What would cause a Powermaster fuse to blow?

[WakkoSS]

I was driving home when my BRAKE light popped on and the pedal went rock hard. Fortunately there was zero traffic and I was 1/2 mile away, so I was able to nurse it home with the pedal and E brake. Sure enough the fuse was bad. I put in a 25 amp I had and now the brakes work. I'll get a 30 back in there when I go to the auto store.

Car has 44k, so I was surprised when I had trouble with it. I had the hood up when I Was working on the cooling system and it rained, so perhaps water had a factor, but the motor didn't get drenched, just sprinkled.

Is this a sign my PM might be crappin out? I'd rather not find out on the highway at 80 mph.

 

[dsa87gn]

sounds like signs of a bad PM motor

 

[Gary Wells]

This is definitely not the "voice of experience" speaking here, but I had a powermaster go out a few years back and my mechanic said that one of the wheel cylinders had been leaking slowly and that was probably making the pm motor work overtime. He put a new fuse in it, telling me that we would try a new fuse first , but he believed that the motor was shot. He said that the fuse might last a minute, an hour , a day, , well, you get the idea. It did go out about a week later. We refused it, with the fuse going out immediately. Truth was I did have a small leak, and had been having to top off the brake fluid resevoir probably more than normal. I sold the car later on down the road, advertising the fact that the pm was out. I have also heard of pm's blowing fuses as an isolated incident/event, not happeing again as a normal occurance. That one would not be my luck, probably.
HTH

 

[Turbo6inKY]

Originally posted by WakkoSS

Car has 44k, so I was surprised when I had trouble with it. I had the hood up when I Was working on the cooling system and it rained, so perhaps water had a factor, but the motor didn't get drenched, just sprinkled.

My car had 16K on it when I bought it. I had to replace the pressure switch and the accumulator ball before it hit 20K.

Never underestimate the ability of a PowerMaster to fail unexpectedly.

 

[WakkoSS]

Ok, I took the car out tonight and when I returned my brake lights remained on. Would a sticking switch cause the motor to continuously run, thereby frying the fuse?

 

[Turbo6inKY]

Originally posted by WakkoSS
Ok, I took the car out tonight and when I returned my brake lights remained on. Would a sticking switch cause the motor to continuously run, thereby frying the fuse?

It is possible.

 

[turbot2112]

there is a way to test the pressure switch with a continuity tester. just cant remember how. id bet on the pressure swich though. i had that happen 2 times before i switched to vacuum.

 

[blackbird]

Powercrapster

Switch to vaccuum

 

[TylerDurden]

Ian, I have had the same prob for the past 5 month, it blows a fuse about every 100 miles or so...all depending on how hard im driving and braking, I have basically ignored it due to the lack of me driving it (awaiting trans). But I have already replaced the ball once, and I dont plan on spending another dime on the PM. Time to hit the junk yards....btw my car as 132k on it.

 

[mark b]

For old gnttype members ... I'm reposting a post from members of that site. Hank Terry sells new motors if you need them. The article also references the switch which can be a source of a leak.

******************************************
he POWERMASTER, a very misunderstood unit. A little
knowledge can save you a lot of money. I have over 95000
miles and 16 years on the assembly line unit and it still
works fine. I am on my third accumulator and had the recall
done on the replacement of the black switch to gray.

Most common mistakes.

The red dash "brake" lights comes on when the main brakes
are applied. This is the time to park the car NOW until the
problem is fixed. The pump motor cannot run for more than 30
seconds without a long rest to cool down. Under normal
conditions it only runs for 2 to 3 seconds on a cycle. The
dash light indicates low pressure and the pump is working
too hard. Driving in this condition will burn the pump motor
up quick. The most common failure here is the accumulator.
They loose their charge over the years and won't last
forever. My experience has been four to five years before
they give out. There is an easy test for them. With the key
off pump the pedal 20 to 30 times until it gets real hard.
Using a watch with a second hand time the motor when you
turn the key on. It should not run for more than 15 seconds.
7 to 10 seconds is desired. It helps to have things real
quiet so close the door, radio off, seat belt buckled and
don't start engine. If the run time is 15 seconds or more
you are loosing the charge in the accumulator or have an
internal by-pass in the cylinder. I perform this test every
two months on my cars. 90% of the time it's the accumulator.

Motor over-worked during pump bleeding of brakes. The system
should be pressure bleed or vacuum bleed. Never pump bleed
with the key on. This will burn the motor up. If you must
pump bleed do it with the key off.

There is a very nice factory service bulletin, 87513B that
will take you thru the problem diagnostics. It's easy to
understand and use. Pump and motors don't fail because they
are worn out, they fail because they are over worked for
some other reason.

18011279 old motor and pump number
18038503 new motor and pump number both include the relay
18038503 switch
18038507 accumulator

 

[Boston GN]

Same thing just happened to me. Drive for the first time in 2.5 years. All was great until I tried to stop one time and the pedal was rock hard. Still able to stop, but a PITA. Parked the car at my destination then came out a few hours later. Brakes worked again. Hmmm. Ten minutes later back to a rock hard pedal, no brake light!
I have no idea what is wrong, just a story. From all my searches it seems that a hard pedal is caused by the accumulator ball.
Anyone have a clue why it would work after sitting for a few and then revert back to a hard pedal? I haven't checked fuses or anything yet.

 

[gofstbuick]

Key on engine off, preferably in a quiet area. Does the PM motor hum low? If so, the then there is a pressure problem, causing the PM motor to run constant. The fuse will blow once the PM motor gets to hot (from running more than 20 seconds). If you hear a low hum from the PM motor, get you PM system repaired as soon as possible. I just rebuilt my master cylinder and found the PM check valve was leaking. Once the rebuilt was completed, the low humming of the PM motor stopped. GMs diagnostic tree is very specific, but you'll need brake pressure gauges to pinpoint the problem. You'll also need to build an adapter to hook the brake gauge between the pressure switch and the master cylinder (with all electrical still hooked up).

 

[gofstbuick]

I posted this in another post, but here are the instuctions to set up a brake guage with adapter to test your PM system.
"The adapter is a 1/4 NPT tee. It will have 2 female ports and one male port. Change female port on the top to M12x1.5 thread pitch. DONOT use a helicoil, use a time-sert, keen-sert or thread insert. The helicoil will leak. Order a hydraulic adapter to change the male port (directly oposite of the female port with the thread insert) from 1/4 NPT to male M12x1.5. Leave the other female port alone, this is the port used to attach the brake gauge. This adapter will allow you to remove the pressure switch, install the adapter, install the pressure switch on the adapter and connect a set of brakes gauges to the adapter. You can then hook the electrical connector back up to the pressure switch and check accumulator pre-charge pressure and pressure switch on time (using the specifications from the GM service manual). By the way, if your feeling really rich, you could order the factory GM tool from Kent-More ($479). The tool I'm using cost approximately $130. Ordered a set of brake gauges from Harbor Freight tools for $93, adapters cost $8 (local hydraulic shop) and machine work was $20."

 

[WakkoSS]

Wow, that's some good information. Thanks. Are the accumulator balls still available? I seem to remember thinking they're not.

I LIKE the Powermonster and don't want to convert to vacuum if I can help it.

 

[gofstbuick]

The accumulator ball is still available and also a less expensive unit is also available. But if you remove the accumulator ball and rest a pen screwdriver through the hole (Don't push on it!!!) and it only drops half way, chances are the accumulator is not the problem. By the way, what color is the brake fluid? Black and gross looking? That black material gets in all kinds of crevices and could cause the check valve to stick. When I took my master cylinder apart, that black goo was everywhere and on everything. The mastercylinder bore is anodized, so if you rebuild it and you see pits in the cylinder bore, then it is time to replace the master cylinder. Because it is anodized, it cannot be honed.

 

[IrvJr]

I like my PM too. My prob is always the accum ball. So i just bought the PM from Autozone for $144 and used theirs. My PM is original with 150k.

Scott

 

[WakkoSS]

Ok, I blew ANOTHER fuse today. I pumped the brake 30 times and it took about 17 seconds for the motor to stop, so that's a problem. Autozone for $144? They wanted $375 when I went in there last night.

Brake fluid is gross...all dark and nasty. Should I just replace the whole thing or just the accumulator? Naturally I'd rather only replace what I have to, considering my cooling system just took a crap and my new tires and wheels rub a little, so I need airbags.

 

[IrvJr]

Sounds like accum ball to me. You can find them used, but people want gold for them. I cant find my box for my PM to give you the part number, but someone must have it.

Scott

 

[gofstbuick]

Originally posted by WakkoSS
Ok, I blew ANOTHER fuse today. I pumped the brake 30 times and it took about 17 seconds for the motor to stop, so that's a problem. Autozone for $144? They wanted $375 when I went in there last night.

Brake fluid is gross...all dark and nasty. Should I just replace the whole thing or just the accumulator? Naturally I'd rather only replace what I have to, considering my cooling system just took a crap and my new tires and wheels rub a little, so I need airbags.

I know lots of members have had good luck with remanned PM, but IMHO, a remanned unit is just a bunch of used parts thown together from different PM units to make one good one. The master cylinder rebuild kit is still available from GM and if you install a new accumulator, you'll have piece of mind, because you worked on your own unit. That black crud can be cleaned out during the rebuild. There was a bunch of that crud on all the pistons and in the bore, (but my 157000 mile GN) and the master cylinder bores still looked good. IMHO, I would not buy a remanned unit unless I saw pits in the bore.