Indicator makes rear lights pulse???

Recently MOT'd my '83 1.8 GTI and the nice MOT guy mentioned that the rear lights pulsed slightly in time with the indictors flashing. Not enough for an advisory (or whatever they are called now).

I'd noticed this a while back whilst undertaking pre MOT checks. I'm assuming this is an earthing issue so cleaned up the earth point in the corner of the boot, the multi-plugs to the light clusters and the under bonnet earths to gearbox/chassis. They all appeared OK anyway and didn't really improve things…..

Rest of the 'lectrics all appear to be present and correct and work fine.

Any ideas where else to look or what else I could check??

 

The grounds for the left right plate are tied to the right side or vice-versa.  Separate them and tie each individual side to the frame.  I use the Bumper Bolts.

Corrosion in the Bulb sockets can cause this as well.  I usually remove the bulbs and clean the sockets with Steel wool to polish the bulb base, as well as the copper finger.  I also clean the attachment of the ground wire to the plate, and tighten the crimp.

Now Pulsing is also a Voltage issue as in how much deflection is there in your VOltmeter.

So you could be under powering them.

Check that you are getting 13.85 to 14V out of your alternator at the battery connection.  

If you disconnect the battery and remove the regulator assembly you can clean the contact points of it.  I have seen as much as a 1/2 Volt gain on my alternator when doing it.

Alternator Tune up (Bosch 2 wire) | Volkswagen Owners Club Forum)

Also you might want to add a ground wire from a case nut on your alternator to the engine I use a Valve cover nut.. as the only grounding of the alternator is from the mount bottom bolt and the pinching action of the bronze bushing moving in steel to an aluminum frame of the alternator so you have 3 different metals being used for the grounding. There is a electrolysis action of differential materials and passing current through them  and  the upper bracket bolt Steel to aluminum and back to steel, and if you don't clean them things you can drop current and voltage because of it.  

Sorry never mentioned but I cleaned the bulbs when I was cleaning the earths and cluster connections, so don't think its that

Will double check battery voltage and the alternator tips

Also if you relay your front headlights you take a lot of the load off the headlight switch by re-directing it from the battery.  You get higher voltage and amperage to the lights which makes them brighter.  You can see a 1-2v jump in voltage available to the lamps, and we know from Ohms law that the increase of voltage increases current flow through the fixed impedance of the Filament of the bulb expressed in Ohms.

This can also reduce the Pulsing of the rears.

As I said separating the grounds from the left and right sides rear to their own Ground path is a really good thing to consider, it's only about less than 2 foot or wire per side and a couple of Ring connectors and a couple of female Blade connections.