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Motor Head
09-20-2003, 07:05 PM
I have an MSD 6A ignition on the car which is a VW GTI. I recently added an rpm activated switch to drive a relay. The desired relay operation is 1500 rpm and above so I am using the adjustable rpm module which the instructions indicated is treated like a "white" rpm module.

The the relay is a 12V 30A single pole. I hooked it up as follows:
White = MSD tach output
Red = switched 12V source
Black = ground
Yellow = terminal 85 on relay (+12V on terminal 86)
Gray = not used

The problem I am having is that the relay chatters below the threshold rpm at about a 58% duty cycle as measured with a meter. Above the threshold rpm the relay does actuate as advertised. I took the following measurements to try and understand what is going on.

- terminal 86 on relay = 13.5 V
- Red wire = 13.5 V
- Black wire = 1.0 ohm to neg battery terminal
- Yellow wire to black wire below threshold rpm = 700K Ohm
- Yellow wire to black wire above threshold rpm = 0 Ohm
- resistance starts a 700K Ohms and reduces to 0 Ohms as threshold rpm is crossed

Before I go any farther I would like to understand more specifically how the rpm switch works and if it is functioning correctly based on the information I have provided. Here are some questions:

Why is the normally open circuit (yellow wire) not "open" but rather high impedance below the threshold rpm

Are solid state relays used in the rpm switch?

How does the relay circuitry function?

How many relays can I drive with one rpm switch (I would like to drive 2)?

Thanks for your help. I love the 6A, my car runs much better with it.

Motor Head

msdtech2
09-23-2003, 04:54 PM
THE RAS/8950 IS BUILD TO PREVENT CHATTER FOR 3000RPM AND HIGHER AND THAT IS ITS RATING. BY USING THE ADJUSTABLE RPM-MODULE
MADE FOR REV-LIMITER YOU CAN EXTEND THE RANGE OF A LIMITER BUT NOT OF THE RAS/8950, AS YOU HAVE NOTICED.

Why is the normally open circuit (yellow wire) not "open" but rather high impedance below the threshold rpm
THE NORMALLY OPEN WIRE IS CONNECTED THRU A POWER FET TO GROUND.

Are solid state relays used in the rpm switch?
NO, SEE ABOVE

How does the relay circuitry function?
IT IS NOT A RELAY, BUT A FET THAT IS CONNECTED TO A CIRCUIT THAT SENSES RPM, AND THE CIRCUIT TURNS THE FET ON

How many relays can I drive with one rpm switch (I would like to drive 2)?
UP TO THE MAX RATING OF THE CURRENT WHICH IS 1.5 AMPS, 2 RELAYS WILL NOT POSE A PROBLEM. LOOK AT THE DATA SHEET (SPECS) FOR THE RELAY YOU ARE USING.

Motor Head
09-24-2003, 10:09 PM
MSDTECH 2,

Thanks for the reply. Any recommendations on how I can prevent the relay from chattering?

Motorhead

msdtech1
09-24-2003, 10:53 PM
An RPM Activated switch can be modified to function at the rpm that you are trying to use it at. You will need to send it in for modification. Private message me and I will issue you an RGA number so I can track your unit as it comes in for modification. You will need to provide us with detailed information on the rpm point where you want the device to activate at and details on the relay/device that you are using.

MSD Tech1