Concerns right side only, LHD. First, turn on right turn signal, and bulb failure indicator on the instrument cluster flashes in sync with the turn signal indicator. Observed from the rear, when the turn signal bulb is lit, the two running light bulbs on the bottom row dim slightly. When the turn signal bulb turns off, the running light bulbs return to their ordinary brightness.

Secondly, with the right turn signal actuated, depressing the brake pedal causes the bulb failure indicator to flash in sync with the turn signal. Also the turn signal and bulb failure indicator rates of flashing are approx. twice the rate that the turn signal ordinarily would flash. Observed from the rear, the right rear turn signal is not flashing, and the inside right running light and right outside brake light are flashing bright and dim quickly, at the same rate as the turn signal and bulb failure indicators. AND, the right backup and fog lamps are glowing very faintly.

I have cleaned all bulb and bulb holder contacts. Doing so did not alter the described behaviour in the least. I should be grateful for any suggestions regarding the cause of this strange behaviour. Vehicle year is 1993.

 

When you get a turn signal bulb out on most cars, they will blink faster. This helps the driver know that a turn bulb is out. Have you tried to replace the bulb?

 

Also when you replace the bulb, use a correctly spec'd bulb - forget about using LED replacements. The "blink" speed is based on the cumulative bulb resistance (less resistance means the capacitor in the relay charges faster - thus the faster blink) and using a wrong bulb will change things.

 

All your symptoms especially those two describe a bad ground. The positive current is back feeding through other bulbs making them glow dimly because the ground for the taillight is bad (loose, not there, corroded, etc)

 

Thanks to all for their replies. Yes, I am fully aware of the double flash rate for turn signals when there is an open ground on one side. It used to be endemic on early '80s 240s here in Ontario, Canada after a couple of winters driving on salted roadways: when the ground straps for the front turn signals were screwed to the interior of the front fenders before Volvo graced us with splash covers under the fenders, the front turn signals would quickly fail from corrosion. Still, none of the replies, as yet, address the two key points:

1. When the right turn signal is activated, the right rear bulb flashes at its normal rate, but the two running taillights on the bottom row alternately flash dim (when the turn signal bulb is lit) and normal brightness (when the bulb is momentarily not lit). This alternating brightness of the two running lights is clearly what is causing the bulb failure warning indicator on the instrument cluster to flash in sync with the turn signal. But there is no obvious connection between the two circuits of the running lights and the turn signal. Or, is there?

2. When the brake pedal is depressed, the turn signal flash rate on the right side doubles. The rear turn signal bulb remains off, and the inside right running light and right outside brake light are flashing bright and dim quickly, at the same rate as the turn signal and bulb failure indicators. Again there is no apparent connection between the brake circuit and that of the right turn signal. Or, is there?

Taken together, the two cases provide experimental evidence that there IS some kind of electrical connection among the three circuits, but a connection which is only activated when the turn signal on and, additionally, when the brake electrical circuit is activated. And I think such a connection has nothing to do with open grounds because one open ground cannot explain these compound effects. Please also take note that bulb mismatch, contact corrosion and the like, the local circuit board ground and the circuit board itself can all be ruled out. I've already done my homework: the entire rear light fixture is brand new (three months old), its circuit board carefully examined and tested for flaws, and the ground strap to it in the wiring harness was fully re-seated where it properly belongs onto the centre of the sheet metal in the trunk with new screw. All circuits have been tested for continuity and abnormal resistance and no anomalies were discovered. As I previously mentioned, none of this work altered the described behaviour in any way.

So, as Churchill said of Russia in 1939, "Itisariddle,wrappedinamystery,insideanenigma;butperhapsthereisakey." And the longer I ponder this, the more I believe it to be the case.

 

Yes and your key has to do with a bad ground on the right taillamp assembly.

Your #1 - the parking lamps change or dim with turn signal because the turn signal bulb is using the parking light bulbs as the ground. Same with the bulb out flash with the TS on. The brake light is using the turn signal or parking light bulbs as a ground so the right brake light is using a different amount of current compared to the left side - Hence the bulb out flash. For a test - take the rear parking light bulbs out (all four) and see what happens. (yes I understand the outer parking bulb is a dual filament that is a brake light too)

 

Problem resolved. Please see https://forums.turbobricks.com/showthread.php?t=364355

Thanks to all who took the time to respond with their suggestions. Much appreciated.