Member Since: 29 Apr 2019
Location: Perth
Posts: 118
FBH Fault 0x0a Flame detector/monitor short circuit
G'day all,
After a few years of inaction, I gave the FBH in my D3 a birthday back in June. It worked great over winter and having the ability to remote start it was awesome although I probably only put 6 hours on it. I figured it hadn't been started since things started to warm up in Sept, so I'd give it a run to keep it working. Or so I thought. Asked it to start to be greeted with an error.
Hooked up the diag software and it's an 0x0a. Flame detector/monitor short circuit.
I don't have a portable 4 wire ohm meter, but my handheld is usually pretty good. I measure the resistance between about 0.8->1 ohm at the white socket, which is considerably high compared to the manual specs.
When hooked up to the software if I unplug and re-plug the burner, the software sees ~0.8xx ohms briefly and then it drops to 0.005 ohm and sits there solidly. I figure I'm probably up for a new burner, which is a as this one only has 125 hours on it.
Before I add yet another expensive item to my LRDirect order, does this sound about right?
It's most certainly not a short circuit, but it's probably twice to three times the resistance it's supposed to be. Given I'm in the other hemisphere and our discos came sans FBH (mine migrated) I can't exactly compare it against someone elses unit.
I'm not particularly in a hurry, given I have about 6 months before it starts to get cold again but if I put it off I'll probably forget about it until I want it.
14th Dec 2021 3:58 am
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4626
I'd say your suspicions are correct and I'm sure you already know the GP has a dual purpose as a flame detector as well. So if the Webasto T Test V2 is telling you 0x0A exists its highly likely the short is within the GP itself. I recently had the same fault and a new GP/burner assy solved it.
Motolab is our resident Webasto Guru, and I'm sure he will confirm or otherwise if he spots this thread.
By the way, why do you need a Webasto heater in one of the hottest places on Earth. yµ (idµ - eAµ) ψ=mψ
14th Dec 2021 5:00 am
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
Because it still gets cold in winter, and the best reason of all "because I can!"
14th Dec 2021 5:02 am
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4626
Cold!....... Come to my back yard tonight and I'll show you cold, mixed with wet and damp air.
You'd be bringing the Webasto to bed with you! yµ (idµ - eAµ) ψ=mψ
14th Dec 2021 5:11 am
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
My Inlaws are in Ayr. My stock response is "don't go complaining to me about the weather, you choose to live there!".
Realistically there's maybe 5 days a year here where it gets genuinely cold enough for the FBH to fire up on its own. Still, it's nice to have and even nicer to use. The D3 does take a while to warm up on a cold night and the heated seat only warms the back half of me.
14th Dec 2021 6:45 am
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
I did a R=V/I test on the glow plug by pushing ~4.5A into it using a 55W bulb as a current limiter, and it's close enough to spec (by the time I got a decent measurement it was about 0.33 ohms, but it had warmed significantly by then. I let it warm up to about 0.44 ohms before calling it good.
If it's not the plug, it has to be the electronics.
I then connected a resistance decade box up to the heater in place of the glow plug, and it's reading out by about a factor of 2000. ~500 Ohms on the decade box causes the thermo top software to read ~0.250 Ohms. 2K reads 1 ohm and 0.3 ohms reads effectively nothing.
I set the decade box so the software was reading ~0.250 ohms and tried to fire it up, and it began the start sequence.
So, it's looking more like an electronic issue than the glow plug. Now to see if I can get the board out without removing the heater.
14th Dec 2021 8:07 am
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
And the answer is "Yes I can". I've also identified the fault.
Click image to enlarge
Click image to enlarge
Quite obvious burning of the pull-up resistor that supplies the plug measurement current.
Click image to enlarge
So the burning (ha ha, so not funny) question is, what liquid got in there and how the hell did it get in?
I've not been anywhere near water. There's an obvious high-tide line. I haven't pressure washed under the bonnet. I don't recall ever hosing the heater. None of it looks like coolant and there's no colour staining.
So, at least I know I don't need to replace the burner. I just need to figure out what value that resistor used to be.
Does *anyone* have a decent high-res picture of an un-molested board? Given every other sensor checks out ok, and it measures the plug (just incorrectly due to a wildly high pull-up resistance) I should be able to fix it. It'd be nice to know how whatever got in there got in there too!
Edit 2 : Right, 1W 6.8 Ohm MELF. Looks like it's switched by a MOSFET PNP transistor to battery only when required to take the measurement. I suspect the corrosion was holding the MOSFET gate transistors base on which with 13.7V across a 6.8 Ohm resistor would see it attempt to dissipate 27W. No wonder it cooked.
Ordered Digikey part MMB02070C6808FB200 which matches the dimensions. Now to find out where the water came from and where it went to.
Edit 3: Ok, so here's how it works.
There's ~5.3V applied constantly to the glow plug to detect it's presence. That voltage is via a relatively high impedance source (~4.7k), so that was what was allowing me to use the decade box to simulate the glowplug. (5.3v in a 4.7k/500ohm divider is about 480mV)
The heater applies a 770us pulse of battery voltage about 9.8 times per second through the 6.8 Ohm resistor to measure the glow plug resistance (and therefore temperature). At ~.25 ohms that would result in a measurement voltage of some ~480mV with the engine running (13.7V supply).
There is a dirty great chunk of corrosion across 3 pins that connect to the daughter board containing the processor and one of those pins is the gate to the MOSFET base of the transistor that drives the 770us pulse. So it probably got damp, shunted the gatebase on and would have cooked the resistor in seconds. The base is held high with a 1K resistor to battery voltage, so with a hFE of ~150 at 2A it'd only take a 14mA leakage to ground to fully switch it on and the base track runs through a via on the part of the board below the high-tide mark, so regardless of the additional corrosion on the daughter board connector, that'd do it.
I have just the board powered up on the bench with a connection to the WTT software and all the waveforms look pretty good, and everything else is working. So I might have dodged a bullet.
Edit 4: For the record the transistor switching the measurement current is an FZT751 PNP and the MOSFET that switches the glow plug current is a BTS282Z N-channel switch. Edited description to suit.
Last edited by BradC on 16th Dec 2021 3:49 pm. Edited 3 times in total
14th Dec 2021 8:36 am
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4626
Very interesting and thorough investigation yµ (idµ - eAµ) ψ=mψ
14th Dec 2021 4:30 pm
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
Repaired, bench tested, conformal coating re-applied and into the oven for cure. Not pretty, but it all checks out.
Click image to enlarge
Click image to enlarge
Still have to clean up the heater cavity and get it all buttoned up, but I might wait until the coating is properly cured.
Due to the water level, there is going to be corrosion in the thermal sensor plug. I gave that a flush with Deoxit D5 a couple of days ago, but I'll probably dismantle it just to check before I put it back together.
I found the power socket on the cap pretty badly broken when I dismantled it, so I have a strong feeling that's where the water got in. I've repaired that, so with all the plugs in place it should be water resistant. Can't understand how so much water got in there, but I've had the car detailed in the last 6 months. Perhaps they gave it a good "wash".
21st Dec 2021 6:19 am
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
Ok, so that was a failure. Not a failure in that it now correctly measures the glow plug resistance, but failure in that it still gives an 0x0a error after it fires up the plug for a little bit. More investigation required.
22nd Dec 2021 10:28 am
BradC
Member Since: 29 Apr 2019
Location: Perth
Posts: 118
Been trying to simulate the glow plug on the bench but this thing is clever.
The fan motor can't be simulated with just a resistance, it needs to see back EMF to detect the rotation and get a handle on the speed. A spare lock motor does ok for that.
The unit measures the plug and seems to want more than 200mohm and less than 350mohm. It then applies power until it sees more than 800mohms. Then it alternates between 80% and 100% on the plug duty cycle seems to expect to see the temperature change in sync. If it doesn't it bails out.
I can get it as far as the first ramp from 80 -> 100% on the plug, but I can't increase the resistance enough (I've only got 10 0.1 Ohm 5W resistors).
Now, I either have a duff plug, or the device is measuring incorrectly because when it fires up the plug in the heater it gets nowhere near 1 ohm. The board is measuring resistance correctly. I can pretty accurately step from 0.1 Ohm to 1 Ohm with my string of resistors.
What I don't want to do is spring $400 for a new plug without some absolute positive confirmation it's stuffed. The plug was working fine before the PCB got waterlogged. Now, I suppose it's possible the plug has been damaged by the electronics, but looking at the circuitry it's highly improbably.
Has anyone managed to simulate a plug on one of these to at least get it as far as attempting to fire?
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as this one only has 125 hours on it.
