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Could someone please explain what the operating voltage of the ignition system primary circuit is supposed to be on a 1974 710K, I am assuming that it is supposed to be 24v but I only measure a little over 9v (running). The service manual indicates 24v and the stock shielded coil has 24v stamped on it. Also: what is the purpose of the in-line cigar shaped metal canister in the primary positive circuit to the coil? It is mounted near the coil itself. Any information at all would be much appreciated, technical details are ok.

The item you pictured is a noise supressor designed to help eliminate hiss and other ignition noise on radios. You can remove it from the circuit if you prefer with no ill effects.

You are correct that the voltage should be in the 22-26 volt range. Items to check would be the voltage regulator and the VDO rpm limiting module. A search of the forum should provide you with plenty of suggestions and instructions on what and how to test these items.

Good luck!

Paul,
Could a dirty Molex cause this too?

pcolette wrote:The item you pictured is a noise supressor designed to help eliminate hiss and other ignition noise on radios. You can remove it from the circuit if you prefer with no ill effects.

You are correct that the voltage should be in the 22-26 volt range. Items to check would be the voltage regulator and the VDO rpm limiting module. A search of the forum should provide you with plenty of suggestions and instructions on what and how to test these items.

Good luck!

Thank you very very much. Will follow your suggestions.

Average voltage at the coil "+" terminal (with respect to chassis) is generally 16-18 volts when running due to the ballast resistor (during cranking, the "+" terminal should almost equal battery voltage since the ballast resistor is bypassed). Anything less than that is 90% likely to be a failing 4500rpm box under the dash - look for a deck of cards sized box with a six terminal connector (two rows of three terminals). Unplug the connector and jumper the harness connector only between the middle terminal of one row with the middle terminal of the other row (do not attempt to re-connect to the rpm module). Now you should have the 16-18V expected, and it will run GREAT compared to 9V. You can safely run this way indefinitely as long as you do not over-rev the engine (it is safe from an electrical perspective). The Molex would not cause this problem; a bad regulator would over charge the batteries (over 30 volts!) and a bad Molex connection would undercharge the batteries (but you would not be able to start the truck if they were down to 9V). But do check the Molex for good connections to prevent problems.