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MG MGA - Third Brake Light with Logic Circuit

Earlier this summer I finally got the last of my ‘59’s gremlins sorted out to the point that she is totally reliable and I don’t have to worry that one of the wheels will fall off. On my morning commute in very heavy rush hour traffic I have been worried that my brake lights will be hard to see in the glare of the early morning sun. So I started looking around to see what could be done for a third brake light .

Barney’s MGAGuru site shows a couple of homemade logic circuits for installing a third brake light on a MGA 1500 plus several other approaches. I found a commercially available logic circuit at JCWhitney, the Pacer Performance Logic Circuit part number 20-702, JCWhitney SKU 1JA 126699 for $10.99 USD. I coupled it with a third brake light, 1JA 126699 for $18.99. This is an incandescent brake light. The logic circuit claims to be “For vehicles without original-equipment third brake light or vehicles without separate brake and turn signal light circuits and prevents brake light from flashing with turn signal”. Comments on the JC Whitney site indicate that the logic circuit has some level of “leakage” so LED type brake lights may dimly light up when the turn signal is on but no brake applied.

For the logic circuit to work properly it needs to be connected to each brake light. At the right rear electrical connections I replaced the White/Brown single bullet connectors with double connectors. From the new connection points I ran a pair of wires up through the gas tank filler gasket into the trunk. I am planning to move the location to come up between the end of the trunk and edge of the body in the future. My car still has the felt in place and there are no gaps to sneak the wires up right now. The wire set was wrapped in non-stick electrical tape and routed along the right side of the trunk. I used self-stick zip tie bases and zip ties to keep things in place and out of the way. The logic circuit is mounted in the trunk just behind the forward bulkhead. From there the power and ground wires run over the top of the bulkhead and around on top to the brake light. Again the wire set has been wrapped to provide some protection. The third brake light ground is on the trunk side of the bulkhead at one of the machine screws where the bulkhead attaches to an L bracket.

The brake light housing did not have a solid bottom. I used a piece of thin, flexible plastic threshold flashing that is very sticky on one side. The flashing is like a heavy piece of duct tape but 10” wide. On the sticky side I dropped four small, super strong magnets. The result is a movable third brake light. The third brake light does still work if one of the other brake lights is out. We will see if the rather inexpensive logic circuit will stand the test of time.

John


jbackman

Or you could just run one wire from the brake light switch directly to the third brake light and forget the logic module. See here: http://mgaguru.com/mgtech/electric/et240.htm

The problem with the logic circuit is up to 1/2 second delay before the third light comes on if the turn signal is operation when you step on the brake. See here: http://mgaguru.com/mgtech/electric/et240a.htm
and here: http://mgaguru.com/mgtech/electric/et240b.htm
Barney Gaylord

Barney is absolutely correct, a single wire, connected to the terminal of the brake light switch that goes to the turn signal relay and routed to the third, high mount brake light does the job nicely. I have been running a third, high mount brake light on our TD for better than ten years wired this way. Cheers - Dave
David DuBois

Wired mine up as Barney suggested. Attached Liftadots to the base so that the light can be easily moved if the tonneau has to be removed.

http://www.mgaroadster.co.uk/Third_Brake_Light.htm

Steve
Steve Gyles

I used the Chrysler LeBaron convertible tail light mounted to the boot lid and wired all the way up to the switch at the front - the (-) wire goes to a terminal near the battery (comes off the negative battery post).

Works great and is very bright!

JIM in NH



AJ Mail

Tested somemore this evening. With the turn signal on and then stepping on the brake there is a very slight delay. Perhaps a 1/10th or 1/20th of a second. However after releasing the brake the third stop light starts to blink.

So I either have to change my driving habits or take the suggestion of others and rewire to the brake switch.

John
jbackman

I designed and built the electronic version shared on Barney's site: http://mgaguru.com/mgtech/electric/et240a.htm I do not have personal knowledge of the internal circuitry of the Pacer Performance 20-702 device. I would suspect that it uses the unlit bulb as low current grounds since it requires no separate ground wires. It may require that unlit lamp as a ground path to "hold off" the 3rd brake light. If my assumption is correct, then it requires both an unburnt bulbs and good buld-socket connections for proper operation. If the bulb is burnt out or loose in its socket, then it may not function properly. I took a lot of care when designing my circuitry to ensure that the worked properly even if a bulb burned out or became loose in the socket, common problems found in 50 year old British cars that I didn't want to rely on.

Your description of the delay when applying the brake is inherent in any decoding circuitry whether relay based as Barney's design is or solid state as is mine. the circuitry needs to see both lights lite up in order to determine if it is truely a brake light event. I considered this during my design process as the "Average" time delay the circuit adds is ~125 mS with ~ 1/2 of the time ther being no delay at all. The time between voltage applied and full brightness of an incandescent brakelight bulb (1157) can be as high as 250mS. So even if there is no circuit delay, a bulb type 3rd brakelight will have up to 1/4 second to full brightness - every time. With an LED and decoder, the average to full brightness is 125ms. That is why I chose LED's for the 3rd light. The only way around any delay (on a 1500 anyway) is to eliminate "decoders", run the extra wire from the brake switch as already suggested and use LED 3rd brakelight. It certainly is easier on a 1600. If you have followed modern cars with LED brakelights or turn signals, they come to full brightness instantly. It really catches you attention NOW! A good thing I think.

I have no idea as to why the 3rd brake light flashes on the Pacer unit after removing your foot off the brake. Very strange operation. I have searched and found one other review where a person found that the Pacer part did not work as expected. He ended up doing the same brake-rewiring job on a Ford Pickup.

I do feel that despite the "modern" appearance that the 3rd brakelight adds to our classis cars, it is a needed modification on today's roadways.

Chuck
Chuck Schaefer

I tried to update that posting but unfortunately time ran out.

I wanted to mention that in a straight ahead emergency stop, the logic circuits act NOW, immediately activating the 3rd brakelight because both brake lights come on together. Only in the case where you turn on the turn signal first, will you possibly get a delay.

In reality, it is far easier to run a wire from the brakelight switch. I did that circuit some years back as a mental challenge.
Chuck Schaefer

This thread was discussed between 25/09/2011 and 27/09/2011

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