In any event, it looks like there might be an option on the horizon for anyone else looking to convert to LEDs. I'm not going to be moving on this quickly; it's something that'll happen as spare time permits, but I am hopeful that we'll have another method for doing this relatively soon.

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 Post subject: Re: Modding the flasher / hazard switch for LED indicators
PostPosted: Sun Nov 11, 2018 1:55 pm 
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And I'm officially calling this a dead project. I did complete the conversion to LEDs, but ended up going the traditional route of replacing the shunt with a resistor. The original thread on how to do that can be found here; notes on why I ended up using a 0.5Ω resistor rather than the 0.1Ω item originally recommended can be found here.

The main reason for burying this one: finding the appropriate ICs to experiment with turned into more of a pain than I was willing to put up with. Had I been able to get them off-the-shelf relatively easily, everything would have moved ahead. However, that wasn't happening and I didn't feel like pulling apart flasher switches at the junkyard to try to figure out who used which ICs... So it's a goner. The resistor method works just fine anyway, so my curiosity will just have to live with my exasperation in this case ;)

_________________
2005 KJ CRD Limited 4x4:
245/75R16 BFG TA KO2s
OME / Clevis 2.5" Lift
JBA Lifted A-Arms
IRO WJ Short Rear UCA/WWDiesel mount
Skid Row Skidplates
HDS Model 001 Thermostat (190°F)
Suncoast TC
Full Weeks Kit
Bosch 5V glow plugs
Hayden 2986 fan clutch / GM 11-blade fan
Samco / Sasquatch Intake Hoses
Carter in-tank pump
Provent 200
V6 Airbox


Last edited by casm on Sun Nov 11, 2018 3:02 pm, edited 1 time in total.

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 Post subject: Re: Modding the flasher / hazard switch for LED indicators
PostPosted: Sun Nov 11, 2018 2:27 pm 
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casm wrote:
And I'm officially calling this a dead project. I did complete the conversion to LEDs, but ended going the traditional route of replacing the shunt with a resistor. The original thread on how to do that can be found here; notes on why I ended up using a 0.5Ω resistor rather than the 0.1Ω item originally recommended can be found here.
The main reason for burying this one: finding the appropriate ICs to experiment with turned into more of a pain than I was willing to put up with. Had I been able to get them off-the-shelf relatively easily, everything would have moved ahead. However, that wasn't happening and I didn't feel like pulling apart flasher switches at the junkyard to try to figure out who used which ICs... So it's a goner. The resistor method works just fine anyway, so my curiosity will just have to live with my exasperation in this case ;)

Thanks for posting this. Updated my records to the change in ohms.
Just curious, did you ever try a 0.3Ω resistor? Would it work? :juggle:

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 Post subject: Re: Modding the flasher / hazard switch for LED indicators
PostPosted: Sun Nov 11, 2018 3:11 pm 
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WWDiesel wrote:
Thanks for posting this. Updated my records to the change in ohms.
Just curious, did you ever try a 0.3Ω resistor? Would it work? :juggle:


Well... I had intended to try a 0.3Ω resistor and work up from there if necessary; they were even ordered along with the 0.5Ω and 1Ω resistors. But when I sat down to finish this project off this morning, I could not find the 0.3Ω resistors anywhere so just went straight to 0.5Ω.

My suspicion is that 0.3Ω is probably safe, though it may be a little close to the hypothetical minimum needed to prevent hyperflash even with 1% tolerance resistors. In real terms, this may mean that it'll work fine in 95% of vehicles with 95% of LED bulbs on the market.

0.5Ω was the best safe guess, with 1Ω being my way-overkill number; I ordered those with the idea that if I was still getting hyperflash with one of them installed then I'd need to look into what else might be going on in there.

_________________
2005 KJ CRD Limited 4x4:
245/75R16 BFG TA KO2s
OME / Clevis 2.5" Lift
JBA Lifted A-Arms
IRO WJ Short Rear UCA/WWDiesel mount
Skid Row Skidplates
HDS Model 001 Thermostat (190°F)
Suncoast TC
Full Weeks Kit
Bosch 5V glow plugs
Hayden 2986 fan clutch / GM 11-blade fan
Samco / Sasquatch Intake Hoses
Carter in-tank pump
Provent 200
V6 Airbox


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