Recently swapped gearboxes, and everything apart from the speedo and odometer is working fine.

Basically, the speedo and odometer "work", but they're scaled down about 1/3. E.g if I'm doing 30mph according to GPS, Speedo shows 20.

I'm guessing this is due to the speed sensor on the box, which I should have changed when it was all apart.

Does anyone know of a little inline module that will magically convert the signal back to the proper one?

If no such thing exists, I think I can make a little emulator/translator with an Arduino, but rather not have to.

Also, apart from the speedo/odometer issues, will the discrepancy cause any issues? Eg ECU limp home mode or something?

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Looks like I need one like this maybe: http://rjes.com/html/vehicle_speed_sensor__vss_.html

 

If you have changed gearboxes "like for like" then the mechanical drive rotational speed to the speed sensor should be the same.(gearwheel in box changes depending upon model)
The actual speed sensor for the newage is the same for all models both 5MT and 6MT.
What else has changed??

 

It's not like for like it seems..

From standard 5 speed WRX 2003 box (UK) to a PPG. I'm not sure what car the PPG was originally in, but I suspect a classic of some sort.

 

Ooh I see what you're saying... The gear wheel must be from a different model in that case...

You wouldn't happen to know what the waveform is like? I think I've found the correct wiring diagram, but it doesn't mention waveform, or indeed if it's 5v or 12v peaks.

I'm hoping to intercept the signal as it leaves the gearbox and modify it.

 

You've opened a proper tin of worms here was the box that came out UK or JDM has it DCCD is it an STI or WRX. There are so many variables as to why this is happening. Drop gears and front diffs just one of them. You would be far better using your own/old case and dropping the PPG gear set in and using your own front diff (and possibly changing the pinion shaft in the PPG gear set to your own) If you use your own/old tail casing you know everything is as it should be. As Don says all the speed sensors are the same so its either out of a classic or the ratios/final drive are mixed up which is only going to end in tears.

 

Mechanically it's got the right diff ratios etc (for my 3.54 rear diff).

I think I should be able to solve it with some electronic trickery.

From some more research it looks like the pulses for most generic speed sensors are normal square wave, so I will generate one and see what happens when I feed it into the cluster

 

Looking through one of my classic paper manuals I came across this.........

 

Don, my goodness, that is extremely helpful!

Thank you ever so much!

 

Hmm, found something interesting yesterday...

This little black box is spliced into the middle of the speedo signal wire (the green wire on the connector)...

I've taken it out, but not gone for a drive yet so I'm not sure what effect, if any, it would have had.

I've also extended the wires now so that I can intercept the signal using an Arduino if required, but will test first without the little box.

Unfortunately the hose fittings to my oil cooler are leaking, so I don't want to drive the car until those are fixed.

 

Also some more info about the speedo drives here https://bbs.legacycentral.org/viewtopic.php?f=19&t=49886

Positives: looks like a square wave at 5v.
Negatives: there's some blurb in there about varying resistance, which I don't understand...

I will try to feed it a 5v square wave at first to see how it behaves.

​​​​​i should really get myself a little oscilloscope so I could see what is going on inside the wire


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For whatever reason it looks like you have a KPH to MPH converter fitted which would tally with your 30/20 problem (30Km =18.6miles)

 

hmm, yes that is definitely a possibility! Perhaps the box was fitted to a JDM import before I got my hands on it.

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If that's spliced into the loom on the gearbox that's your problem. Is yours a UK car.

 

Yes it is, yeah.. a 2003 blob WRX

 

Solder all back and heat shrink and your good to go