I have a GM vacuum pump on my car that burnt the contacts in a 40a relay, I am using a SSB vacuum switch to activate the 30/40 relay.
The relay didn't last 2500 miles before going bad?
I is a 5 post relay made in USA, the expensive one with the weather pack housing.
Do I just need to go to a larger amp relay or ya think the one I had was junk?
I hate to go to large for fear of burning the wires, I am using 12 gauge wire.
The vacuum pump is the GM made for continuous run.(like new)
Any input or ideas are appreciated.
T.C.

Sure sounds like you need a higher amprege rating relay.

Using a larger relay will not cause the wires to burn.

Put a snubber on the relay or across the load.

Some relays are designated continous duty or intermittent duty

There are a couple of parts inside a relay. Do you know what failed ?. If it is the coil then the windings may not be able to withstand continous use (like some of the Ford starter solenoids).
If the contacts have arced and welded together then excessive amperage was being passed more that what it could handle (even though a part is "rated" at a certain value who's standards if any do they use ?).
If the return arm or spring for the contact points has broken it could be from a bad metal or something else.

I would disect it and see what has failed.

Jim

Thanks for all the replies,
Jim that's exactly what I did "took it apart" what I found was the point set was burnt and there was no continuity from the points to the pins on the motor side.
The relay coil was still working and nothing was burnt, melted or smelled like burnt?
The points were black though?
Do you guys know where I can find a higher rated relay, radioshack doesn't have them, 40A was all I could find at NAPA.
Although the motor is meant to be continuous run in the GM application, the way I am using it is not.
The motor will build vacuum and only run when needed, (on @ 15 off @ 22) so if I am on the road it's not running all the time.
Whats a snubber?
Thanks T.C.

Whats a snubber?
Thanks T.C.

http://en.wikipedia.org/wiki/Snubber

Thanks for all the replies,
Jim that's exactly what I did "took it apart" what I found was the point set was burnt and there was no continuity from the points to the pins on the motor side.
Thanks T.C.

Guess I need to read before I speak :) . I even read it the first time but it didn't sink in.

I wonder if even though the pump is not burning the wires they are handling a slightly higher than normal current demand (amperage) when the motor first starts up but the relay contacts cannot. Add into this the motor starting and stopping a lot and it might be stressing the relay contact points. I know I've read that when wiring up some of the Taurus fans you have to run fusible links on them instead of fuses due to the start up current demands.

Jim

Thanks again for the reply Jim:
I am thinking it's the load on the other end of the duty cycle when the pump is at it's peak vacuum?
That's when the motor labors the most, when the vacuum is high just before the switch turns it off.
Could it be a low voltage situation?
Like if the dual fans,A/C,lites,and vac. pump are all running at the same time?
105A alt. setup to MAD three wire specs.,Optima red top batt.
T.C.

Did the snubber reduce the arcing of the contacts when the relay drops out?

What is doing you in is the difference between startup current and operating current. These two currents can vary widely for electric motor circuits. For example my electric cooling fans pull close to 70A at startup and operate at 15A. What causes this is the resistance to current flow of the motor changes as it changes RPMs. As the motor turns faster it actually works as a generator and generates voltage with a polarity reverse to the supply. This impedes current flow until there is a natural balance between current and mechanical load.

By running your vacuum pump in a on/off cycling mode you spend a lot more time in that startup very high current mode. If there is additional mechanical load, the startup will be lengthened and you will see an even longer high current spike. If GM designed the pump to run continuously, it is a good bet that they did not optimize it around low startup current or startup with a load. It could well be that, even if you upgraded the relay with a perfect relay (does not exist), running in this mode will shorten the life expectancy of the vacuum pump motor. The high startup current may build heat up in the motor windings causing them to fail early.

I would recommend running the pump in continuous mode the way GM intended and designed for.

A continuously running vacuum pump? Most have an internal pressure switch.

Thanks again for the replies:
Oneofakind67 I haven't a clue where to buy a "snubber" or how to install it if I could find one?
Maybe ya can help me out ?
vrooom3340 I will have to check the motor for heat next time I drive the car?
I really don't want continuous run cause it's to noisy and is more drain on the battery when cruising.
I am gonna replace the relay and see how long it last, heck it could have been as simple as a poor ground, the next one may last for years?
T.C.

Poor ground would have made the relay last longer.

Here is something to do to get a bit more information here: disconnect the pump motor and put an ohm meter across it to measure the resistance of the windings. Then calculate the startup current by dividing 14 by the number you measured.

It could also be good to measure the running current.

You may be able to reduce the startup current with a suitable inline current limit resistor. This is basically what I now have on my electric fans. The fans by themselves measure 0.2 ohms, thus the 70A startup current. While running the impedance is closer to 0.9 ohms or 4X higher. With a large ceramic resistor of about 0.2 ohms the startup current is reduced significantly to 35A and the fan speed lowered slightly. Overall operating current remains the same at 15A.

Note that running continously is not a drain on the battery at all. The alternator output will be increased to handle the extra load. So maybe it will take 1-2 hp extra to drive the alternator.

If it seems noisy... then perhaps some sound management is in order. Like louder mufflers :-)

http://www.fordmuscle.com/archives/2003/02/electricfan/schematic.gif

TP,

The diode around the fan motor is a snubber. You can get a 1N5408 diode at Radio Shack.

http://www.americanmicrosemi.com/tutorials/images/1a.jpg

I am not sure a diode in this configuration will help on a vacuum pump. It can help a lot on a fan because of the momentum of the fan keeping the motor turning. This generates more reverse voltage potential. I doubt that a vacuum pump runs/coasts very long after power shutdown.

The other reason for that diode on the fan application is to cause the fan to stop spinning much sooner. You could accomplish the same affect by swapping around some of the relay connections:

fan positive lead to 30
fan negative lead to 87A as well as ground
Power supply in to 87

This will connect the fan power input to either ground or to +12V. When it connects to ground it also connects to the fan ground side making a complete circuit. The reverse voltage generated by a spinning fan will cause a current trying to run the fan the other direction. The motor will stop spinning in about 1/3 the time it takes in the open circuit configuration.

Just a little something I learned many years ago messing around with HO slot cars :-)

The purpose of the diode is to absorb the inductive kick from the fan, keeping the relay contacts from arcing over. Whether the fan stops in 1 second or 10 seconds makes very little difference. It's the same with the vacuum pump motor, making and breaking the relay causes inductive kickback from the motor. Here's some info on maximizing the life span of relays:

Motor loads — When an electric motor starts up, it has very low impedance
and requires a large in-rush current to begin building a magnetic field and
begin rotating. Once it is running, it generates a back electromagnetic
force (emf), which can cause a large inductive spike when the switch is
opened. The result is a large in-rush current at “turn-on” and arcing at
“turn-off.” When you are switching a motor load, typical industry practice
is to derate to 20 percent of the resistive rating.....
You also can place clamps, a diode, a zener diode, a varistor, or a resistor/
capacitor (RC) network in parallel with the load as a snubber or suppression
circuit. In the next section, we’ll take a closer look at RC networks and
varistors (Figure 1).


http://cp.literature.agilent.com/litweb/pdf/5988-6917EN.pdf

Heat will also kill them. I know before I put a heat shield on mine, they would get hot and stop working untill I would let them cool down for half an hour. Mine where located close to an exhaust pipe.

Once again thanks for all the replies
Thanks for the drawing onovakind67, the diode looks simple enough to install.
I also have twin Deralle fans that have relays in the adj. temp.controllers, if this works as you say should I put the diodes on the fan motors also?
Wonder why manufactures don't suggest putting these diodes on any part using a motor and a relay?
Sounds like cheep insurance?
Scotty I have these mounted on the front part of the fender well so heat isn't a problem.
T.C.

Clamping diodes are used on most autos today, mostly on solenoid drives like transmission controls and injector drives. Inductive spikes in the system raise havoc with computers. You can buy Ford starter relays with a built-in diode. Aftermarket fan controllers often have diodes built into the controller.

Thank you all for the help.
T.C.