I have a 1982 F100 with a 300 straight 6. I installed a factory tach about a year ago and it worked fine. Afew weeks ago it stopped working. I check to make sure the wire going to the cluster was still sending voltage with a old rpm and dwell meter and the rpm gauge on it worked. So I know I'm still getting good signal to the cluster.

I went to the salavge yard today and found 2 more so I bought them both. I have tried both of them and neither works.

Does the tach and other gauges use the same ground? All my other gauges work fine.

Is there a way to test the tach and circuit board to see whats not working?
Has anyone had good luck repairing this units?

 

The tach uses the same ground as the ignition module, which is a black/lightgreen. The diagram shows a plug near the cluster with the black/lightgreen and the darkgreen/yellow.

The black/lightgreen goes out and ties in at the plug to the ignition module, and at the plug is another black/lightgreen, which goes to the distributor, and the dist housing is where it ties in to get a ground.

The darkgreen/yellow is the signal wire, which is really the negative of the coil. It also goes out to the ignition module plug, where it ties into the darkgreen/yellow that goes to the - of the coil.

 

Thanks Franklin,

Have you ever tried to repair a tach?

I'm going out to check the wire colors on the cluster circuit board and see which wires got to what connection.

I know it is getting a signal to the tach connection. If I ground the connection for the tach ground and the tach still doesn't work then the tachs have to be bad right?

 

photos and tach info here.

Ford Truck Enthusiasts Forums - JimsRebel's Albums

To test the junk yard tachs, just hook it up like you did with the dwell/RPM tester. Just B, G, C... or power, ground and coil, this is how I tested the one in the photo as I also have a 6 cylinder engine.

Will post more info later as dinner is ready now.... Jim

 

My diagram doesn't show the power connection, but maybe you can figure it out by looking at the circuit board to see where it comes from.

 

1 or C = Hook the "Tach" (neg side of coil) wire here.
B= This goes to 12v switched power
G= Ground if your truck has an Inline 6 or V8.
And also
8= Ground only if your truck has a V8.

 

Ok I added 2 more photos to my photo album on DS2

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Duraspark engine harness.
Note: the top right plug is the tach feed, this harness is for a V8 engine as it has the ground wire for the "8" connection on the back of the tach, a inline 6 will only have the green tach wire on the engine side of the harness in this plug as shown in my truck photos of my 300 ci engine.

Jim

 

The ground wire you are referring to is the ground for the “8” connection on the back of the tach. This wire goes to the engine DS2 harness then…. If it is a 6 cylinder engine it stops at the plug, or if it is a V8 engine harness it goes to ground as you stated above.
<O</O
Here is a photo of my truck with the 6 cylinder engine harness. The center plug (tach feed) in the photo has just the green wire on the engine side of the DS2 harness and both wires on the truck side of the plug.

 

My truck is wired up properly. I checked the wire colors on the connection and it checks out with what you guys say it should be.

Dark green/ yellow dash comes coil and is a signal for the tach.

Black /light green dash is the ground which I don't use ince I have a 300 six.

Red / yellow hash is the power from the fuse box.

Light blue /red which it part of the interior lights.

I have checked the DG/Y wire and I know that the coil is sending a signal.

I need to check and see if the Red/Yellow wire is sending power to the connection. If it is then I know that the tachs have to be bad.

I'll go out and check to see if I have power to it or not and post the results.

Thanks for all your help and pictures, sometimes they are worth a thousand words.

 

Well I figured it out.

I went out and checked the red/yellow wire and it didn't have any power to the cluster connection.

I checked the fuses and sure enough one of them was blown.

It was # 18 15 amps - seatbelt buzzer, warning indicators, carb circuits, tachometer, choke heater.

Never dawned on me that it might be a power problem. Always check the simple things 1st.

A couple of good things did come out of it. I got my tach working again and I now have 2 extra working tachs for the future. They are getting hard to find in the salvage yards.

Thanks for all the help.