Erratic Speedo and bouncy Tach

bornrich · 01-07-2005, 01:32 PM

My Speedometer was updated about 3 years ago from 85 mph to a 160 mph speedo and has work fine since that time. Until now. In the Morning, at low speeds (0-20) it bounces between 0 and 160. Anything over 20 and it is pegged at 160. After a few minutes on my drive to work it starts to work correctly. Any ideas wht could cause this activity?
Also my tach, while seeming to be working fine, bounces on rpm changes. Meaning I running at 3k and I lift for a down shift the needle bounces until it settles at 1k with the clutch in. After I complete the downshift the rpm wil rise past, say 4k, then settle to 4k (correct RPMs). This is irritating at the track when I'm contantly up shifting and down shifting. Is this normal or is ther a fix?

Early_S_Man · 01:56 PM

Matt,

I suggest that you start monitoring your system Voltage, and see if the aberrant instrument behavior isn't related to some high Voltage excursions ...

Both electronic speedos and tachs have been known to go 'wild & crazy' when the Voltage regulator is misbehaving!!!

bornrich · 01-07-2005, 01:59 PM

Is the voltage reg. separate from the alternator? I had my alternator replaced a couple of years ago with a new unit.

bornrich · 01-11-2005, 01:26 PM

I had my car checked by Bob at RMG here in Sunnyvale and the cars electrical system checks out fine. Bob said my tach is acting normal. But he couldn't find any reason for my speedo to be acting strange. Of couse it was working fine when he had the car. Any other ideas?

tshih · tshih's Garage

Depending on what speedo you replaced the OEM one with (for example, a later carrera 3.2 speedo) the sender unit which is attached to the side of the gearbox may need replacement.

Or it could just be a bad electrical connection check the wires under the rear access plate in the interior tunnel between the rear seats.

Tom

bornrich · 01-11-2005, 02:29 PM

The speedo I have is my original one that was re alibrated and refaced to 160mph. I check the connnections in the interior and check the sender next. Thanks

Early_S_Man · 01-11-2005, 03:09 PM

Matt,

Are you saying that the speedo misbehaving is ALWAYS temperature dependant ... that it always acts up when cold, first thing after starting up overnight? If so, it may be resonably easy to test for a sticking reed switch inside the sender.
Put the rear end up on jack stands when the car is cold or hasn't been driven for several hours ... remove tunnel cover over shift coupler and find the two Red/Brown sender wires in the tunnel. Disconnect the two-wire connector and connect a multimeter set to low Ohms or continuity check to the sender wires going back to transaxle. Put transaxle in neutral and rotate rear wheel until you have continuity -- reed switch contact closure. Next, rotate rear wheel EXACTLY one revolution and count eight contact openings and closures. If reed switch is balky/sticking or open during a majority of the one wheel revolution ... the sender needs to be replaced!

bornrich · 01-11-2005, 03:43 PM

It seems to be temperature dependant as it hapens first thing in the morning or after siting at work. Some times it corrects itself in a couple of minutes other time it take 5-10 minutes. I willtry your suggestion over the weekend. Thanks