Mechanical brake switch for 47 Plymouth

[central52]

I put in a Watson's Streetworks mechanical brake switch on my 47 Plymouth. Now, are the two wires that come out of the old hydraulic switch a positive & negative thing? When I went to connect them to the new switch, I hit one of the wires to one connection on the new switch, and it sparked. Checked the 30 amp fuse, and it was blown. (I know I was supposed to disconnect one of the battery cables before doing any electrical work). Anyhow, I put in a new fuse, and I get nothing now. Before, I had at least tail lites, intrument lites, interior lites, Hi-beam lite, (no low-beam). What in the world did I do? How do I troubleshoot this? Ed P.

[Don Coatney]

Did your brake lights work with the original switch?

Sounds as if there is a short-to-ground somewhere in the brake wiring.

[Plymouthy Adams]

Are you saying you have no lights? dash, tail or ortherwise?

The original just transferred voltage to the other contact when pressure was generated by depressing the petal..there is no ground on this switch..both wires hot when operating..it cares not which wire goes where as long as both went to the switch.

Th mechancial switch should do the same function..

In changing your fuse..I know this sounds silly but some get broken inside near the cap and will not show as bad..recheck this..also..do you have the key on?..

Coatney may be right..the old original wires are subject to be brittle..often moving these can result in insulation breaking off and going to ground..as this happened when you touched the hot wire..suspect is the wire going to the rear of the vehicle...

[Norm's Coupe]

I have a Watson brake switch too on my 48 coupe. As Don & Tim pointed out, it sounds like you have a wiring problem. If you still have the old original wiring to the switch you should cut at least an inch or two off from the bottom, then add new wire to the switch. That way, you'll get rid of the old wire at the weakest point where it connected to the old switch. Also, check to see if you have a headlight relay in your car. If you do, it also has a fuse and you may have blown that one too.

[Lou Earle]

The mechanical switch operates just like a toggle switch you might put on your dash to turn something on or off. It has a hot wire leading to it and another wire leading to the -in this case the brake light(s). when u push the brake it "flips" the switch to convey power to the wire leading to the brakes- exactly like the older in line pressure switch. So I suspect you might have touched the Hot wire to the switch itself by accident or the switch is in fact bad. It makes no difference how you attach the wires to the terminals on the switch-remember it is just a breaker or toggle switch.

As an aside I have never understood why anyone switches to the manual- Chevrolet used it in 41 and abandoned it sometime later. You can buy a new pressure switch almost anywhere. I believe I have replaced one in about 35 years of messing with these old cars- so I stick with the pressure switch. I wish to hell my sons ' mustang had a pressure switch- you will get a ruptured disk trying to replace the mechanical one under the dash!!!

Lou

[Norm's Coupe]

As an aside I have never understood why anyone switches to the manual- Chevrolet used it in 41 and abandoned it sometime later. You can buy a new pressure switch almost anywhere. I believe I have replaced one in about 35 years of messing with these old cars- so I stick with the pressure switch. I wish to hell my sons ' mustang had a pressure switch- you will get a ruptured disk trying to replace the mechanical one under the dash!!!

Lou

Lou,

Chevrolet must have kept the mechanical switch for some years after 1941. They still had it in the new 1970 Impala Custom Coupe company car I had. That one I know had the mechanical switch because it was mis-adjusted and I had to have the dealer adjust it properly.

I guess it's all a matter of opinion on the switches. Yes, I'll agree with you the hydraulic switches don't go bad any faster than a mechanical one. However, I switched to the Watson switch so I could adjust the switch to come on with just a light touch to the brake pedal, without slowing down if I want to. That way I can warn the guy behind me faster that I may be slowing down. Can't do that with the hydraulic switch because you have to build up the pressure by actually putting the brake on. Because of that I just think the mechanical switch is safer.

[Niel Hoback]

I'm with you on that, Norm. It's nice to be able to warn cel-phoners that you are going to change speed.

The real reason I went to a mechanical switch is bcause the hydraulic switch on my 49 DeSoto popped apart just as I was pulling in behind my other car after work. No damage, but it convinced me immediately that there is a great advantage to mechanical switches; you don't lose your brakes when the switch breaks.

[RobertKB]

I finally put in a mechanical switch in my '38 Chrysler as I had put in three hydraulic ones and they all quit working after a short time. They were all brand new units and gave me nothing but grief. Both my '48 and my '53 have their original hydraulic swithches and they work fine. Go figure!

[PatS....]

"This high quality switch fits most cars, with 20-50 lbs. of pressure required instead of stock units that dont come on until 60-120 lbs. of pressure. Standard 1/8" pipe thread and is compatible with silicone brake fluid. Requires use of a brake line tee. $26.95"

http://www.hotrodexpress.com/index.php?l=product_detail&p=20

[central52]

I have no pressure in the brake line. Pedal goes down to the floor. (I'm getting complete brake overhaul done next month). I'll check the main 30 amp fuse again, but as for the second headlite relay fuse, where exactly would it be? I can't seem to find it, if I have one. Did all the 47 Plymouths have these 2nd fuses? Ed P.

[Don Coatney]

I have no pressure in the brake line. Pedal goes down to the floor. (I'm getting complete brake overhaul done next month). I'll check the main 30 amp fuse again, but as for the second headlite relay fuse, where exactly would it be? I can't seem to find it, if I have one. Did all the 47 Plymouths have these 2nd fuses? Ed P.

A headlight relay would have been an aftermarket add-on. If your car is an unmolested original it most likely does not have a headlight relay.

[bob westphal]

My P-15 had a stock headlight relay and so did the car I stole the one I'm using now. Every Mopar of this vintage I have worked on had relay too.

[Plymouthy Adams]

I know they had a horn relay...schematic show only a headlight switch, CB, dimmer switch, front rad shell distribution terminals and the headlights. Schematics hold true through 1954 in my factory book.

[Norm's Coupe]

Ed P,

I wasn't sure if every P15 had the headlight relay. Guess Bob Westphal gave you that answer though.

As far as location, it could be mounted almost anywhere. However, mine is located on the top left side of the firewall, under the hood. I posted a picture of it some time ago on the forum. Will see if I can dig out that particular picture and post it again for you. It's just a small box with a fuse on the bottom or top (depending on brand). The one on my coupe is a Delco brand, but I do have a new spare that is a different brand, and looks different.

[Norm's Coupe]

Ed P,

Here's what the Headlight Relay looks like. As mentioned different brands may look a little different, but basically the same. This was taken before I cleaned up the engine compartment and rebuilt the engine.

[Lou Earle]

I have 12 plymouths and dodges ranging from 34 to 54 and not one has a relay- some have circut breakers on the switch and most have fuse on the switch.

I have several 6 volt relays for lights that I plan to put on my cars but none came equiped to my knowledge with them - see if you see any on the wiring diagrams in the shop manual. I have found that just because something is on a particular car does not mean it was standard equipment on htat or any other car.

Lou

[Norm's Coupe]

Have had my head under a half dozen P-15's at the yard. Do not remember any with that setup.

As mentioned, I wasn't sure if they were on all P15's. Actually, I was told that the relays came out in the 40's and were used to increase brightness of the lights on 30's and 40's cars. Also, that during the 30's & 40's they were add ons. So.........that could be why you and Lou do not have them on yours. Sometime in later models they became standard equipment.

However, the fact remains that if Ed P does have one and blew that fuse, he wouldn't have any lights, and that's why I brought the subject up.

[Don Coatney]

Dayton Engineering Laboratories Co. (DELCO for short) was associated with General Motors (Cadillac initially) beginning in 1910. If your car has a DELCO relay I would think it is aftermarket. The function of a headlight relay is to remove the high amperage load from the headlight switch. Has nothing to do with brighter headlights. Headlights can be equally as bright either using or not using a relay.

[Lou Earle]

YOU SAID

"I put in a Watson's Streetworks mechanical brake switch on my 47 Plymouth. Now, are the two wires that come out of the old hydraulic switch a positive & negative thing? When I went to connect them to the new switch, I hit one of the wires to one connection on the new switch, and it sparked. Checked the 30 amp fuse, and it was blown'

WHAT 30 AMP FUSE? THE one on the headlight switch? or where was or is it located? The 30 amp fuse attached to the headlight switch HAS NOTHING TO DO WITH THE BRAKE LIGHT SWITCH.

The brake lights work on a separate circuit and operate if the headlights are on or not. Think of it this way:

Battery to headlight/ tailight switch to headlights and tailights.But not to brake light.

Battery to break light switch to brake lights. but not thru headlight switch.

Entirely separate circuits.

So I am no wizard but the 2 problems are not related in my view. It would be nigh impossible to blow the headlight fuse by shorting the brake light wire.

Lou

[central52]

Wow, you guys are fast on the replies. I guess today's New Years' bowl games aren't that interesting. Anyhow, thanks for the information on the relay. I checked again, and, definitely, do not have one, as that one pic shows how it would look. BUT,I did check my 30 amp fuse to see if it was defective at the ends. It wasn't. So, just for fun, I matched it up with the original blown fuse. AND IT WAS A QUARTER INCH SHORT! Even tho it fit in the holder and set in properly, it didn't go high enough to make good contact with the plate above it. I then inserted a small piece of metal in the holder and that made up the difference, and, voila, I got back my electricity.

So even tho you can buy 30amp fuses in any auto parts store, they may not be right for our cars. Ed P.

[Lou Earle]

You are correct there are 2 30 amp fuse sizes- take your old one to store and you will find new ones the same size as old ones. I made that same mistake.

Lou

[Norm's Coupe]

I continue to be amazed at how people can say something can't have anything to do with something, when they are not looking at the whole picture.

Here is another picture of the headlight relay and also the way it's hooked up, ON MY CAR. ALL LIGHTS GO THROUGH THIS SYSTEM!! Blow the fuse on the relay and YOU HAVE NO LIGHTS!! The wires for the ALL lights go through the connection board to the right of the relay, then to the relay for power.

As for making the lights brighter or not. That's not my bag. It's the way they were advertised and sold in the 30's and 40's. If you have a beef with that, contact Delco, or other manufacturers.

[Plymouthy Adams]

Well..Autolite was primary to Chrysler till I think 1956 when Ford and Mopar traded about..Ford got Autolite and Mopar got PPG paint..and no offense to anyone here but I have never seen an early Plymouth with stock headlight relay and it is not covered in my repair manual in the schematics..for the Dodge might I add..a higher line..and I agree with Coatney on the Delco part number..icksnay..further the books both Plymouth and dodge makes no reference to any relay in their trouble shooting..

Now no one is going to argue the fact that these may have been installed after manufactuer..the disagreement centers around being stock...I do not think this would be an oversight of Mopar over the entire range of 15 years I have original publication for. .to me it would be inconcievable for MoPar to omit reference to a part of this significant value in either text or schematic.

I will conceed this...what was done in Canada..unsure..have no Canadian Chrysler books..not even sure if there would be a difference even there.

[Norm's Coupe]

As mentioned' date=' I wasn't sure if they were on all P15's. Actually, I was told that the relays came out in the 40's and were used to increase brightness of the lights on 30's and 40's cars. Also, that during the 30's & 40's they were add ons. So.........that could be why you and Lou do not have them on yours. Sometime in later models they became standard equipment.

However, the fact remains that if Ed P does have one and blew that fuse, he wouldn't have any lights, and that's why I brought the subject up.[/quote']

Tim, I believe I agreed on the stock or non stock question in the quoted post earlier.

However, that's not what this thread started out to be about. It was about why Ed P lost his lights installing his brake light switch. Had nothing to do with whether or not the relay was stock or non stock. Just that if there, it could be why he lost the lights.

This seems to happen on a lot of threads. Someone ask a simple question, then someone will post a simple answer. Then before we know it we have a thread with 20 to 50 post on a theme that is usually, "that can't be, that can be". In the mean time the poor person who ask the simple question has to read through all the arguments on stock or non stock, etc., and how or how it doesn't work. In some cases, that will just serve to confuse the person asking the original question. He probably doesn't care if it's stock, or in the book if he gets the answer to fix the problem.

I think that the questions about the item not working, or working, stock or not should be started in a different thread. That way the person with the original question gets just the answers that pertains to the fix, or possible fix to his problem.

I am not pointing fingers at anyone for doing this. I've done it myself. But...we should change that and make a different thread to discuss such things.

[Plymouthy Adams]

Norm..did not want to pinch the toes of anyone ..but the simplist task is to address the stock condition..then if it should be found that the system has been modified then we approach the task of resolving the problem with the gone awry mod..and just for the record, I addressed the issue of the relay only after the comment that Every Mopar of this vintage I have worked on had relay too...this is reason enough to address the subject, very misleading. I researched the books and schemtaics and reported what I knew to be a fact before opening the book..they are not discussed as being stock in either text or schematics. As we have a number of new owners finding their way here, the forum should have/display specs and facts supported by technical information. In this instance I point the finger at me..this approach tends to take guess works out and be less confusing. I work with schematics and tech orders daily, they exist for a reason..I would hate very much for someone to go looking for an item on their car that does not exist.