Speedometer correction!
 

Well,
I had been unimpressed with my speedo accuracy from day 1. Didn't really wanna pay much to fix, and sending it into Hollywood Speedo every time I changed tire size was even less appealing. Found these chaps Down Unda, Official home page of the Yellow Box Speedo Recalibrator. Ordered one up and patiently waited until they strapped it onto a seagull and it got here(couple a weeks). Install was easier than a stereo, and calibration was very straight forward. The only downside is they speak Australian which is different than 'murican! Just kidding!
I made my own harness so I wouldn't have had to cut my wires, easy peazy. Got it in and fired up corrected the already gps verified speedo error 11%. Went for a drive, crap no speedo, the sender uses 2 wires both brn/red, pulled over swapped them and bingo speedo! Verified with gps, spot on! 3.1416 banana job!
http://forums.pelicanparts.com/uploa...1490654359.JPG
http://forums.pelicanparts.com/uploa...1490654359.JPG
http://forums.pelicanparts.com/uploa...1490654359.JPG
P.S. As seen 4 wires +, -, sensor in, out to speedo. Basic stiff!

OK now, this looks like a winner. Can you show a few more details on how you did this? I assume my '87 has the same sensor wiring set up as yours (2-wire inductive coil). How did you connect to the box with the harness you made (do you tap it at the trans, or at the speedo)? From the website, it seems like an electrical neophyte like me can even do this. Thanks for finding it!

Looks like a great solution. Investigating for purchase.


Sent from my iPad using Tapatalk

problem with mine, its a RUF, is it reads fine up to 60 then reads low to around over 100.
have not checked it at 150+, did not have the GPS in the car and have not been that fast in a while.

its a good 5mph low at least at 80.

Correct, all the 915 trans equipped, and I would assume the G50 trans cars as well.
I made a "harness" that taps into the existing wiring by adding a spade and 2 female crimp for each power connection. Its really so simple its goofy. Tap into the +, - terminals with the 1in 2out, part of the "harness", then take the sensor output and attach to the yellowbox input and attach the yellowbox output to the speedo. It is intuitive once you start looking at it. The speed compensation details are crazy accurate. The Aussie's love to mod their cars and this isn't really a suprise.

Sounds like a speedo related issue is in play, this only works as a signal modifier.

I solved this problem (and a bad signal from my 915 tranny) by installing an AutoMeter GPS Speedo Sensor that is easily calibrated by hand. Its DOBA.......Dead On Balls Accurate. :D

how do these affect the odometer readings? Seem to recall that the speedo is optimistic by design/german law requirement -- but that the odometer is generally spot on. Would this mod make the odometer pessimistic?

Great...I just ordered one...couple of questions:

Where exactly did you tap power from (looks like the Yellow Box needs a power source)? Not sure what you mean by "1in 2out". According to my schematic, the sensor wires into the speedo are brown (ground) and green (signal?).
Did you do all of the splicing right by the back of the speedo, then leave the yellow box behind it in the back of the dash? (Their instructions indicate to do it down by the sensor pick-up at the trans, then run a cable all the way up to behind the dash, but this seems silly).

Any photos for us that are electrically challenged would really be helpful.

Thanks again...

This looks expensive until you ask NHS to make the change (ask me how I know).

If you look at this schematic you will see the speedo at the right side.
http://forums.pelicanparts.com/uploa...1491103307.PNG
Note the colors and where the wires come and go to.
The red/black stripe comes from the fuse box.
The browns are ground
The red/browns(2) come from the speed sensor, one comes from the sender and creates the pulses and the other one is the "ground" for the circuit. Neither wire carries any significant voltage or amperage so switching them around won't do anything other than cause your speedo to not work, until you switch them back.
The power is required for both the speedo and yellowbox, hence the 1 into 2 pigtail.
The ground is required for both the speedo and yellow box as well.
It's actually easier than it sounds. The reason I did pigtails and auto spade connectors was to eliminate "splicing"
The pigtails allow plug and play.

True! I go through "R" compound tires like crazy, usually trying different combos for different handling effects. I could get it closer with tire/rim diameter changes, but that was for non speedo effects. For truly small Porsche cash, I can adjust my speed to the accuracy I want. And the unit looks bullet proof.

Got my Yellow Box the other day, and installed it this evening. All I can say is YES!! My speedo is now dead-nuts accurate (had to make a 12.4% correction). What a great joy; the speedo has bothered me for years. Install was easy, as Click says above; very intuitive, and easy to program. Only difference from above is that my car is an '87, and the signal and ground wires are different colors from '83-'85 (signal is green, ground is brown). Great instructions to do various diagnostic tests, etc. At $93, it saved me hundreds, I am sure, from having to send to NHS. Thank you for finding this solution, Click. Even my wife wants me to give a shout out to you, 'cause she has heard me complain about it for so long. If any of you have a calibration issue with your speedo (electronic only, though), this is the ticket for sure.

Glad it was as easy for you as it was for me!

Hi I just received my Yellow Box and removing the Speedo I found it had 4 wires ,Red,Blue,Red/Black and a double Brown , My 911 is 74 but has a late 915 box with output to drive the electronic speedo which is 11%to high.http://forums.pelicanparts.com/uploa...1527937897.JPG
http://forums.pelicanparts.com/uploa...1527937938.jpg

So their website says it produces a square wave and works with their speedometers, not with ones that expect a sine wave. Do our speedometers (I have a 1980 SC) expect a square wave?

Also, does this drive the odometer as well as the speedometer?

Thanks!

The electronic speedometer works by receiving 8 pulses per wheel revolution from the reed switch (puck) mounted to the outside of the transmission. There is a disk inside the transmission that has 8 magnets mounted to it. When a magnet is aligned with the red switch, magnetic flux causes its contacts make which provides a ground input to the speedometer. Inside the speedometer there is a pull up circuit that provides a 12 VDC signal input that gets interrupted to 0 VDC every time the reed switch provides a ground. So, this creates a square wave signal to the speedometer and odometer drive circuits. A speedometer calibrator intercepts the pulse signal from the reed switch and alters the pulse rate by a percentage to increase or decrease the pulse rate. For most Porsche speedometers, the pulse rate needs reduced because most read 5 to 7 MPH too fast. But you must check the speedometer readout a different speeds, because the speedometer may not be linear. My speedometer reads dead on at 60 - 70 MPH, but still reads a little fast at lower speeds after setting up a speedometer calibrator.

Both the speedo and the odo are driven from the same signal input. If you correct the speedo signal, you'll also correct inaccuracy for the odo.

I fitted a Yellow Dog in 2012 or thereabouts; built-in period-correct legally-mandated optimism[*] with the extra inaccuracy added by 17" tires was really annoying...

It was especially jarring/noticeable because the PO had adjusted the trim pot on the original speedo so it was dead-on accurate - but I replaced that when it broke the odo gear with 237K on the clock; every other factory speedo read much, much, higher and it was driving me crazy...

Measured the rolling diameter, calculated the conversion factor/set the dip switches, made up a little spade terminal pigtail harness to provide power/intercept the speedo feed and tucked it behind the gauges. Checked it out against GPS, called it good...

I'd actually completely forgotten it was still nestling in the dash just doing it's job, hadn't given it a thought in years/many 1000's of miles...

Question that seems to come up is whether the electronic speedos/senders interchange; I've had everything from the '77, to an SC, 3.2 and a 930 speedo fitted - and went from a 915 to a G50. No difference that I could see.

Also worth mentioning that if your speedo needle is erratic and you've checked everything else, the 100uF cap in the speedo board is known marginal - rated for 16V. If your voltage regulator has ever gone over-voltage.. The speedo will likely eventually fail entirely (mine did), but can behave oddly (like steady reading then drops down & back up a few seconds later) for years first...

The cap can be easily replaced with a 50V part - old thread with bonus Early S Man content: http://forums.pelicanparts.com/porsche-911-technical-forum/247776-carrera-vdo-speedo-sucks.html

[*] The Wikipedia article on speedometer accuracy https://en.wikipedia.org/wiki/Speedometer indicates requirements all over the map - so the factory likely just went with the Euro ones, as they were tightest...

Current EU Type Approval regs state: The UK is different: The 1997 US Federal standard states (emphasis mine): But no idea what the laws actually said when these vehicles were made - e.g. Australia apparently had no accuracy requirements at all until they introduced speed cameras in 1988...

I too have issues with my Speed/Odo and looked into this company however they didn't answer any emails I sent asking for clarification on their product. Has anyone else recently heard from them?

Hope you can help a neophyte...

I have a 77 targa with 915 transmission. I am off by 10% linear (speedo reads high). I dont really want to drill into the speedo to adjust the pot..likes like giving a knife to a mass murderer. I assume I have a sensor on the transmission that picks up the pulse (should be a square pulse then) Where do you install the yellow box....behind the dash? Which wire goes where if I do, I can power source and ground easy enough...I figured if I could take the inbound to the speedo and re-route to the yellow box, then back to the speedo, I could even mount the box in the smugglers compartment or behind the blower.

Click, Chris, 555......I am a pictures guy..any chance you can grab some photos of your set up and show me all the connections? so I get this right, Lyle..i got your PM..thanks

rcraske@cogeco.ca

Thanks