ajc283

Dear 928 gurus,

Hopefully there are a few members with input on my problem. I have an early 5spd '78 928, everything has been working well until the tach stopped working recently. I have replaced the flex circuit about 20 years ago when I did an extensive restoration on almost everything on the car. I have recently removed the POD and cleaned the flex circuit (which was in good shape), cleaned the three brass terminals on the back of the gauge, replaced the gauge with a known working gauge from a fellow member here (928 Classics - Jim) however this hasn't fixed the problem, checked and cleaned the ECU connection, and now I am trying to determine the next step. I am thinking of checking the electrical path from the ECU to the next leg, I assume the CE Panel, and then from the CE to the right side POD connection (green wire). I am not an electrical engineer and I have difficulty reading the electrical schematics on the shop manual. Can anyone with better knowledge please help me determine the wires to check coming in to the CE Panel and out to the POD? Any advice with this problem would be greatly appreciated. -Anthony

The Forgotten On

What exactly is not working aside from the tach?

The tach gets its signal from a hall sensor inside of the distributor, that then sends its signal through the green wire (replace this if you haven't) to the ignition computer.

It then goes from that to the tachometer and the fuel pump relay.

You most likely have a bad green wire as that is the only link between the sensor and the rest of the system.

Once you replace that wire, if it still has that problem it is probably the ignition computer.

I just helped a friend on here trace the problem and fix his car which had a failed ignition computer after installing a good used green wire (4 years of use)

I gave him a spare I had and it has been working great for him since.

ajc283

Thanks for your help. My green wire is at least 20 years old. I will order one of these first. The only gauge not working in the cluster is the tachometer, everything else works fine. On the plus, I can now remove my POD, gauges…etc and reinstall it in under 30 mins. I will provide an update after I replace the green wire, any recommendations on sourcing a new one?

ajc283

Update on my tachometer problem. I swapped a good green wire and ignition computer and still have a dead tachometer. Any thoughts of where to proceed from here?

The Forgotten On

Only thing else to do is to get a oscilloscope to see if the hall sensor in the distributor is generating a signal. They rarely go bad, but you never know.

Without that info, all I can tell you to do is to throw parts at it which wont necessarily fix the issue.

I would however clean the pins at both ends of the green wire and at the connection for the ignition computer. Here is a simple-ish schematic of the ignition circuit:

ajc283

I don’t have an oscilloscope, however I can check voltage at the green wire on the Ignition computer. My multimeter has a Hertz function; so I am guessing maybe the signal from the Distributor will be coming as a spiking frequency or voltage? And then this signal is converted to an RPM using the circuitry on the back of the tachometer (resistors, capacitors…etc) to make a readable wavelength on the gauge?

My thinking here is to first determine if I am getting a signal from the Hall Sensor in the distributor. Then if yes, maybe I have a break in wiring somewhere from the Ignition computer forward to the tach. So I should test continuity of the wiring from the ignition computer plug to the CE Panel and then from there to the back of the gauge. Does this make sense? I have recently replaced my CE Panel with a better one about 2 years ago.