My '78 blinkers and hazards flash rapidly with no noticeable 'click-click' sound you'd normally hear from most relays. I can't remember if the blinker relay ever made noise or was it always silent? I have no bulbs out around the exterior of the car, so maybe the relay IS toast? I've read different posts here where it's located. Any help on this is appreciated.
Mike

 

Two different flashers failing at the same time? I would look at commonality like wiring, corroded sockets, etc.

 

Ahhh...OK. I failed to mention that when I put the hazards on with the door open and interior lights on, the interior lights would dim slightly with each pulse of the hazards. Pretty weird huh?

 

95% of the time that is caused by a bad ground.

 

There are two flashers--both are installed in the fuse box in your '78. They look like round plastic cans about an inch in diameter. They're not relays. One contact is a thin strip of metal designed to bend as it heats. The heat comes from current passing through the contact. Once hot enough it bends and you hear a little click as it suddenly springs open. Next it cools and you hear another little click when it snaps closed. Some click louder than others.

The flashing speed is completely dependent upon the load (i.e. wattage of the lamps). This is why there are two different flashers -- the turn signals usually flash two lamps while the hazards usually flash four.

Slow flashing on one or both sides usually means that a lamp is burned out or that the wrong size (too low wattage) lamp(s) are installed. This is why standard LEDs DO NOT WORK with mechanical flashers!

Slow flashing turn signals on both sides can also be caused by the wrong size flasher. This often happened when a trailer was hauled and a "heavy duty" (hazard flasher) was installed and not replaced after done with the trailer.

Rapid flashing blinkers means too much load on the flasher. Oversized or too many (that trailer thing again) lamps can cause this but there is another cause: poor connection(s).* Do make certain that the proper lamps are installed but given your symptoms I bet that you have grounding points and likely lamp sockets in GREAT need of cleaning.

*Why do poor connections (they don't have to be ground) cause rapid flashing? It's all about the very fundamental relationship between voltage and amperage. A poor connection (be it power or ground) causes voltage in the entire circuit to drop. When this happens the filament in the lamp still tries to draw the wattage it is rated for. As a consequence of the lowered voltage, the amperage (current) increases. This this amperage that causes the contact in the flasher to heat so it behaves as if more or larger lamps are installed.

-------------
Suggestions:

1) First clean your lamp sockets. All you need is some spray contact cleaner (auto parts store) and a small brass brush.

2) Examine the lamps. If the base has any but the most minor corrosion, replace them as they're getting really old anyway. Clean the lamps you keep with contact cleaner and the wire brush.

3) Next clean the grounds. There is only one grounding point for the rear. It's near the radio antenna and easy to access and clean. A still wire wheel on a drill the thing to use for the bolt, the hole and the wiring contact. The front grounding points aren't so easy. There is one in the center of the nose between the headlamp door vacuum relays. It's not bad to get to. The other is on the front, lower driver side corner of the radiator support. It is an ABSOLUTE pain because it's blocked by the big round framing member that doubles as the vacuum storage tank.

---------------------------------

Once you begin this electrical contact and ground point cleaning process I HIGHLY advise you continue to do so until ALL have been cleaned! This includes all of the electrical connectors under the hood--and especially those near the firewall on the passenger side. If you don't get everything clean even more bizarre problems show up as things find one of your nice clean grounds via a route they should not use. You're also likely to find marked improvement in the operation of the windshield wipers, blower motor and essentially everything electric.

 

Mr Mike said it all. Follow his instructions. I might also add that once you have the sockets cleaned put a dab of dialectric grease on the bulb socket. It will stop corrosion and make it a lot easier to remove the bulb in the future. Do it in the plugs too.