At 02:24 PM 6/27/05, Siggi in Frankfurt, Germany wrote:
"I'm trying to install a hazard flasher to my otherwise correct working circuit. .... 1500 MGA, and what type the additional relay should be used?"

There are a few different ways to do it. Start by reviewing the original 1500 turn signal circuits here: ET-101f (print this one) and here:ET-104

As a beginning learning experiment, you can hook a jumper wire between terminals 4 and 8 on the relay box, switch on the ignition, and switch on the turn signal switch (for either direction). The jumper wire will cause both left and right signals to be energized at the same time, giving you 4-way flashers. Alternately you can jumper all three terminals on the turn signal switch together and switch on the ignition, which does the same thing.

There are a few problems with this approach. One is that the ignition has to be switched on, which will also supply power to the fuel pump, ignition coil, and the charging control box. If left on for very long this could burn out the ignition coil, and the combined loads will run the battery down faster (after a few hours).

Additionally the original flasher unit is designed to flash at the proper rate only with exactly two bulbs connected for electrical load. On power up of this flasher there is a short pause to heat up before it switches on the lamps. The lamp connection bypasses the heater, allowing it to cool off, at which time it goes open circuit, and the cycle starts over. With only one bulb connected there is insufficient current for the heater element, and it stops flashing.

With more than two bulbs the flasher unit heats up faster and switches on the lamps quicker. It also gets a little hotter than normal, which keeps it in the on condition longer before it cools off enough to switch off. This results in a slightly shorter off time and a considerably longer on time, giving a noticeably long total cycle time. The original flasher unit is not intended for this application and could suffer premature failure with the extra load. I have never had a problem flashing a third lamp with my trailer hooked up (in 50,000 miles of trailering), but it does cause the lamps to stay on longer with each flash. I have not tried any long term test with the original flasher unit operating four bulbs at once. In short term test it does indeed flash considerably slower (long time on and short time off). I have no idea how long it might run without failure of the original flasher unit (or burning out the ignition coil), but it might be worth hot wiring it this way if you think you need four way flashers in an emergency.

If you are doing this as a permanent installation, it is advisable to move the flasher unit supply wire from the A4 terminal of the fuse block to the A2 terminal so the flasher unit can work without having the ignition switched on. Also move the turn signal switch supply wire from the fuel gauge "B" terminal to the ignition switch supply side (having to separate two wires from one ring terminal to do this). This last move will allow the turn signals to work without having the ignition on, but we do have the original self-canceling turn signal switch, right? The more important reason for these power feed changes is to eliminate electrical feedback back into the ignition circuit during operation of the four way flashers when the ignition switch is off.

You will also need a two-pole two-position switch (toggle switch is okay) to actuate the four-way flashers. When this new switch is actuated is should connect battery power to both left and right relay signal terminals at the same time (relay box terminals 4 and 8). If the two previously mentioned power feed wire changes have been done, the new switch can simply be wired to connect all three terminals of the original turn signal switch together when actuated. Using the original flasher unit in this manner would maintain function of the dash indicator lamp.

An alternative to the double pole switch can be a single pole switch and a double pole relay. If you do not have fog lamps on your MGA you can use the original "F" switch on the dash for the flasher switch to operate the new relay. The two common terminals on the relay can be connected to always-on power along with one terminal of the relay trigger coil. The other terminal of the trigger coil will run to the "F" switch, which will be connected to ground on the other side. The two normally open terminals of the relay get wired to the turn signal switch L & R terminals. Here the single pole switch and double pole relay together do the same thing as the double pole switch noted above. It's just the relay allows you to use the single pole "F" switch for the four-way flashers.

You could also replace the original flasher unit with a heavy duty flasher unit to assure that the flash rate will remain constant regardless of load. The HD flasher should be able to handle up to six bulbs at once (up to 150 watts of load), in case of a trailer hookup. The HD flasher would disable the non-flash mode of the turn signals if one bulb burns out (or has a bad connection). You would then have to remember to be very diligent at regularly checking the operation of the outside turn signal lamps, as there would be no inside indication of failure of one lamp. Also the heavy duty flasher unit may not have an output terminal for the dash indicator lamp. This can be accommodated by installing two (cheap) diodes to connect original relay terminals 2 and 6 to the dash indicator lamp (which only uses 1/3 amp).

So far this article has been about how to get hazard flashers on the cheap for your MGA 1500 model. For a better (but slightly more complex) method, turn the page and read more.