Tach help, Please
 

I have had a no start problem after rotating my gauges for better viewing. I have tried many things to fix this. See thread titled "If alarm is set, what is shut down?/ No start".

In this thread Tyson said "If the tach is grounded, you won't have a spark (should be the black/purple wire."

Can someone tell me how to test the tach, to see if it is grounded. (My next option is to by pass the factory alarm at the two #61 terminals). Any other suggestions would be welcome. I have done many searches over the last few weeks and am stumped.

Thanks

Jeff:

Given that you have power to the coil and no spark and engine turns over, I would be willing to bet that the small black wire with a pin connector that goes to the Alarm control unit has come loose or is not getting power.

I do not know what the Alarm unit looks like, but I do know from doing my conversion, that if the black wire is not getting 12v when the key is on or in the start position the car will not fire.

The black wire runs from Pin number 5 of the DME relay SOCKET to the alarm control unit (87A). Find that wire and disconnect it from the control unit and connect it to fuse number 8 (turn signal/backup lights).

Turn the key and keep your finger's crossed.

If this works, there must be something wrong with the alarm control unit.

As to grounding of the tach, your tach has a black/purple wire (pin2)and a black wire (pin1) that power the tach. The other wires are for lights, and other items not related to tach functions. If you are worried about the tach being grounded write down the position of the wires and pull them all making sure they don't come in contact with one another or body metal, and see it that makes a difference.

Let us know what you find.

Jeff, Marc stated exactly what i meant by my reply.

You mentioned in your other thread that the last thing you did before your engine wouldn't start was remove the gauges. That's always the place to start. I was guessing that you may have put the wires on the back of the tach in the wrong places. If the Tach signal wire is touching ground, it will kill the spark.

Marc, Can the car run by bypassing the alarm your way (this sounds much easier than removing the air handler and by passing at the alarm unit) or is this just to test.

Also as I was testing the engine speed and reference sensors I noticed the insulation on the wires were falling apart. Looks like I will be replacing them. May be my problem. I taped them up as a very temporary fix so I can diagnos problem.

Jeff:

Since I don't have the alarm system in my car ('77 911) I'm not sure.

But I am sure that as long as that black wire has power as I indicated above, the car will run.

The black wire is the only wire associated with the DME that goes to the alarm control unit. Other wires go to other places. I will go back and check the wiring diagram, at this point you have nothing to lose.

The question in my mind is what, if anything, won't work without the control unit working.

I'll keep checking, meanwhile see if you can rig a patch cord to that fuse. :)


Marc

Ok I checked, one lead from the alarm control unit goes to the alternator, not sure why. You will find out if the battery is not charging in short order.

All other connections are to detect a break in and set off the horn.

Marc