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MG TD TF 1500 - Stop Light Switch

I looked through the archive and found that I can get a new stop light switch from Nappa - Echlin SL 144 or 147 for my TF. My question is - are the threads compatible?
Ron Coates

it's a tapered pipe thread ie North American but everyone said it threads ok into the brass fitting. Worked for me, and strangely no fluid leakage while changing.
T W Moore

IMHO, you will be much better off if you use a Ron Francis switch
They last a lot longer than the others.

Steve Wincze

Dave Duois sells a relay to mount along side of the brake switch, which makes the poorly made switches last a long time. The switch then just controls the relay and it supplies power to the lights. I have one on my TF and it works great. PJ

Are you sure that is is American pipe thread? All other threads in the brake system are BSF.
Ray TF2884
Ray Lee

Ray, I might be wrong, but I think mine is BSF. I guess it would be easy to standardize them in that brass with a std US tap. PJ

The 144 will have connections closer to the original if you go that route. The 147 has spade terminals I believe. It threads in fine into the fitting but I have no idea what the threads are. The Moss supplied switch is one the right.

I removed the 144 from the TD before I sold it and replaced it with another switch because it required near lock up pedal pressure to activate the switch. Most likely due to arching/corroded contacts. Would a relay have prevented this? Maybe but I doubt it. I'm still running the original Lucas switch in the TF and its still going fine....knock on wood.

MG LaVerne

Checked again, the brake pipe tube nuts are BSF.
The pressure switch thread is pipe thread but whether it is BSP 26tpi or NPT 27tpi I can't tell. With it being tapered it is hard to tell.
Ray TF 2884
Ray Lee

I think I read that all of the switches are made by Echlin, including the ones sold by Moss. Does that mean the Moss unit has the same threads as the ones from Napa?
Ron Coates

The threads on all of the brake light switch are NPT. I don't have a thread chart handy right now, but I would guess that NPT is probably 27tpi. With the thread count being so close, the switch will thread right in and seal just fine (not what a purist would like, but it is the only game in town and it works, which is the bottom line)

Ron - Hope you got two or more switches. If your is like the experience of nearly every other person who has replaced brake light switches in the last ten years or so, you will be replacing that new switch within a year if the car is driven regularly. I have even heard recently, that the Ron Francis switches are often failing in a short period of time. Your best bet is to make up a relay/arc suppression circuit and install it at the same time you replace the switch. Information on making up the relay circuit can be found at: Cheers - Dave
DW DuBois

Is anyone familiar with the BWD switch sold by O'Reilly? It looks just like the Echlin switch and I am wondering whether it is made by them.
Ron Coates

Aa many will attest, DO THE BRAKE RELAY circuit and save yourself grief and expense no matter what physical switch you get!!!

R Biallas

Just for the record 1/8 BSP is 28 TPI and 1/8 NPT is 27 TPI. They are not the same, but as Dave says, they will likely fit with a slight interference.

About 13 years on my NAPA VW brake switch, no relay, no problems.
Len Fanelli

I have decided to do the relay with the Napa switch.
Ron Coates

I second Steve Winzce's suggestion of a Ron Francis switch. I had too many failures of the newer common variety ones and none with the Ron Francis switch. Plus this switch is set to close at very low pedal pressure giving an additional margin of safety.
John Quilter (TD8986)

Will it last, who knows, but I have a new Moss switch and Dave's relay. Light pedal pressure. I'll get back in a year and let you know if it's still working as good as it is now. Grin. PJ

Did the Ron Francis switch. Easy install works much better since it is a low pressure switch.
Bob McLeod TD 5618

I just punch my feet through the floor. No switch or relays needed. The shower of sparks..... well, they light up the sky better than any brake lights. ...And that one time, the gas tank.



Mitch, I mounted a stick on the side of the car with a nail. When I pull on the top of the stick the other end rubs against the tire. Fewer sparks and saves wear and tear on the sneakers. Jud
J. K. Chapin

People are recommending using relays with the brake light switch, the reason being that it shunts the heavy current through the relay, not the switch, extending the life of the switch.
An easier approach is just to use led bulbs. The current requirement is much smaller than incandescents, so it achieves the same goal without complicating the car's electrical diagram and adding more connections to fail.
Either way, the switch is engaged the same number of times - whenever you press the brake - so the wear on the switch does not change, all that changes is the current being passed through it. In both cases, it is greatly reduced.
Geoffrey M Baker

Did you patent that design Jud, or can anyone build one? I had a coaster when I was a kid that worked just like that! LOL. PJ

Bob - "Did the Ron Francis switch. Easy install works much better since it is a low pressure switch."
All brake light switches are low pressure. The problem lies in the contacts. For whatever reason, the contacts in today's switches are no longer capable of handling the current required for the brake light bulbs (which is not so high that I would consider to be a "high current" device). I also have to wonder if the problem is not the result of a high reverse voltage spike then the brake light switch opens when the brakes are released.

Whatever the reason for the switches burning, it has gotten worse in the past 10 - 15 years. Our TD had the original brake light switch in it when we purchased the car in 1974. I didn't have any problem with the switch until sometime in the 90s. The new switch that I put in failed in less than a year and subsequent switches failed in equally short time periods. My theory (and it is just a theory), is that cars over the past 20 years or so, have made extensive use of relays to switch current loads within the electrical system, enabling them to use lighter grade switches on the dashboards.

For whatever reason that today's brake light switches not holding up the way they used to (with the possible exception of the Ron Francis switch - which I think that the jury is still out on their reliability), the brake light relay/arc suppression circuit is a proven way to insure that the brake lights come on reliably, every time the brake are applied. I have even gone so far as to install a tattle tale light in both our TD and MGB so I have a visual indication that the brake light bulbs are getting power whenever I apply the brakes.
Cheres - Dave

PS. Geoffery's recommendation of using LED replacement for the brake light bulbs is another reliable (albeit expensive) to protect the vulnerable brake light switches available today.
DW DuBois

I purchased a brake light switch through NAPA 7/5/2010, part number SL147SB for 11.09. I took in my old switch and they said they could get an exact replacement, which I think was Lucas. However, my old switch may have been from an MGA or MGB, since quite a bit of the rest of the system is.


Bobby L.
B. F. Loughridge

Dave D.:

If the problem is a high reverse current upon releasing the brake pedal, wouldn't a 50 cent transil eliminate the problem by being placed across the contacts? I see all the new pump kits coming from Burlen now use a transil instead of the diode or capacitor.

Of course, the use of a relay is the correct solution if the problem is simply low current contacts in the new switches.

Lew Palmer

Lew - "If the problem is a high reverse current upon releasing the brake pedal, wouldn't a 50 cent transil eliminate the problem by being placed across the contacts?"

I am just throwing that out as a possible cause of the failures and at this point it is strictly speculation on my part. My relay/arc suppression circuit uses a diode across the relay coil and a capacitor across the brake light switch as added protection (whether or not it helps prevent switch arcing is helping or not, I have not researched). Suffice it to say, the setup has prevented any further brake light failure for the last 10 years on our two cars and I have not had any negative feedback after all of the relay/arc suppression circuit that I have sold. I am sure that there are a number of other circuits that people have put together from the instructions I have on my Homepage. If someone wants to try putting a transil across the bare switch to see if that will cure the problem and report back on it in the next ten years, have at it. Cheers - Dave
DW DuBois

The problem is not just the contacts. I purchased 2 switches from Moss which had "uroparts" on the box. the first one blew out the bakelite top and diaphragm. The second one never went on the car because I could see that the bakelite was off-center and tilted to one side.
This was clearly a manufacturing problem. I wonder how many millions were produced on the same line?
PS. I cleaned up the original switch and it's worked fine for 3 years now. If it ever fails I'll go with a mechanical switch.
cj schmit


Keep in mind that brake lights are a resistive load not an inductive load. There should be little if any reverse current (inductive kick) when the circuit opens. It is strictly a matter of poor quality switches since many cars in the past operated from showroom to junk yard with the original switch. If you can find a yard that does not crush old cars they are a good source of reliable switches. My TD runs a 1957 Ford switch.


Jim Haskins 1953 TD
J. M. Haskins

Dave & Geoff,

This forum is littered with suggestions/recommendations to switch to LEDs.

Sounds great except that not all of us have changed the ground polarity, including myself, and very few LEDs will work with a + grnd.

Its on my bucket list, but if it requires any bending (of my back), I'm afraid it will remain on my bucket list for a while. Switching the ammeter, and other goodies that are buried, makes it problematical, at least for a while.

Gord Clark
Rockburn, Qué.
Gord Clark

Gord, I'm known to be a solid advocate of negative ground systems, but not in your case. With your collection of cars I think it would be foolish to get into such a modification. I think you would be best off sticking with the OEM systems. If you want to go the LED route you can do it by purchasing LED assemblies that are wired for a positive ground configuration. Bud
Bud Krueger

Very true, Bud. Positive ground LED lights are not complicated, it's just switching one wire. In fact, I built my high power rear LED lamps for positive ground, and when I decided later to switch to negative (for all the reasons that have been mentioned in many threads) I just switched the wires and presto, they work fine with my new negative ground system.
So wanting to switch to LEDs does not require a negative ground conversion - although if you want to do a complete switch including front running lamps, you may find negative ground is needed only because finding a positive ground LED-compatible flasher may be hard to do. I could not find one for several years, but there may be something on the market now. I found that running rear LEDs and front incandescents worked fine because the front lamp current drew enough to trip the positive ground flasher. Of course, there are other ways around that problem - adding resistors will do it.
As Gord says, I think the hardest part of converting to negative ground was switching the ammeter wires as that required pulling out the center console.
Geoffrey M Baker

I have installed the relay and have not had any issues with the brake light switch purchased from Moss. I have recently converted all of the lights in the car except the headlights to LED - Interior lights positive ground from Veloce Solutions

He makes the lights up in either negative or positive ground configurations.

Tail lights and fender lights fro Lite Zupp


This thread was discussed between 13/08/2015 and 25/08/2015

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