yellowbox speedo ratio adjustment

[7783911]

after reading several posts I decided to try the YellowBox ratio adjustment unit. 75% of the time its spot on now compared to GPS but on occasion it will oscillate wildly then return to correct speed. Bad BOX or should i be looking elsewhere...prior to yellowbox, the speedo never oscillated but was reading 10% off and the ratio box corrected that except for these aberations

any ideas out there?

[spuggy]

Originally Posted by 7783911

after reading several posts I decided to try the YellowBox ratio adjustment unit. 75% of the time its spot on now compared to GPS but on occasion it will oscillate wildly then return to correct speed. Bad BOX or should i be looking elsewhere...prior to yellowbox, the speedo never oscillated but was reading 10% off and the ratio box corrected that except for these aberations

any ideas out there?

Hmm. Actually, I had an issue when I first fitted the Yellow Box; which is why I never posted the traditional triumphant installation post with pictures that I'd expected to...

Specifically, what I saw was that the indicated speed reading would abruptly drop ~8 MPH, stay there for like 1/4 of a second or so and then return almost immediately to the previously-indicated speed - but it'd overshoot it and sink back down to the "correct" reading. And some seconds (maybe 10-15?) it'd do it again.

I removed the Yellow Box - and noticed that without it fitted, the speedo did the same thing with about the same frequency, although not as pronounced; more like 3-4 MPH drop - which actually just looked more like a "tremble" in indicated speed when driving along than the "drop-and-kick". Not as noticeable.

I bought a pocket DSO, aiming to trace the input/output of the YB to track down the source of the anomaly - but got distracted. I also try not to spend time staring at the speedo in normal driving, and having it indicating correct speed >99% of the time was still a win.

One day, somewhere between 6 months to a year after fitting the YB, the speedo that was fitted (not the original, as the odo stopped on that @ 247K miles) just stopped working. Zip, nada, zilch etc.

I fitted a random used (and maybe repaired) 3.2 speedo I had picked up years earlier - and it worked perfectly. No jumping at all.

"Oh, sweet..."

Some years before this, I had had a voltage regulator fail. Which I only noticed because the fuzzy growth on the negative battery terminal was insane (clean it off and it'd come back in less than a week). Yeh - battery was boiling; 18V across the terminals at idle. Never a moment's instability or weirdness with the tach BTW...

Replaced alternator (and thus the built-in VR), thought no more of it.

Comparatively recently, I was reading a thread here and found mention of components in the speedo rated at 15V or 16V. Which is fine on a 12V car - until your VR starts kicking out 18V on a prolonged basis...

So concluded that over-voltage had probably damaged the speedo, but that it still mostly worked. Until one day it didn't.

Don't know why the YB seemed to make the damaged speedo more sensitive - but the replacement is still soldiering on without issues a decade later, sooo....

[ClickClickBoom]

Yellow box is sensitive to overvoltage. My car spit out a short term overvoltage of 17+ volts for a few moments. Yellow box failed shortly thereafter. I have a post on the adventure.

[spuggy]

Originally Posted by ClickClickBoom

Yellow box is sensitive to overvoltage. My car spit out a short term overvoltage of 17+ volts for a few moments. Yellow box failed shortly thereafter. I have a post on the adventure.

Wouldn't surprise me. The Yellow Box can use either a 5V or 12V supply. Perhaps a little isolation (eg stepping down 12V->5V) would protect these better. Something intended to step 12V down for USB power should do the trick nicely.

My overvoltage happened & was completely rectified some time before the YB came along. Pretty sure that damaged the speedo itself - even though it seemed to continue to work - given that when I looked closer, I saw erratic behavior without the YB in circuit. But it was less pronounced/obvious so I'd never really noticed it before. The speedo I fitted after wasn't erratic at all, with or without the YB.

Early_S_Man (RIP Warren) sums it up succinctly in this post Carrera VDO Speedo SUCKS., where he says:

Originally Posted by Early_S_Man

Actually, the same component [input filter cap] Paul found bad ... is well known for failing in all of the VDO speedos [Volvos, BMW, Mercedes, Audis, Ford, Nissan] using the UAF2115 chip. It is a bit marginal, at best, with the 16 Volt rating, so I would recommend replacing it with a 100 uF 50 Volt cap ... like this one:

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=29963&pa=29963PS

It would seem that failing alternators that are boiling batteries with 16+ Volts output are also frying that marginally-rated capacitor ... and the indication of a pending failure is a jumping, or erratic speedometer needle!

There are two 16V caps in the speedo. These threads: Sewer backs up, and speedo smokes. Carrera VDO Speedo SUCKS.

The 100uF can short internally when it fails - which leads to the 33 Ohm resistor burning out (usually badly enough it can't be identified from the color bands).

For example, this thread: Help !! My Speedo Fried :(.

The conclusion:

Originally Posted by drola

i went to radio shack. bought a 47 & 100 50v caps and a 33 ohm resistor.
speedo works like new.
Cost - less than $5 including tax
Thanks again !