RWD - Bulb sensor delete -follow-up

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Bulb sensor delete -follow-up

Excellent photos and diagrams so that others can see what we've been talking about.

Thanks Art, for your detailed analysis and explanations of my bulb sensor problems. Yes, some of my early speculation on what was going on was rather implausible when I go back to read it, although things were perhaps more complicated than you imagined. The jumpering you did to make your Test sensor is more or less exactly what I accomplished the other day, except your soldering skills are perhaps a little tidier than mine (I wasn't planning for a photo shoot).

To belabor this thread more than I should, and what with the brickboard messaging facility perpetually broken (and me being too lazy to find your email address), what follows is not for people's future technical reference, just for you as a backwards look at my diagnosis and speculation in relation to what you've said and a distillation of the pertinent diagnostic steps I took. Being thrown a few curves along the way, I was very much grasping at straws when I turned to you and the brickboard. There are also some details that I didn't properly clarify as I wasn't expecting you to be doing such a thorough job of remote diagnosis (like I should expect anything less).

Like you, I totally suspected a cold or cracked solder joint on the brake light circuit (54->54R/54L/S). All my initial symptoms said so. It was the first circuit I carefully inspected and resoldered. The whole time my meter said I had continuity, so I did indeed turn to a load test. Out of the relay tray, with 12 volts directly from the brake light switch to 54 (also proving the integrity of the brake switch), the brake circuit in the sensor functioned exactly as expected, a nice 12 volts out on all the brake light pins. When installed in the relay tray and with a few test wires looped around the pins, things were very different, only residual millivolts out to the brake lights. Just to be sure, I tested the wiring from the relay tray back through the brake lights, but that all checked out.

[BTW, 740/940 relay trays aren't that horrid to work with, better than playing hide and seek and needing to remove the glove box and trying to reach for some of them wedged in a rats nest of wires under a 240 dash. The smaller front row relays have at least some access when the tray is unclipped and pulled forward a bit. Removing the above storage tray allows you minimal access to everything and with a bit more effort you can pull it right out and flip it over if the wire harness is completely free and properly folded at the back.]

One thing I did not clarify that may have caused you to hold your open brake light circuit thought while I was off on assorted wild tangents was that when my DVM saw continuity change (beeped) during handling while grabbing and twisting the stack in my hands, it was not while being clipped to 54 and 54R/54L/S, it was between 54 and K-31 (K-31 being the bulb sensor circuit with all the components on it like the transistors and diode that should not at all be in contact with any of the bulb circuits). My apologies, I was not specific enough and should have clarified that for you when I saw your diagnosis hold firm.

On the bench, I tried to replicate the short so I could find the highly suspected solder bridge (with all the lumens I could muster, magnifying glasses and dental pick in hand). As I tried to indicate, what I surprisingly eventually found was that just gently bending the reed switch wire going through the brake light coil was all it took to make/break the short between 54 and K-31 (K goes to ground so that was presumably how current from the brake switch was bleeding off to ground and not going through the brake bulbs). During previous repairs a year ago (to the headlight circuit), attempting to give it a bit of a refurbish, I would have re-soldered and re-centred all the reed switches to make everything good as new. I may well have accidentally cracked the reed switch ceramic coating and later on accidentally bent that wire closer to the inside of the brake light coils and it's taken over a year to wear through the thin coating on the coil wires enough to allow a short under load that only got worse during current testing and handling. When the reed switch was removed that was the end of the short and all other misbehaviour. Not content to just leave it at that and put it back in the car to live without the brake lights being sensed, I decided to slip an insulator on the other reed switches so that nothing like this would ever happen again. They likewise decided to fall apart, so at that point I declared it toast and decided to give it a lobotomy and jumper the base board for temporary use and future testing purposes.

Hopefully I've now been able explain this to your satisfaction. As always, it's been both fun and educational having such discussions with you again!

I'll leave it to some other post to get the brake light fuse amperage from you (I did blow it once during testing) as I'm sure your manuals are closer to your fingertips than mine and it's also raining out. (please just grit your teeth and try to smile at this attempt at a joke)
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now


New 1 Bulb sensor repair -Art? Anyone else?
posted by  Dave Stevens  on Sat Apr 6 22:27 CST 2019 >

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