87 VW Cabriolet - turn signal fails when headlights on

Maintenance/Repairs
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1

I’ve got an 87 VW Cabriolet with an odd problem – the right-hand turn signal does not work when the headlights are on (there’s a buzzing noise from the flasher). The left works fine and both work when the headlights are off. I haven’t checked the bulbs in the taillight assembly, but I will. Any other obvious places I should be looking?

2

I would replace the bulb first. Otherwise check the wiring to the bulb socket. Look for a bad ground connection.

3

I would guess that either the front or the rear bulb is a dual filament bulb and the filament for the turning signal is sagging and making contact with the filament for the parking light in the front or the tail light in the back. I believe that the parking lights go on with the headlights. If you turn on just the parking lights, I would guess that you would have the same problem with the right hand turning signal. I would replace both the right front and right rear turning signal bulbs.

4

I never would have thought of that. I’ll check it out. After doing some more research, here’s what I’ve found:

  1. The ground for the right-hand assembly SEEMS Ok – at least mechanically. I haven’t tested it yet;

  2. When the turn signal is working (headlights off), it also activates the same rear tail lights that are normally on when the headlights are on (the lights flash along with the turn signal lamp). This isn’t the case when using the left-hand turn signal.

So, it would appear that the wire that feeds the turn signal is somehow feeding into the headlight circuit?

5

You have a bad ground at the headlight or taillight assembly. Common issue.

6

You have got it right. There is no ground for that bulb so when the turn signel voltage is trying to light the heavy filament the voltage goes through the heavy filament to the ground end of the bulb and goes back through the light filament to reach a ground through all the other bulbs attached to head light circuit. Since the light filament has the most resistance the majority of the voltage drop is across that so it lights.

You may notice that when the headlights are ‘on’ that the lighter bulb filament is lite and goes out when the turn signel voltage tries to turn the heavy filament ‘on’. Since both the running light circuit and the turn signel circuit have the same voltage there is no current flow. The turn signel flash does not see enough current draw to allow it to work correctly.

Trace the ground for that bulb to see where the break is.

I hope I have not completely confused you.

7

Hi there just wondering if you found out what the problem was my 87 is doing the same thing on the left side

8

Hi —

I did resolve it, but I don’t remember exactly how, since I posted that in 2009. But I recall that one of the suggestions was to check grounds and that seems to ring a bell. Funky grounds seem to cause a lot of problems in these cars.

BTW — If you haven’t discovered the Cabriolet forum on VWVortex (www.vwvortex.com) I highly recommend it. There’s a guy by the name of Brian who posts frequently and answers questions and he knows everything there is to know about these cars. He’ll have lots of suggestions.

John

9

The flasher functions requires a certain number of amps to flow in the turn signal circuit. Anything less will cause it to do nothing or buzz when you turn the signal lever on. If right on the current threshold already, it could well work with the headlights off, but not with them on, since the headlights are drawing some of the available current supply away from the turn signal circuit. A circuit is what it says, a loop, and anything that hinders current flow anywhere in the loop can cause this to happen, including a weak battery, a bad bulb, a bad bulb connection, or a bad ground.

btw, folks here have complained this occurring when they replace the bulb with an LED version. Some LED versions aren’t designed for turn signal circuits b/c they draw too little current. But other LED designs compensate for this somehow. So for turn signal circuits be sure if you switch to an LED bulb to buy one that works w/turn signals.

10

Ok thanks man
U still got the cabby

Send from iPhone

11

2) When the turn signal is working (headlights off), it also activates the same rear tail lights that are normally on when the headlights are on (the lights flash along with the turn signal lamp). This isn’t the case when using the left-hand turn signal.

With that detail from the OP one of the turn signal sockets on the problem side has lost its ground. When the ground circuit is lost the turn signal will ground though the tail light circuit when the tail lights are off. When the tail lights are on that circuit is powered and not an available ground path for the turn signal.

12

You got it . . . the flasher relay has to be compatible with LED lamps

Grote has several flasher relays which are LED-compatible, for common relay configurations, anyways

I know I went off-topic, but I still felt it was good information, and not totally irrelevant

13

I do — she’s running great. The turn signal issue was followed by many others, but none that were surprising for a car that age or insurrmountable. Still a great car to buzz around on a nice summer day! How long have you had yours?