Dash pod repairs

[Sonny]

Hello, I have a 1987 928 that has numerous instrument problems; notably the speedometer is very inaccurate( needle does move but erratic) ; odometer does not work at all. Is there instruction on DYI repair for these problems? I am located in the eastern PA area so local shop that can repair or supply parts is appreciated.

BYW, this is one of the 3 surviving SPYDERWERKS cars ( Spyder body on 928 chassis ).

[dr bob]

Hi Sonny, and Welcome to Carpokes!

There are a few components to look at as part of your work. The speedometer is driven by a pulse sensor in the rear upper right of the differential cover. That sensor is a reed switch, triggered by magnets in the ring gear carrier inside. From the speedo 'motor' in the gauge cluster, a small current passes to that sensor, which closes that end of the circuit when a magnet passes by. You can test for function at the instrument cluster connector, but it may be easier to start at a connector in the spare wheel well. Depending manual or automatic gearbox option, this is a 2-pin (automatic) or 4-pin (manual) plug connection. Regardless, from the body side of that local connector at pins 1 and 2 (both brown with red stripe) you should be able to see the switch. Key off, it should show alternating zero ohms and infinite ohms as the differential turns.

The connections continue to a 14-pin connector in that well, at pins 3 and 4 now, same insulation colors. Pin 3 goes to chassis ground from the body side. Pin 4 from the body side is still brown with red stripe to pin 14 of connector B on the cluster backplane, from which the signal passes to the speedometer/odometer instrument via that backplane.

The speedometer itself translates the speed of the pulse to needle movement, while the pulses also trigger a little clockwork ratchet that moves the odometer. There's a fairly fragile nylon gear in the actual odometer shaft, and that is subject to damage based on age, heat, and from 'abuse' that happens if you try to reset the trip meter while the car is moving. There are replacement gears available in the aftermarket, plus you can search out the files needed to 3D print them yourself. This fragility makes buying pre-1989 cars a little more of a crapshoot. '89+ cars have an electronic speedo/odometer/trip meter, in addition to the relatively analog needle speedo driven by the pulses.

The known good instrument service places for them are Palo Alto Speedometer and North Hollywood Speedometer last I looked. https://paspeedo.com/ or https://www.nhspeedometer.com/ respectively. Neither is cheap, both are excellent, and unless you've already cut your teeth on VDO instruments I can recommend both as best options. You may find other services locally with a little searching, or maybe a referral from your local PCA chapter folks. Once I have to put it in a box to ship, the quality of the work is the most important part of the decision process.

Removal of the gauges assembly from the cluster is pretty well documented. I encourage you to look hard at the upper steering column bearings while you have the steering wheel off, and respect the combo turn signal switch especially alignment on the shaft when you put it back to make sure you don't damage the plastic turn signal reset piece.


Again, welcome to Carpokes!

[dr bob]

I should also mention that many instrument and related irregularities are the result of corrosion and poor grounds to the body. Your spider conversion might be even more susceptible. The instrument cluster and sensor grounds are common at MP IV, on the firewall above the pedal assembly bases in the cabin. There's a general instruction to maintain the ground connections on a one- to two-year interval, as part of a regular electrical inspection and contact cleaning exercise. We live in a dry high-desert climate, after a dry SoCal coastal desert climate, and generally have no issues with corrosion. Still every couple years it gets a couple hours of detailed PM on wires and connections.. No issues to report so far in my almost 30 years of stewardship, something I attribute in part to the electrical maintenance protocol.