using tach signal for VSS

I know this seems strange, but bear with me.

I'm using a TBI (donated from an 87 5.7L truck) on my boat. It runs great! However, I was having throttle follower issues where it doesn't come back to idle fast enough when I rapidly decelerate from a high RPM down to idle. I have not been using any type of speed sensor, so the signal is missing. Also, I do not use the park/neutral switch, so this system acts like a manual transmission. I've read that the throttle follower depends on feedback from the VSS to work properly.

The transmission in the boat is a 1:1 velvet drive. No shifting gears. The RPM of the crankshaft is the same RPM at the prop shaft. I thought if I used the tach signal to satisfy the VSS signal, my issues might go away. When I used the tach signal, the data lot showed 96 MPH when idling at 800 RPM. This was not real surprising.

So here is my question. Is there anyway I can trick the ECM or lower the pulses so the speed is closer to reality?

When I am running at 4600 RPM the speedometer on the boat reads 39 MPH (this is not linked to the ECM). The way I have this set up now the datalog would show 552 MPH at 4600 RPM.

I have never liked idle speed control in the 7747. It is not straightforward and often does not operate as expected.

IIRC 7747 doesn't have the ability to adjust pulse count internally. It expects either 2k or 4k pulses per mile. You could try using a DRAC to adjust the signal to a more reasonable value.

Your speedometer must be electric, yes? Try using that signal?

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Originally Posted by 1project2many

I have never liked idle speed control in the 7747. It is not straightforward and often does not operate as expected.

IIRC 7747 doesn't have the ability to adjust pulse count internally. It expects either 2k or 4k pulses per mile. You could try using a DRAC to adjust the signal to a more reasonable value.


Your speedometer must be electric, yes? Try using that signal?

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I have a DRAC (never needed it) how do you adjust it?

No, its a mechanical pitot tube that uses water pressure to push the speedo needle.

try a std trans bin and see if it changes .

http://gearhead-efi.com/gearhead-efi/doc/DRAC2.pdf

I know electric speed sensors are available for boats. They look like small paddlewheels.

Not certain, but most logic requires a "stop" motion or very low to get to the "idle" condition.
You would need to adjust where the minimum speed for idle is at in the bin.
DRAC is the best option to play with the number of pulses.

Correct me if I'm wrong:

DRAC takes an analog pulse signal from the VSS sensor and converts it into a digital pulse signal. It also allows the digital signal to be manipulated so the ECM see's 2000 digital pulses/mile.

If I send the digital signal from the ignition module to the DRAC, will it get confused, since its not a analog signal?

Using the tach signal as VSS eference can be done with a few simple components if you can build small electronic circuits. Something with a 555 timer and some capacitors and resistors should do the trick. But you will, as mentioned, have to modify the bin as well. The DRAC expects an analog input with varying frequency and amplitude in regards to "vehicle speed" and although I haven't tested such conditions, I'm not certain it would perform as desired.

Also, correct me if I'm wrong but you will need to condition the tach input to the ecm"vss" to a 5v signal as opposed to the 12v coming from the coil terminal. A simple voltage divider circuit should suffice.

Buddrow

You are suggesting that I use the 12 volt pulse signal from the coil (not ref signal the ignition module). Use a voltage divider to knock it down to a 5 Volt DC signal and input the 5 volt pulse signal into the DRAC?

Sorry, I'm not very knowledgable of how this works.

No I am not suggesting you use this signal into the DRAC. The DRAC expects an analog input with varying frequency and amplitude in regards to "vehicle speed" and although I haven't tested such conditions, I'm not certain it would perform as desired. The 5v ref from the module as well as coil tapping will provide a digital on/off type signal and I am not sure if the DRAC will accept it correctly, but its worth a shot if you wanna try it.

Buddrow

Why not just fit a VR sensor into the 1:1 drive and trigger off the gear teeth or bolt heads, then adjust the pulses per mile using the DRAC/some tuning?

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Originally Posted by buddrow

No I am not suggesting you use this signal into the DRAC. The DRAC expects an analog input with varying frequency and amplitude in regards to "vehicle speed" and although I haven't tested such conditions, I'm not certain it would perform as desired. The 5v ref from the module as well as coil tapping will provide a digital on/off type signal and I am not sure if the DRAC will accept it correctly, but its worth a shot if you wanna try it.

Buddrow

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Thanks, I'll give it a try and see what happens.

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Originally Posted by Xnke

Why not just fit a VR sensor into the 1:1 drive and trigger off the gear teeth or bolt heads, then adjust the pulses per mile using the DRAC/some tuning?

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This option has crossed my mind. It would definitely work. I was just looking for something easier, where I wouldn't have to:

-make a speed sensor mount
-make a flag mount
-buy a speed sensor
-run wires back from the speed sensor to the ECM

I'm not looking for speed accuracy. I just want the throttle follower to work properly. If I can get something close to reality, I can manipulate the bin.