Well Dang!

It has worked in the past.

Have you tried calling MSD??

 

yeah and this was the bad news........talked to MSD tech and he said yes that what I have is a very small amount of current leaking to ground, he has seen it before in the "analog" switch, i asked him what i could do to fix it and he said that the MSD "digital switch " will fix the problem.....turns out i have a small amount of current leaking to ground and its causing the chatter because the relay is so sensitive....they will swap units if i want and i can buy the new more costly unit it and install it, all the connections are the same....i don't want to go down that path yet.....

i wonder if i could have a faulty relay?

 

Sorry it took so long, I was trying to find more info (no luck) on the MSD switch. Judging from the fact that the tech you spoke with reffered to an "analog" & "digital" switch, I'm guessing what you have is actually some kind of transistor circuit doing the switching as oppossed to actual contacts (if anyone has info to the contrary - please let me know).

Based on this assumption, I would try wiring it per the drawing below. Diodes D1 & D2 can substituted with the 1 amp diodes you already have. They aren't neccessary for the circuit to work - they just clamp the voltage spike when the voltage is removed from the relay coil.

Relays have a rated minimum "pull -in" voltage, however they also have a rated voltage to stay "on". I think your MSD switch may have some resistance in it's switching circuit with a path to ground which lets the relay coil turn on / but not enough to "hold" / which makes it buzz (fast switching on/off)

My schematic includes a 100K ohm (1/4 watt) resistor (R1) to "pull up" the switching circuit when it's in the "off" state. This effectively puts +12V on both relay coil terminals keeping it off because the voltage drop across the relay coil is zero volts. When the MSD switches, it grounds the relay coil & resistor which turns the relay on. The current through the resistor at this point will be minimal ~ 120uA, so it's not a concern.

I can't post the pic here because my Photobucket bandwidth limit has been exceeded for this month due to all the HOT ROD Power Tour pics I posted recently but you should be able to use the link below to go there directly - click on the thumbnail and get a large view.

Again, this is based on a guess as to the MSD box, so if it still doesn't work right, let me know & I'll dig
for some more info on it.

link to schematic

Hope this helps, let me know how it works out.

 

thanks i got the resistors on the way home from work and will try it tonight....thanks again, bob


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ok well tonight i brought new clean electrical power to the relay and to the MSD switch... i made the resistor circuit Todd68 thought of and tested it ...i did not put the diode in ....and i'm still getting the same buzzing....with the resistor in place it now is up to 43m amps to make the relay buzz....i am at my witts end....i even tried measuring the resistance through the MSD switch to the chassis ground and i was able to measure resistance but when i moved the dial switch through the different ohm setting 10k, 100k etc the needle stayed where it was? any ideas? anything i can measure? its getting personal now and i'm digging in.....thanks for the help

 

Have you tried reversing the power to bring the relay in as you suggested in one of your earlier posts. This is the preferred method. Switch the hot instead of the ground. There should not be any voltage on the relay at that point. Maybe check with MSD before trying this.

 

Does it buzz with key on engine off? Wiped out diode in alternator (generator) allowing ac voltage into circuit.

 

only happens when the engine is running and their is a tach signal...no tach signal no buzzing...


i don't see how i can do this, the MSD switch only controls a ground circuit.

 

I dont know the style of relay you have but if it is a cube relay that you can get the cover off of you can put in a slightly stonger spring that will overcome more magnetic force.

 

its a sealed cube

 

Bob, the more I think about your problem the more I think your msd switch is not right. If a switch made for automotive use can not completely open enough to turn off a relay what use is it? the msd rep wants you to buy a digital switch when there is no reason an analog switch should not operate a relay. If a switch cant cut off 12volts enough to open a relay its a bad switch.
Greg.

 

I would have to agree at this point. Have you tried just swapping relays? If the other relay doesn't buzz, then the first relay was bad. If it does the same thing when connected to the MSD then I would say it's the MSD unit. I'll see if I can find a schematic for the internals of the MSD switch tomorrow. If I can find that, diagnosing would be a lot easier...

Edit: I just found this quoted on another website:

"We discovered that the MSD rpm switch was not 'snap action'. It shuts off in a 'pulse width modulated' fashion over a range of a couple of hundred rpm. This would cause the solenoids to 'chatter' on and off in that rpm range."

They were trying to control fuel flow with a solenoid & had what sound like the same result you had with the relay. I'll keep trying to get the schematic & help with a solution, but it's getting late here now, so I'm done for today!

 

can't argue with that logic....i have cleaned up all the wiring so there is clean power going everywhere....and when i disconnect the ground connection the relay stops buzzing...



ok i spoke with a different MSD tech and he said i should have no "ground circuit leakage" and he thinks its a bad switch, i will pick a new switch up on the way home from work, and a new relay from an auto parts store..where do i find those relays that require full voltage to pull them in? do they have a specific name? i can't get that expensive MSD relay until tomorrow and i'm wondering if that is set up to work with MSD differently then regular relays?


and i will test my new MSD switch before i put it in using this methodology......hook it up to ground, +12v, connect to Tach signal....(yellow wire is switched open normally, and closed at selected RPM this is the wire that goes to the new relay) so when i test it, it should have an "infinite amount of resistance" between the yellow wire and MSD ground at under the prescribed 1300 RPM indicating an open ground circuit....at the prescribed RPM the resistance should go down to zero indicating a completed ground circuit. what do you think?

heres my PLAN B if my new fancy MSD RPM controlled switch and relay doesn't work i may just use a SPST switch and hook up the SPST switch to the relay ground circuit and turn it on and off by hand as i enter/leave the highway it would also save me $$$ from my fancy RPM hook up....
thanks for al the help and if theres any more ideas i would like to hear them, i kind of enjoy the chase.....i guess i really am touched....thks bob

 

do you suppose a SPDT relay is less inclined to chatter then a SPST relay?

 

do you suppose a SPDT relay is less inclined to chatter then a SPST relay?
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Shouldnt make any difference. They are the same coil but have 2 sets of contacts instead of one. Your problem isnt the relay, its the switch.
Greg.

 

When I design circuits using 555 timer chips to drive relays, I often have a similar problem. The solution that has worked for me is to put an electrolytic capacitor in parallel with the relay coil. The value of the capacitor (microfarads) varies with the impedience of the relay, but use a capacitor rated for at least 25 volts and observe proper polarity (or it will explode). May be worth a try.

 

well first off thanks to everyone for trying to help me out....the good news is i just saved $92......tonight first thing....switched relay...no change in results....second thing : temporarily installed second MSD RPM switch, checked resistance to ground on the two switch contolled wires....both showed considerable continuity to ground at the same time, interesting since one is suppose to be open and the other is suppose to be closed????? so third test: hooked up a test light to each switch contolled wire to ground....both tests glowed BRIGHT.....now i'm not the brightest bulb on the tree but the new RPM switch showed no signs of being able to function properly, so it didn't get installed, so i then ripped the old MSD RPM switch out and installed a regular SPST switch to control the relay and i can now controll lockup by the switch, which i mounted in the bracket next to the headlight override switch (there still is the brake light relay in place that will overide the new manually switched relay)...well not exactly the results i wanted but thanks for all the help....i can't believe i'm the only guy running into this with the MSD RPM activated switches....so bottom line i saved $92 and spent lots of hours trying to make this work, i am still interested in getting the relay to work off of the Tach RPM signal so keep me in mind if you have another way of doing this. oh i almost forgot the last test....opened refrigerator and cracked a beer... thks Bob

 

Bob, you've got mail (PM)