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Bulb sensor repair -Art? Anyone else?

posted by  Dave Stevens on  Sat Apr 6 22:27 CST 2019
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last visit: Sun Jan 1 02:36 CST 2012
[RELATED]

I'm attempting to repair a goofy bulb sensor -the later red-orange one made by Hella for Volvo. These were used in all the later RWDs. I've isolated it to voltage from the brake light switch badly leaking out into the dash indicator reed switch circuit. Barely getting 1/2 volt at all three brake lamps (obviously none will light). I've successfully re-soldered this bulb relay before (based on notes I write on the case), but this time no such luck. Single diode tests okay, single 47 uF cap tests okay, four resistors likely okay, that pretty much leaves either a physical short somewhere or a leaky transistor (T1, T2, or T3, all are NPN audio amplifier transistors worth an entire 1 cent each in large quantities). On close inspection and cleaning, there are no solder bridges or corrosion seen at or between any solder joints or traces, plus I re-flowed all I could easily get at. I'm not about to waste more time and pull the transistors for testing unless I figure someone else has succeded doing this.

Art, it's clear from your old turbobricks posts that you know (or once knew, laugh) a hell of a lot about these troublesome little beasts. Any thoughts? Anyone else?

I've also read in old posts that a simple bypass bulb sensor with the base board jumpered to bypass all the circuitry makes for brighter bulbs. Opposite to modern cars with LED bulbs, our old Volvos are well known for their weak lighting. I'm having a bit of difficulty believing that bypassing the bulb sensor will make much difference as I'm not seeing much resistance in the circuits. Anyone care to comment on that thought?

Anyone care to comment about the aftermarket Kaehler bulb sensors compared to the original Volvo ones made by Hella? IMO, the Kaehler fuel pump relays are far superior to the original Volvo white ones made by Stribel so I figure Kaehler has an above average reputation for quality. Hoping that holds true for the bulb sensor as I can get them considerably cheaper.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now






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