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DISCUSSION => Electrical => Topic started by: nanotech9 on July 10, 2011, 05:04:36 PM

Title: Speedometer not working but Tach & MPG still do
Post by: nanotech9 on July 10, 2011, 05:04:36 PM

I pulled my VDO gauges out to mess with a glitchy tach problem the other day..  everything else worked great other than the tach.

I swapped out for another that i had laying around for years.  I have a set of needles off an e36 i think and i had transplanted one onto the tach years go (not a straight fit - required modifying the backside of the needle), so i modified another needle and fit it to the Speedo.

I reinstalled the gauges and now everything works but the speedo. Even the MPG works...

if i understand right, the MPG gauge makes it calculations off of RPM's, and Speed, and TPS.... right?   So if any one of those three weren't producing a signal, the gauge wouldn't work right?

If thats right, then its the speedo gauge itself thats bad right?

I've already pulled the speedo, logic board, and the little blue "key" (in the front) and re-soldered them all.  No luck.


What i dont get is that it was working fine until i R&R'ed the gauges... the ONLY thing i changed on it was the needle.   I think the needle is a TINY bit heavier, but surely not so much it keeps it from moving?

Title: Speedometer not working but Tach & MPG still do
Post by: nanotech9 on July 10, 2011, 05:06:25 PM

also, the odometer does not work, but the tiny gear had a tooth missing, and the big gear has a couple broken off...

I fiddled around with the diff sensor...  its got crimp type butt splice about 4" from the sensor itself.  seemed strange to me, like maybe its been replaced before?  Wires seemed firmly attached - i actually pulled on them a bit to see if they would fail under stress (assuming the would be corroded) but they didn't budge.   Again, i think its working because the MPG gauge is still working.

Title: Speedometer not working but Tach & MPG still do
Post by: DesktopDave on July 11, 2011, 07:31:30 AM

The MPG gauge calculates off the speedo signal, tach signal and injector bandwidth from the DME.

I've replaced those diff senders the same way several times over the years.  I usually do a solder & heat shrink as it's very vulnerable to water & salt.

Moving those gauges around might have been enough...those springs on the SI board aren't very strong.  I'd get back in there with some contact cleaner & see if that does the trick.

There's an Ebay seller that has those odometer gears for under $20 IIRC.  I'll see if I can dig that posting up.