?? Tach stopped working
 

During my commute this morning, I noticed the tach was not working. A search on the subject gave me the impression that it's a electric and not a mechanical driven device. Also, many bouncing tachs are symptoms of alternator issues. Can anyone give me an idea of what's going on. I'd hate to get stranded on the 25+ mile commute home tonight.

Tach needles on these cars may stick in position from time to time due to the plastic (yes,plastic) on the gauge face warping from heat and binding tach needle. May not be the case if it occured in the morning but I suppose it's possible.

I just checked and there is no obstruction between the needle and the face. I turned the car over and the needle registers nothing. When I tap on the glass, it bounces a little so I know the needle isn't stuck.

Could be related: PO installed two 10" electric fans on top of the AC condenser located under the tail. They were attached with zip ties and the left one broke free while driving. The fan slipped down and burnt/cut into one of the plug wires. It caused the car to cut out while driving but I popped it into gear and it fired back up. I got it home and checked it out. I noticed that one of the plug wires was cut to the core but the core shielding seemed to be intact so I put some liquid electric tape on it till I get a new set. (best bet Magnacore for $200?) I have been driving it about 60 miles a day since with no apparent problems so I didn't think it was too serious.

Anyone with suggestions please advise

On a Motronic car the tach is a seperate output from the DME controller under the seat. Do you have any way to check the signal at the tach, o-scope or meter with freq counter?

I have no way to test it here as it is in the parking lot at work. My main concern is that if it's related to the alternator, then I might not make it home with a 30 mile commute. I'd hate to get stranded somewhere. Is the tach an indication of the alternator not working? The battery light is not illuminated.

Any suggestions/ advice?

Drive home

If it breaks down maybe it will be close to where you can get a beer and a sandwich.

I checked all fuses before I left. All were good and I made it home without any problems.

Rick, do you know if the tach is directly wired to the DME or is there anything else that might be broken inbetween.

Isn't there a sensor on the tranny?

Straight to pin 21 of the DME http://forums.pelicanparts.com/porsche-911-technical-forum/397830-911-carrera-year-1985-ecu-pin-out-info.html

Narrows it down to Tach, interconnect or DME driver circuit. :(