What a difference a watt makes...

[Oiseau DeTonnere]

So, decided to replace the aging bulbs in my '57 taillights and parking/turn signal lights. Both are originally 1034's but now the recommended bulbs are 1157's. Problem is, here (Europe) 1157's are difficult to find, as that's a US spec bulb. I bought what I thought was the closest match here - a P21/5W. Found in another forum:

The P21/5W is the European version of the dual filament lamp. It is 21 watts for the major filament (brake or turn application) and 5 for the minor filament (parking lamp).
The closest retrofit for the P21W is the 1073, and for the P21/5W it's the 1034, but these two lamps can be hard to find.
The 1157 is a U.S. spec lamp which is 27 and 8 watts. Although the light output is roughly the same, the 1157 is designed to last longer. The 1157 is rarely used in new applications since the all-glass wedge base lamps (3057, 3157, 3357, 3457, etc.) perform better, are cheaper to manufacture, and can be installed more easily with automated assembly equipment.
There are other performance characteristics such as luminescence and nigrescence rates that distinguish these two lamps. The European-spec lamps always have a nickel-plated brass base which is superior to brass or aluminum.

For the taillight/parking light function, they light up just fine, but the turn signals are a different story. The flasher now flashes veeeeery slooooowly.

I tried to figure out why and I think I got it:
The flasher unit is a thermal one (original to the car I guess), and it's designed to heat up and cool down a bimetallic strip making/breaking the contact. For that to work efficiently, there needs to be a specific amount of current passing through the flasher that does the heating. Although as per the above article, the 1157 and P21/5W bulbs are considered to have more or less the same brightness, they are not the same wattage. The 1157's have 27/8 watts, whereas the P21's are only 21/5W. Thus the two P21s are only consuming 42 watts during the turn signal phase, whereas two 1157's would normally consume 56 watts. That's 33% more. So I surmise that with the reduced power draw, the flasher is just not heating up fast enough. It wasn't all that fast to begin with compared with today's cars.

So, the 1157's are going to have to go back in for now. I guess my next step is going to be to find a suitable electronic flasher unit to replace the thermal flasher unit. Also seem to be a bit hard to find here. Hmmmm.

[Rusty57]

You thinking that the turn signal flasher is a thermal unit is correct.

I believe that difference for the turn signals is greater than you show in your analysis. The load going through the flasher is only the wattage for the turn signal filament. The electricity for the parking tail light filament does not go through the flasher.

The original flashing lamp load was 16 watts. Using the P21 bulbs reduces that to 10 watts. That is a very significant drop in thermal load through the flasher.

An alternative to consider is a solid state flasher. It does not depend upon the size of the load to operate.

These are often used when someone converts to LED bulbs. They draw such a small load that a thermal type flasher will not work at all.

Edit: 27 Aug 2021. I was wrong with my thinking that the smaller filaments were the flashing filaments. It is the higher wattage filaments that flash.

[CSavaglio]

I think you're correct that the reduced load is making the flasher's bimetallic take longer to heat up and flash.

As said, a solid state flasher is a solution, just make sure it can handle the load of incandescent bulbs and not just the dramatically lower load of LEDs . Also, be sure to ground (earth) the solid state flasher or it won't work at all.

Another possible solution is to use a flasher designed for single bulbs, altho that may flash too quickly since it looks like your load is kind of in the middle.

[Oiseau DeTonnere]

You thinking that the turn signal flasher is a thermal unit is correct.

I believe that difference for the turn signals is greater than you show in your analysis. The load going through the flasher is only the wattage for the turn signal filament. The electricity for the parking tail light filament does not go through the flasher.

The original flashing lamp load was 16 watts. Using the P21 bulbs reduces that to 10 watts. That is a very significant drop in thermal load through the flasher.

An alternative to consider is a solid state flasher. It does not depend upon the size of the load to operate.

These are often used when someone converts to LED bulbs. They draw such a small load that a thermal type flasher will not work at all.

Hmm, I am never sure which filament is used for what (I don't have a wiring diagram here yet). So what you're saying is that the low wattage filaments are used exclusively for the turn signals, correct? I'm just trying to figure out the logic, but I think I'm confused...

Fronts:
Lights off: the turn signal flashes the low wattage filament on-off.
Parking lights on: the high wattage filament is used for parking lights and the turn signal adds the low wattage filament to make it flash bright-dim?

Rears:
Lights off, brakes off: the turn signal flashes the low wattage filament on-off.
Lights off, brakes on: both filaments light up?

Lights on, brakes off: the high wattage filament is used for taillights on and the turn signal adds the low wattage filament to make it flash bright-dim?
Lights on, brakes on: both filaments are already on, so it doesn't appear to flash at all? Or is there some magic that flashes the low wattage filament off in that case?

An alternative to consider is a solid state flasher. It does not depend upon the size of the load to operate.

As said, a solid state flasher is a solution, just make sure it can handle the load of incandescent bulbs and not just the dramatically lower load of LEDs . Also, be sure to ground (earth) the solid state flasher or it won't work at all

I found this:

As far as I can tell, it's designed to replace the two pin original flasher... no ground that I can see though. Supposed to handle up to 150W.

[Joe Johnston]

This is how it should be.

The tail lights and front "parking lights" (as we call them) are the lower wattage filament (or less bright filament) in these bulbs.

Rear brake lights and front and rear turn signals are the brighter filament in the bulbs. This is so the brake light and/or turn signals will over power the view to other drives.

Turn signals and brake lights work no matter if light switch is on or not.

Brake lights work even if ignition is off but turn signals must have ignition on or in accessory position.

The front "parking lights" originally did not illuminate when the headlights were on, but tail lights did. These "parking lights" only illuminated when the light switch was pulled out to the first notch in the switch and would turn off when the headlights went on.

The backup lights originally only would illuminate when the the headlights were on and the shifter in reverse.

You should easily be able to find an electronic flasher from something that will work, but it may not exactly plug into the old wiring harness and need some jumpers to make the connections.

Lots can change with several owners over the years. Best of luck in sorting it out.

[Oiseau DeTonnere]

So, I changed out the thermal flasher for the electronic one I showed in the post above. Not an easy job - the old one, which was just stuck up behind the dash not attached to anything, was completely corroded...

That thing is probably original to the car. I couldn't even get the connectors off the contacts so I ended up cutting them off and making a couple of small extensions with new insulated connectors. Getting them attached to the original wires was the hard part - had to get under the dash face-up to work work on it - major pain in the a$$ - and the back... Plugged the electronic flasher in and tried it out - bingo! It now flashes like a real modern car. And I get to keep my P21/5W's.
Small victories...

[Daddio]

Every victory is great! Did you ground the new flasher? Many folks here say they need to be.

[Oiseau DeTonnere]

Did you ground the new flasher? Many folks here say they need to be.

No, there is no provision to do so. I don't know how it works exactly, but there are only two flat connector pins (+12v in, +12v out I assume) like the original flasher - no ground pin. The original does not have a specific ground connection either, but it does have a metal case - I'm not sure that needed to be used as a ground, if it did, it was a miracle that mine worked at all before, as it wasn't attached to anything, it was just laying on the lip at the bottom of the dash.

The replacement has a plastic case, so there is nothing to ground to. The image I posted above might be misleading, what looks like a possible third contact is actually just a shadow.

There are all kinds of replacement flashers out there, some may indeed need to be grounded, but this one doesn't. If the pins weren't corroded like mine were, it would have been a 5 minute replacement job. I think the diameter is even the same so it might clip back in where it was originally - if not, they supply a clip with it.