Ignition Switch, Tach and Speedo Question

[L Sjoberg]

83 SC, bone stock save for MSD coil. circa 169k miles. rebuilt Bosch alt, new coil, new battery, etc., after the alt died while driving. have cleaned all grounds, as well. now the question:

intermittent no tach - sometimes tach will start and stay dead, other times will fire up at about 1500 rpm and then slowly fade to zero while driving, other times will function normally. also intermittent speedo - sometimes dead, sometimes will kick in only above 20mph, other times functions normally. also intermittent dead ignition, i.e. key will turn all the way right but no triggering of a start operation. otherwise, car runs perfectly normal.

curious if there could be any root cause of an almost-certainly dying ignition switch? it's always possible of course that the fried alt also killed the tach and speedo, and of course all kinds of things can go wrong with a 40 year old car (especially since I know the quality of the mechanic, namely me).

apologies if a stupid question, even ancient electrical systems are often Greek to me, regardless of how many diagrams I try to read. trying to assess any potential for commonality, before I start tearing about things individually.

[dictoresno]

I don't have the wiring diagrams for the 83, but for the 84-86 the instrument cluster all grounds to the ground point in the frunk behind the drivers side trunk lid strut. perhaps try cleaning it and regrounding everything?

[L Sjoberg]

thanks for the earlier reply! have cleaned all of the grounds, the grounds in the frunk near the hood strut were for sure not fun getting to.

have pulled the ignition switch and will swap in a new unit over the weekend. will swap out the dizzy cap while I'm at it and do some other regular maintenance items and then see how she runs. the original switch was definitely ready for a change.

my guess at this point is that the new switch will solve most of the stuff and the speedo will be an independent problem, but will update the thread after the weekend (hopefully).

[L Sjoberg]

just in case someone finds this thread 20 years from now and wonders what happened...

new ignition switch solved the obvious problem with the switch (no spring action to start the car when the key was turned all the way to the right), but had nothing to do with the tach or speedo. cleaned all grounds again, and then also pulled the tach and checked / cleaned all the connections. also confirmed the tach is getting power when ignition is on (it is). this seems to have solved the tach problem for now, the speedo kicks in only once the car has run a little bit so I imagine this will mean putting the car up on the hoist and checking things underneath.