My Dodge Cummins truck has an exhaust brake and it uses a signal from the ECM to activate the vacuum relay to engage the brake. I would like to make it a little smarter by having it automatically disengage below 15 MPH. I can get the signal from the VSS (Vehicle Speed Sensor) that is used for the ABS system. I can then process this signal to give me an enable or disable signal in relation to vehicle speed. I'm thinking of using a 555 and using the VSS signal to reset the timer if it's fast enough. Any ideas on other ways I might accomplish this?

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What type of signal do you get from the VSS - voltage based, or frequency based (or something else, maybe??). How does the signal change in relation to vehicle speed?

 

It's frequency based. The pulse train increases in frequency in relation to the speed of the vehicle.

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Well... I have no idea how to use 555s , but I know a great deal about the system you are using. (Vehicle Speed Signal)

Its a 5V signal that is connected to a hall sensor on you ABS tone ring. When a tooth goes by the sensor your signal wire is driven low (body ground).

I'm doing somthing similar that uses the vss, but I do all my work on PICs.

My guess is that the hardest part will be getting your timer to know when X number of grounds per second is 15mph. For me, (using a pic) I just set an interupt to count lows and then every second I check an see how many I have... Multiplied by the number of teeth I have, divded by my gear ratio, etc etc I know how fast I'm moving.

I cant be much help with a just hardware approach. I'm dumb and can only figure out processors (go figure).

But it sound very possible.

 

I wish I had paid more attention in the Microchip seminars! ops:

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