jeffsnook Posted August 10, 2021 Share #1 Posted August 10, 2021 We have recently acquired a 1971 240 and have been going through it's various problems. As delivered to us the tachometer was not working. The power wire from the ballast resistor to the tach (green/white) and the return wire from the tach to coil (black white) were disconnected and a short jumper from the ballast to the coil was installed to allow the engine to run. There is no continuity through the tach circuit as measured at the connectors hanging loose near the coil. If I use a test light to check for voltage I get a good bright light at the resistor, but if I properly connect the tach circuit the light is very dim which I am assuming is a direct short somewhere in the vehicle. I have the factory shop manual and wiring diagrams, but am lost as to which multiplug in the passenger side footwell contains the wires from the coil connections, as the color codes are used for more than one system. I am very familiar with this type of inductive tachometer, as I work on many British cars that use a similar system, but tracing the circuit path in this car has proven quite frustrating. Any guidance would be greatly appreciated. Link to comment Share on other sites More sharing options...
Zed Head Posted August 10, 2021 Share #2 Posted August 10, 2021 You might find something in the Electrical chapter of the FSM. https://www.classiczcars.com/files/category/9-factory-service-manuals/ Link to comment Share on other sites More sharing options...
Captain Obvious Posted August 10, 2021 Share #3 Posted August 10, 2021 I looked through the FSMs a while ago and never found a complete diagram showing whole system including the tach. I whipped one up though using the 72 manual as a starting point. Doesn't show wire colors, but at least shows how things should be connected. Does this help at all? : Link to comment Share on other sites More sharing options...
siteunseen Posted August 10, 2021 Share #4 Posted August 10, 2021 Totally off topic, you all know me! What does this little high five mean? I've seen it lately and have to know everything so what says @Mike? I'm thinking "High I'm a new member"? But it could be some deeper Illuminati BS. 1 Link to comment Share on other sites More sharing options...
SteveJ Posted August 10, 2021 Share #5 Posted August 10, 2021 25 minutes ago, siteunseen said: Totally off topic, you all know me! What does this little high five mean? I've seen it lately and have to know everything so what says @Mike? I'm thinking "High I'm a new member"? But it could be some deeper Illuminati BS. When I hover over the icon, "jeffsnook recently joined" pops up. 1 Link to comment Share on other sites More sharing options...
siteunseen Posted August 10, 2021 Share #6 Posted August 10, 2021 Okay it's a "high I'm a new member". That's great! Friendliest forum I've ever been a part of. ❤ We're better than Spongebob. Link to comment Share on other sites More sharing options...
SteveJ Posted August 10, 2021 Share #7 Posted August 10, 2021 To piggy back on @Captain Obvious's post... The white/red wire goes to the ignition switch and is connected to the black/white wire coming out of the ignition switch when the key is in the ON position. The black/white wire travels down the dash harness to the engine harness. As you noticed, there are 3 black/white wires at the dash harness/engine harness interface. 6-wire connector - Also has a green/white wire. 4-wire connector 3-wire connector - I believe that is the switched source for the voltage regulator. My 73 has a different harness, so I can't help you with tracking down the connector directly. What you could do is disconnect both the 6-wire and 4-wire connectors. On the engine harness side, measure resistance between the black/white and green/white wires. The black/white that has low resistance (3 ohm or so) with the green/white wire is the one that connects to the ignition switch. Anyway, the black/white wire travels out to the ballast resistor, and there is the green/white wire on the other side of the resistor. (Do you see why I suggested checking for resistance on the engine harness side?) The green/white wire travels to the tachometer and comes out black/white. The black/white wire goes back through the dash harness, to the engine harness, and on to the positive terminal of the coil. I hope this helps. Link to comment Share on other sites More sharing options...
Zed Head Posted August 10, 2021 Share #8 Posted August 10, 2021 (edited) 3 hours ago, Captain Obvious said: I looked through the FSMs a while ago and never found a complete diagram showing whole system including the tach. I almost added something about the Owners Manual but didn't. Didn't want to get too wordy and thought the FSM might have it. That's where the full diagrams are for some reason. Page 37. Edit - actually I didn't post the link to the diagram because he's looking for the actual harness connections. Best to just stick your head up under the dash and poke and pull and follow. https://www.classiczcars.com/files/category/17-owner-manuals/ Edited August 10, 2021 by Zed Head Link to comment Share on other sites More sharing options...
jeffsnook Posted August 11, 2021 Author Share #9 Posted August 11, 2021 Thanks for all the suggestions. We're still poking around! Link to comment Share on other sites More sharing options...
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