My thoughts on the Nason Parking Brake Switch

[nodine]

We have only had our coach six and half years and have replaced the Nason parking brake pressure switch once and it looks like it is going bad again. The part number on the switch is SM-1C-66R/WP28. According to the Nason catalog the part number breaks down like this:

SM - low pressure switch SM
1C - 1/4" NPT Male SPDT (single pole, double throw)
66 - Fixed pressure set point 66 PSI
R - Rising pressure switch
WP28 - Weather Pack connector

The specifications indicate this switch should have a cycle life of 1,000,000 cycles. If I take a very conservative guess of 20 cycles of the parking brake in a driving day I should be able to drive my motorhome everyday for 137 years before I have cycled that switch 1 million times. Something stinks here. I have the old switch and decided to do a test. I attached an air chuck nipple to the pressure switch and ran my air compressor up to 100 lbs. This switch had three connections, A,B, and C, with A (black wire) being common, B (Red wire) normally open, and C (Blue wire) normally closed. I connected the VOM set on the 200 ohm range between A and C and as expected got continuity of about 0.5 ohms. Seems a little high but never the less useable. When I plugged into the air hose the contacts at A and C opened as you would expect but there was not any continuity between A and B. I cycled the switch several times and never achieved continuity between A and B but A and C always recovered when the air pressure was removed. This tells there is a high probability that the contacts for A to B are burned.

Looking at the chassis schematic I see that the B contact connects to one side of the relay coil for Park Bell 1 and the other side of the coil is connected to 12 volts. The coil of a relay is an inductor and inductors do not like for the current flowing through them to change. When the contacts energizing the coil are opened the inductor will increase the voltage across the contact in an attempt to keep the current flowing. This usually results in an arc that can burn the contact. It is my opinion that there should be a diode across the coil of the parking bell relay to collapse the field when the contacts open. There is no reason this switch should fail so early in its life. The contacts are rated at 5 amps at 30 VDC. I know I am not the only one who has had a problem with switch.

If we order a new switch I will either use a diode across the coil or wire in a solid state device between the Nason pressure switch and the Park Bell relay.






Bob

[danes-on-tour]

Bob - thank you for this update. I didn't 100% understand it all, but nevertheless. Some of it did make sense to me.

Have chased a replacement switch for few days, it turns out that this is a stock item with Monaco Parts. Monaco part number is 16614841. A bit more expensive than most other places at around $46 plus shipping, but beggars can't be choosers. I and happy not to have to wait the 4-5 weeks everybody else quoted me.

[Dutch Star Don]

Bob....I'm with the Danes....I was good right up to the word inductor before my propeller quit spinning ....I would have thought cheap switch until you said 1 million cycles. You have just created a nice side business....a diode wired with spade terminals that you could package and sell as plug and play.

The week before we sold our coach, the switch went bad again. This was my second replacement. I didn't want the new owner to have a mysterious issue.

[Camelot Camper]

Having just replaced my Nason switch of the same part number, I also measured the resistance between A and C as described above by nodine. I first choose to use my older RCA VTVM in the calibrated R x 1 scale and then with my Fluke 75 digital meter. In both cases the resistance between A and C measured zero or less than 0.1 ohms.


While I haven’t yet performed an autopsy on the switch, it’s important to understand that the switch contact problem was when the air pressure was removed from the switch (handle pulled up) and NEVER when the air was applied (pushed down to release the brakes). Moving the internal switch contacts in two different directions (brake on and brake off) is normally done with a spring (brake on – no air) and air pressure (brake off) applied against a small diaphragm. My experience is that the first failure is always the spring that wears out and loses its strength after so many cycles. In my case, if the parking brake light didn’t come on, I just recycled the handle until it did. If and when I perform the autopsy, I’ll post the photos and results.

[danes-on-tour]

Originally Posted by Camelot Camper

Having just replaced my Nason switch of the same part number, I also measured the resistance between A and C as described above by nodine. I first choose to use my older RCA VTVM in the calibrated R x 1 scale and then with my Fluke 75 digital meter. In both cases the resistance between A and C measured zero or less than 0.1 ohms.


While I haven’t yet performed an autopsy on the switch, it’s important to understand that the switch contact problem was when the air pressure was removed from the switch (handle pulled up) and NEVER when the air was applied (pushed down to release the brakes). Moving the internal switch contacts in two different directions (brake on and brake off) is normally done with a spring (brake on – no air) and air pressure (brake off) applied against a small diaphragm. My experience is that the first failure is always the spring that wears out and loses its strength after so many cycles. In my case, if the parking brake light didn’t come on, I just recycled the handle until it did. If and when I perform the autopsy, I’ll post the photos and results.

I feel compelled to translate the above for the benefit of the non technical readers, so here goes:

The switch is a cheap piece of ****, and it starts breaking the minute you start using it.

I hope this clarifies matters

[William Willard]

Henrik -- Thanks, your reply works for me -- William Willard

[Dutch Star Don]

Has anyone ever called Nason to complain and see if they'll pony up a replacement. Someone should at least try.

[nodine]

Originally Posted by Camelot Camper

.......
While I haven’t yet performed an autopsy on the switch, it’s important to understand that the switch contact problem was when the air pressure was removed from the switch (handle pulled up) and NEVER when the air was applied (pushed down to release the brakes). ......

We have always experienced the problem when the parking brake is released (handle pushed down). Pressure applied to release the parking brake and should open A and C and close A and B. We get a warning bell indicating the transmission is in gear at the same time the parking brake is applied but the brake is released. Sometimes we drive for miles before the alarm stops.

Has anyone else experienced the alarm while the parking brake is released and the transmission is in gear?

Bob

[Gorlininc]

My parking brake alarm went off when driving yesterday. It lasted about 5 seconds and shut off. It did this last year around the same time of year. When the weather us still cold. I know the brake was off so it didn't bother me too much.
How difficult is it to change that switch
I have a 2001 diplomat 38A.
Thanks
Rick

[speerfamily]

Nodine - yes, I did,,I just waited 6 weeks for a replacment and just installed it. Before it could be a couple miles before the stupid ding would stop. Cost about 40 bucks...

[danes-on-tour]

Originally Posted by Gorlininc

How difficult is it to change that switch

That would be one of the easiest jobs you can think of.

BTW- the switch is a stock item with Monaco Parts. I just ordered one yesterday. $46 plus shipping.

[Dutch Star Don]

Mine only alarmed after I released the brake and put it into gear. Sometimes lasting a few blocks. I found that if it didn't stop when the brake was released, I would reapply the brake and then slam it down hard. This usually made it stop alarming.

[TEDDE]

I changed my brake switch last year 2013 and the switch releases and engages just fine. My new problem is the indicator light on the dash never goes out. Anyone have any ideas....I thought it was the switch but I installed a new one but after reading your comments perhaps the switch is faulty.....Ted

[nodine]

Originally Posted by TEDDE

I changed my brake switch last year 2013 and the switch releases and engages just fine. My new problem is the indicator light on the dash never goes out. Anyone have any ideas....I thought it was the switch but I installed a new one but after reading your comments perhaps the switch is faulty.....Ted

Do you also have an alarm when the transmission is placed in gear or is your problem just the Parking Brake Lamp on the dash stays on?

Bob