I have here a combined flasher and hazard flasher relay for a truck, called "Bosch Warnblinkgeber 24V (3+1) x 21W", part no. 0 335 215 250. It has 11 pins to connect:

  • 31
  • 49, 49a
  • C, C2
  • L, R, RL, LL, 54L, 54R

This pinout is fairly common among manufacturers (Bosch, Hella, Monark etc.). But what do these pins mean, esp. how do I connect it so that dash indicators work?

On 4-pin flasher relais, 49 is input (positive from ignition / battery), 49a is the flasher signal output, and C and C2 are positive for dash indicator lamps for vehicle and trailer, lighting up when "enough" load is connected to consider it having working flashers.

Trying this same connection on the 11-pin relay (and 31 for ground of course) results in a flasher signal on 49a, but no signal whatsoever on C or C2. How do I connect this thing, and why?

This is my flasher relay (case removed, pinout shown on case):

Flasher relay in question. Case removed, pinout shown on case.

1 Answer 1


How 11-pin flasher relays work internally. See this wiring diagram:

Components and connections for hazard flasher / flasher relay BOSCH 0 335 210 250

It is for the BOSCH 0 335 210 250 flasher relay (same as yours, only for 12V). The diagram shows the internal components of the device logically (physically speaking, there are some more inside).

How to connect 11-pin flasher relays. According to the above diagram, this is how you connect the relay:

  • Connect 31, 49, 49a, C and C2 as you did already.
  • Connect the cables with signals for left and right flasher which return from the control stalk switch to L and R, respectively. These are inputs for another independent set of relays.
  • Connect LL and RL (outputs for flasher signal) to the vehicle's turning signal lamps. Means, to where the cables from the control stalk switch were going to before.
  • Connect 54R and 54L to the trailer socket indicator signal pins.

Now, when your turning lights light up, C and C2 should also light up the dash indicator lamps. If they don't, make sure you indeed have the load connected corresponding to at least the flashers for one side: 3x21W at LL or LR for C to light up, 1x21W to 54L or 54R for C2 to light up. If you connect less, the relay will not show the indicator lamp to hint to a possible lamp failure.

The reason for 11-pin flasher relays. The difference to the 4-pin relays is: with a 4-pin "(3+1) x 21W" relay, the amount of load on a common line triggers the dash indicator lamps: for 3 x 21 W, lamp C is lighting up, for 4-5 x 21W, lamp C2 is lighting up additionally. The problem is, if you have a trailer with working indicator lamp and a vehicle with one of three broken (on one side), lamp C will light up and lamp C2 will not, supposing a problem with the trailer lamps. This is fixed with the 11-pin relays, since it detects loads for the trailer and vehicle flasher circuits separately.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .