Tach signal

[DMC 2692]

Where does the Tach get its signal from? My Tach has been stuck at 7000 rpms for years now and I initially thought it was just a bad Tach itself. However this morning when I started the car I noticed the needle jump a little higher I've never noticed that before. I thought before I get into taking the binnacle apart to replace it I might try to eliminate an easier problem first. Any ideas?

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[Bitsyncmaster]

The tach signal is the same signal that fires the ignition coil. That signal comes from the ignition ECU. So if your engine runs, your tack is bad or it does not get that signal or power and ground.

[DMC 2692]

The tach signal is the same signal that fires the ignition coil. That signal comes from the ignition ECU. So if your engine runs, your tack is bad or it does not get that signal or power and ground.The engine runs just fine so I guess I'll be replacing the Tach and see what happens

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[Rich]

Before you tear out the tach....The pinout at the cluster/binnacle would be a typical starting point for trouble-shooting your symptom. You ask about the tach signal but both a signal and a ground are needed for the tach.

Tach Signal: There's a white/slate wire shown on the wiring diagram feeding the tach - and several other components in parallel - from the Ignition Ctrl Unit like Dave said.

http://dmctalk.org/showthread.php?17-DMC-12-color-wiring-diagram

Pin A12 is for that wire - it is on the larger of the two inst. cluster plugs.

Tach Ground: Blk wire to pin A13 - and that's it's only function. You can verify the wire is grounded by doing a continuity check with a T-pin into that ground wire.

If you're lucky you might just need to unplug and re-connect the plug to renew either of these connections and make some progress.

[DMC 2692]

Before you tear out the tach....The pinout at the cluster/binnacle would be a typical starting point for trouble-shooting your symptom. You ask about the tach signal but both a signal and a ground are needed for the tach.

Tach Signal: There's a white/slate wire shown on the wiring diagram feeding the tach - and several other components in parallel - from the Ignition Ctrl Unit like Dave said.

http://dmctalk.org/showthread.php?17-DMC-12-color-wiring-diagram

Pin A12 is for that wire - it is on the larger of the two inst. cluster plugs.

Tach Ground: Blk wire to pin A13 - and that's it's only function. You can verify the wire is grounded by doing a continuity check with a T-pin into that ground wire.

If you're lucky you might just need to unplug and re-connect the plug to renew either of these connections and make some progress.Thanks! I'll definitely give this a try....once it's warmer than 10 degrees outside here in PA

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[David T]

The engine runs just fine so I guess I'll be replacing the Tach and see what happens

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The tachometer itself is not a high failure rate part. Before spending a lot of money on a part that may be good follow Bitsyncmaster's advice and verify power, signal and ground to the tach. It can also be a bad connection to the flexible PCB in the binnacle. If you do take the binnacle out you should replace the illumination bulbs as long as you are in there.

[DMC 2692]

The tachometer itself is not a high failure rate part. Before spending a lot of money on a part that may be good follow Bitsyncmaster's advice and verify power, signal and ground to the tach. It can also be a bad connection to the flexible PCB in the binnacle. If you do take the binnacle out you should replace the illumination bulbs as long as you are in there.That's a good point. While it's out might at well make sure I don't have to take it out a few months later for a burnt out bulb

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[Jonathan]

Off the top of my head? I'm not sure. Not from experience anyway. I am familiar enough with the car and schematics to see if we can figure it out. Or until someone else just chimes in and tells us. :)

The schematic for the instrument cluster shows the tach on pin 12. Pin 12 is a W/S coloured wire, which is white and slate.

The full wiring diagram for the entire car shows the instrument cluster and our W/S wire. If you follow it, it combines at a node with other W/S wires. These go off to four other places. Those four are: the RPM relay; the idlespeed relay; the ignition coil; and the ignition control unit itself.

There are no obvious markings on the wiring diagram showing the "direction" of the signal, i.e. what is coming and what is going. Meaning, if that is all the same signal/information, it is coming from one location and going to all the others. My interpretation would be that it comes from the ignition coil connection point and goes as an output to the others, including the control unit, RPM relay and idlespeed relay. Oh, and of course to the Tachometer in the instrument cluster too.

Other forces or inputs speed up or slow down the engine, and our tach signal sees the results of that. That's our feedback loop. Give it more air/gas, RPMs increase, get measured by the ignition coil connection, feedback to the control unit and then it decides to add or subtract air/fuel. The instrument cluster tachometer is just for display/info to the driver and is only taking information in one direction. In much the same way the engine temperature gauge does.

The bulkhead connections diagram shows this W/S wire for the ignition coil (on the -ve side of the coil) being in the yellow connector and at position #5.

So, I would check the connection on the top of the ignition coil first to see its condition. Then the yellow bulkhead connector. If those look ok, the W/S wire going up to the tachometer itself might have gotten pinched or cut somehow. I don't know exactly where the routing is, but it's likely in the big jumble that goes down under your elbow in the console. Wherever it pokes out again up around the dash is another place to see if you can find a problem. You can check all of those things without taking anything apart or spending any money on new parts.

EDIT: ^^ hahaha, man I type way too slow!!

[Bitsyncmaster]

That's a good point. While it's out might at well make sure I don't have to take it out a few months later for a burnt out bulb

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Get the new "flex" circuit board before you pull everything out. You will be happy you got it because the OEM ones the copper just pulls off at the connectors.

DeLoreanGo has the new boards if your looking for any other parts also. I still have my OEM flex board in my car but have the new one waiting till I pull the dash for any reason.

[dmcnc]

My tach occasionally jumps slightly during highway driving. I cleaned the contacts at the connector and the circuit board looks decent.

[Elvis]

for the broken one:

LM1819