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Old 04-21-2011, 01:12 PM   #1 (permalink)
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Speed and distance slow to start

I really like this MpGuino! My average tank mpg based on my odometer has improved from 15 to 18 due to driving style and some aero changes on my 88 Bronco II 4x4. My only problem is that it doesn't start measuring speed or distance or mpg until I get to around 5-10 mph. As a result, the tank miles read less than the odometer if I make any stops. As long as I am moving above that speed they agree. Also If I slow down to under 10 mph to make a turn for instance, the instant mpg goes to zero and the average mpg is counting down as if I am stopped. Is there a setting I should change? Thank you for all your hard work on this device! Norm

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Old 04-21-2011, 01:26 PM   #2 (permalink)
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it may be the type of speed signal you have. if it is inductive then it won't work well at slower speeds. If there is another speed sensor on the tailhousing that might work better, or if there is a conditioned signal coming out of the ecu or something. Got a schematic?

Worst case you can get a gear tooth sensor and point it at an axle bolt or something like that:
http://ecomodder.com/forum/showthrea...tml#post178690
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Old 04-21-2011, 01:42 PM   #3 (permalink)
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Thank you for the quick reply. I am using the vss signal to the ecu. I don't know what sensor it uses yet.
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Old 04-21-2011, 09:25 PM   #4 (permalink)
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Thanks dcb. I think we have those very sensors where I work. I think I'll try that route for the vss on my Jetta. I do have a cruise control on it, but it controls engine speed not vehicle speed.
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Old 04-22-2011, 12:51 PM   #5 (permalink)
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If you're lucky you car might have a speedometer output signal. (And by that i mean an actual output signal from the speedometer.)
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Old 04-22-2011, 01:07 PM   #6 (permalink)
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You know you might be right, I'll have to check that. It appears the VSS signal comes from the ABS sensor on the rear differential and is probably just inductive. I may play with a Schmidt trigger to condition the signal so it at least lowers the speed at which it stops counting. If I could get it down to 5 mph or so I think that would help. Not sure if anyone has done that before.
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Old 04-22-2011, 11:13 PM   #7 (permalink)
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Two things I would try are removing R6. If that does not help then try installing a resistor in parallel with D1, try values between 20K and 50K.

ABS sensors are mounted at each wheel and often do not read down to zero. The sensor in the differential should be for speed and must read down to zero to keep the odometer accurate.

Mike
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Old 04-23-2011, 01:10 PM   #8 (permalink)
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Thank you for the reply. I connected my oscillocope to the junction of d1 and r6 and at speeds below say 15 mph it is more of a rectified sine wave with a peak to peak voltage of about 2 volts. Once it hits 15 mph it is more of a square wave with a voltage of 3.5 volts which climbs to about 4.5 v as I increase speed. I tried a pot in parallel with the zener but still couldn't get the level high enough at low speeds. I also put a pot across the resistor but even at a pretty low resistance it didn't change much and I was afraid of it drawing too much current. I think I could make it work with an op amp set up as a schmidt trigger to make it a square wave. I guess that's the advantage of adding you own hall sensor since it is more of a digital output.

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Old 04-24-2011, 05:06 PM   #9 (permalink)
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Yahoo! I finally got it to work using the breadboard and some components from previous projects. I converted the sinewave from the Bronco's VSS magnetic sensor goining to the ecu to a nice squarewave and fed that to the MpGuino vss input. It now starts registering speed as soon the truck begins to move and steadily climbs with speed. Should be better than thinking it is idling at anything less than 15 mph which I know was making my tank mpg and tank miles read low when making lots of short trips. I just need to solder it all together on a small circuit board. Do you think anyone would be interested in a little diy writeup?
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Old 05-02-2011, 08:16 PM   #10 (permalink)
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So, how about that write up, or did I miss it? My MPGuino should be here soon, and we only have the inductive pick-ups at work.

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