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Speedo and Tach not working S4

2K views 12 replies 6 participants last post by  Norseman50 
#1 ·
I just picked up a 1991 S4 Spider today. It's super nice in every way except the speedo and tach do not function. I have just replaced the speedo sender with the unit available as a conversion. I have also verified that there is a current from the coil by the fact that the car will not run with the plug up on the fender unplugged. I'm going to post some pictures I took of the back of the cluster when I took it out of the car. Everything looks pretty good so I'm really at a loss. Every other gauge works besides the most important two. The lights also all work in the pod so it seems like the grounds are good. Is there somewhere between the plugs for the speedo on the firewall and the tach on the inner fender that the failure could be occurring? I also swapped in a previous cluster that came with the car and it also didn't work in exactly the same way. BTW the seller says when he was sold this cluster it was said to be good. Lastly, fuse 8 was blown so I replaced and no change. So was fuse 4 which fixed the radio that had been removed, most likely due to not functioning but didn't fix the speedo/tach by osmosis. Nature provided thunderstorms all afternoon so instead of driving my new car I spent the day tearing into this issue with no joy. I've read all the threads and I think I've done pretty much everything except for the adjustment procedures which don't seem like they really would apply in this case due to complete non-functionality. Thank you for any insight!
 
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#2 ·
if two separate clusters don't work in exactly the same way, then you can be fairly sure it is not the cluster (especially as one was sold as working good)
btw. fuse 7 and 8 need to be checked.
I presume the odometer/mileometer is also not working......?

you will need to check the connector plugs for any wires that might have pulled out....not only the ones clipped to the back of the instrument cluster, but also the connectors further down near the steering column.
this is what that shortish wiring loom looks like so you know what type of plugs to look for.
Cable Electrical wiring Technology Auto part Wire


Do you have the coloured wiring diagram for the S4? It shows the monopod wiring so you can trace wires to speedo and tach units.
If you need one, let me know I can send a pdf
 
#3 ·
Hi thanks for the help and I don't have the color wiring diagrams. I would really appreciate it if you could send me one! I'll track those wires down this weekend and see if the plugs look good. Thanks!
 
#6 ·
Updated status (small update). The light doesn't work in the speedometer. Perhaps that indicates a break in there somewhere? Understand it could be a burned out bulb. I think I'm going to ship my cluster to my father who has a '92 spider with working gauges. If it works in his car, the cluster works! If it doesn't, we know it is the cluster itself.
 
#7 ·
I don't think one single speedo illumination light not working says anything other than a bulb has blown.
you can see in the wiring diagram that I sent you (you got that, right?), that all 5 illumination lights are on the same daisy chain, so all of them (in the lower segment) would be out, if it was a break in the ribbon...

I suppose you could ship it off and ask your Dad to try it in his S4....seems a lot of work (well, for your Dad anyway)
 
#8 ·
Ok big update from today. After doing pin to pin conductivity checks for all the tachometer and speedometer wiring (from the coil to the plug on the cluster and then from the plug on the cluster to the sensor line for the tachometer), I determined that there were no breaks in the line so the fault must be in the gauge itself. Sure enough I pulled the tach out and discovered that the previous owner had turned the adjustment screw all the way to the maximum level. I think it was past the usable range so when I turned it down it sprang to life. I then used my digital timing light to calibrate it. It is still adjusted pretty high in the range and does lag a bit above 4500rpm but it's dead on from 1000-4000 or so so I'll call it a win.
break
Speedo has good conductivity as well from the plug all the way to the gauge. I put on one of the new sending unit modules that Classic Alfas is selling, but I noticed it has a blade connector on there. Does anyone have experience with these? Do I need to rig up some kind of ground strap to the silver blade on the side of the body of the unit? If that doesn't work I may remove the gauge and see if the same mistakes were made with that one.
 
#9 ·
I then used my digital timing light to calibrate it.
I would be interested to know a bit more about how that works, if you have time to post.

Its a shame you are not closer. I have a spare speedo I would be happy to loan for testing if you were in the UK. The original sender does need a ground connection. I recall it has a three wire connector - pulse wire, positive feed and ground. I assume the new one will need something too.

EDIT: I just looked at the ClassicAlfa site and your part seems to have a three pin connector so I don't know what the silver blade on the side would be for. Probably best to give them a call, they're always very helpful.
 
#10 ·
Yeah, as you found, one of the main failure modes for the monopod instruments is the potentiometers. They either build up resistance and the gauge reads low or they fail outright. As you found, sometimes you can fix them by cleaning with contact cleaner and turning them a bit. But I suspect you're also going to find that over time it's going to start reading low again.

I eventually had my speedo guy replace the pots with better ones and then calibrate the units. They've been working well since.
 
#11 ·
Update from today. Tachometer continues to perform well. I put in the speedometer that was a spare with the car. The odometer works but the gauge doesn't read anything. I think this means the signal is there but the gauge is broken. Do you guys concur with that assessment?
 
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