need help designing a square wave modifier circuit

[jclaudii]

Here is what I want to do. I want to take the input of a square wave and change it a little bit and then output it. The application is to help my vehicle "sense" the correct speed so it can shift correctly. This is on a Jeep Grand Cherokee, it uses a 3 wire Hall sensor that generates a square wave dc signal. This signal is generated by the ABS circuit and is broadcasted on the vehicl's bus system. Now from studying this thing forever, it seems the signal first goes to the ABS ECM first so it can do it's thing independently of the ECM. The abs computer has an output, and that is where I will tap into it before it goes to the vehicles ECM.

Someone mentioned using 555 timers to do this and just adjust a pot or something to get the desired offset to make it work right. It's been done, check out SpeedoDRD but for 80 bucks i can make several of them more than likely!

Thanks for the help!

 

[crutschow]

You haven't stated what this "change it a little bit" for the square-wave means. What exact characteristics of the wave do you want to change? The SpeedoDRD changes the pulse frequency so it that what you want to do? If so do you want to increase the frequency or decrease the frequency? By what percent?

 

[Mickster]

IIUC, the only reason I can see for changing Vehicle Speed Sensor pulse frequency would be due to using tyres with a different rolling circumference than that which is to be expected as standard, thus incorrectly displaying/sensing actual road speed.

Is this your current problem?

 

[jpanhalt]

Exactly. I helped a fellow hobbyist adjust the speedo in his racing VW minivan using the NJM4151 (or KA331 or similar) F --> V and then V --> F converter to get almost any proportionality he needed. There are other, probably easier ways to do it, but he wanted to do the F/v then V/F conversion.

I hope the OP will more clearly define what he wants to do. These are fun projects.

John

 

[jclaudii]

You guys have it. I have put 285/75/16 from the factories 245/70/r16 which is a little over a 1.5" difference or around 10.15% difference in the diameter of the tire. according to the below website it says my speedo may be 11.28% slower than with the factory tires. So at 60MPH speedo may show 54MPH. According to GPS, my speedo seems to be off about 5MPH instead of 6. The problem that needs to be corrected is the Powertrain Control Module (PCM) thinks the transmission should shift at certain programed speeds, instead it shifts early. This correction would be two fold for me. 1. fix the speedo to be more accurate, and 2. make my shift points back to where stock should be.

Tire Size Calculator - tire & wheel plus sizing

 

[shortbus=]

Not sure about other makes but with GM cars they can re-flash the memory in the computer to change tire diameter to control speedometer. Might want to check with dealer/service department.

 

[Mickster]

I'd second that advice also.

 

[Roff]

I would think that shift points would be based on tachometer input (among other things), not on speedometer input.

 

[Jaguarjoe]

IIRC when I put low profile summer tires on my pick up years ago, it took about 30 minutes for the dealer to reprogram the PCM to adapt to them. $45.00

 

[Mickster]

IIRC when I put low profile summer tires on my pick up years ago, it took about 30 minutes for the dealer to reprogram the PCM to adapt to them. $45.00

Probably more likely 10 mins doing the programming and 20 mins doing admin, such as a body damage report, brake & lights & levels check, writing up the job etc. (remind me again why this job pisses me off!)

But, wouldn't you class that $45 as a bargain, considering you could have otherwise spent a few hours building and tweaking a circuit to do the same thing?

A few hours which could have been spent doing paid work...

 

[Jaguarjoe]

But, wouldn't you class that $45 as a bargain, considering you could have otherwise spent a few hours building and tweaking a circuit to do the same thing?

Absolutely

Some things are not worth fooling with, especially when my wife drives the truck more than me. If the dealer would have said it couldn't be done, then that's a different story.

 

[jclaudii]

I think that would be an option...but from what i have read, aftermarket tuners like the superchips and others along with the dealer can go above the "stock options" for '99-'01 models...after that, they have the option to type in the tire ratio's and let the computer make the adjustments. Models before '99 was the ZJ body style and it used speeodometer gears to drive the hall effect device. so changing out the gears would get you back to where you wanted to be.

I don't know if it matters what frequency it is operating at, but the device just needs to take the incoming frequency and step it up or down via a pot or digital stepping. This way I can use the gps and drive and adjust it some until it is dialed in and then stick it under the dash.

Does anyone know what the speedodrd is using to accomplish this?

 

[Jaguarjoe]

Here are 2 different approaches to take:
1) Buy a cheap PIC, wire up a simple interface and power supply, then spend hours making a bulletproof program and let an adjusting pot feed the PIC's ADC.
2) Buy a F/V convertor (National Semi), feed the pot, buy a V/F convertor NS again) spend hours wiring it and debugging it but do no programming.

 

[jclaudii]

Hmm...No "simpler" way?