After nearly running over a co-worker (I'm blind in the right eye) I bought one of those after-market "beeper backup lights" for my 945 and it works like a charm

HOWever, the same beeper bulb will not work in my '98 s90. The normal backup lights work fine, but when I substitute a beeper bulb, neither of the backups will come on. The beeper bulb is OK, lights up and beeps when I put 12v to it on my workbench.

Is this some arcane Volvo voltage-balancing thing?

Or could it be that the beeper bulb has a higher resistance to current flow and is upsetting the intelligent Volvo?

It doesn't make a lick of sense, otherwise

Maybe I should just buy a 12v piezo beeper from an electronic parts house and wire it in parallel across one of the stock backup lamps.

Did the regular bulbs work after you put them back in?

Could you have blown a fuse?

yeah, backup lights work fine with the original bulbs, just not with the beeper bulbs

I'm gonna use a separate beeper; they don't draw much current so should not upset the bulb failure monitor

Hi GAcyclops,

A 1998 Volvo 90 series? Sedan or Wagon?

There may be an arcane Volvo voltage-balancing thing. Doubt it, through.

From the 700-900-90 Volvo FAQ:

https://www.brickboard.com/FAQ/700-900/ElectricalLightingHorns.htm

Click FAQ abopve to access the whole thing and read it all.

Also, we do have some wiring diagrams for your 1998 Volvo 90-series:

http://www.volvowiringdiagrams.com/?dir=volvo/S90%20V90%20Wiring%20Diagrams

In this directory is a 70 megabyte PDF of the wiring diagram for the 1997-98 Volvo 90-series.

So, I can't guess as to the circuit layout. I'm unsure of the white reverse lamps are wired into a bulb out sensor on your 98 90-series. Doubtful.

This is a beeper that fits like bulb into the tail light assembly. You have only one? Can you test the beeper from your 990-series in another car so you know it works.

If checked for good connection to ground between the tail light?

Else, study the wiring diagram. Someone else that also owns a 90 series may chime in with the fix it solution.

Hope that helps.

Duff.

"my '98 s90. The normal backup lights work fine" so it's not a wiring issue in the s90.

"The beeper bulb is OK, lights up and beeps when I put 12v to it on my workbench" so it's not a beeper bulb issue.

the s90 has a "bulb out" warning light, which I believe is controlled by a sensor that checks either current or voltage balance in the driving lights system.

Wish I could be there to share in your puzzle.

Assuming you recognize the common sense Occam's Razor wit offered by James below and have dismissed that most likely reason, I think I would get a meter and watch what happens to the voltage and current while it is in use.

Not sure how convenient that would be in your particular taillamp housing, getting at the backup lamp socket, given we don't have any RWD anywhere near as young as a '98, but short of building an extender cable, that would be the approach I would take to solve this puzzler.

Did these modern RWD Volvos move to CAN bus?
--
Art Benstein near Baltimore

Yogi purportedly said "When you come to the fork in the road, take it."