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Something blew the OE fusible link and I wouldn't think it would be the constant draw from the tach being being on all the time but if you have no draw now cross your fingers & continue on.

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Nacho's parallel bypass is to run those 2 wires straight thru the bulkhead rather than using 2 pairs of connectors in the bulkhead to join them (drill out the 4 bulkhead cavities (2 in each half) & solder in 2 pieces of wire).

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I would power the tach from the heater circuit in the fuse box.

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I've already connected the tach to the 3-way tap above the radio (also described by 72roadrunnergtx, above) - but I will try to find out how much current it draws, and change if need be.

Thanks to all for the info!

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So we can all learn something from this problem. Your pics show the big red wire was connected to the acc terminal on the right side of the fuse box. I was going to suggest you unplug it too but someone else asked about it before me. To my knowledge it should only be hot with the key on. After you said you found the problem you said it was plugged into the left side {batt}. I hope you have found that part of your problem but you may want to check that acc terminal to see if it is hot all the time.

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Sorry, my bad. Actually, there was a wire in each of these terminals - I didn't post a pic of that. The BATT side, always hot, was powering the tach. The ACC side, hot only with key in ACC position, is still being used to power the aftermarket gauges (oil press, temp, vacuum). If I had a clue as to what I was doing , I would have zeroed in on these wires from the start. Oh, well - lesson learned for next time.

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The dash harness accessory power tap originates at the fuse panel ACC bus bar via the 20a fuse labeled “ACC”. It is the same location used by the factory to power the optional factory tach, and other “key-on” electrical accessories on this model. Both the heater feed/fuse and the male spade at the bottom of the fuse panel are also powered by this bus, of the various suggestions mentioned here, little difference where the aftermarket tach is powered from other than the fuse panel male spade is unfused. And no, the tach did not “blow” the fusible link. The reason the link was replaced with a fuse holder appears to be unknown to the current owner, no point in speculating now.

The bulkhead connector by-pass described has been taking place since these cars were new and there have been a few variations over the years. The Nacho referral relates to a helpful current Moparts member who regularly contributes here on the electrical issues such as this and has his own slight variation of what has been described. Main point is get the charging current away from the factory spade connectors in the bulkhead connectors.

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Thanks very much for the clarification on both of these points. I did manage to dig up this post from NachoRT74, which provides a lot of detail on this modification:

Nacho's charging and wiring upgrade

Btw, I have intentionally avoided discussing the PO, or why any of this may have been done. I'm not interested in finger-pointing or assigning blame; I just want to fix these issues and make sure I don't damage anything. I knew the car had a few issues when I bought it. Dealing with all of this stuff is part of the "fun" (Ha!!) of the ownership experience, hmm?

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Well at least you know we are paying attention!

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True! Actually, I'm amazed that this post has gotten so much attention - nearly 800 views so far.

Anyway, I'm going to let this post die. I'm compiling a list of questions on the charge/fusible link wiring upgrade, so I'll create a new post just for that issue. Thanks again to everyone for helping me out on this problem - this forum continues to be an invaluable source of info!!

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Anyway, I'm going to let this post die. I'm compiling a list of questions on the charge/fusible link wiring upgrade, so I'll create a new post just for that issue.

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Whatever you do dont delete it or you will (likely) incur some severe wrath (not from me tho). I'd suggest keeping it going & just adding on addit'l related electrical Q's & it'll all be together.

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Whatever you do dont delete it or you will (likely) incur some severe wrath (not from me tho). I'd suggest keeping it going & just adding on addit'l related electrical Q's & it'll all be together.

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Ok, we'll save it as a case study in what NOT to do - LOL!!

My first question concerns re-installing the fusible link: A Powermaster alternator was added by the PO, but I don't know its output. I couldn't find any labeling or printing anywhere on it:





Assuming it's a higher-output alternator, is there any concern in using the standard (Dorman 85627) fusible link? Or is there a higher-capacity fusible link that I should use?

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Assuming it's a higher-output alternator, is there any concern in using the standard (Dorman 85627) fusible link?

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Fusible links are sized in relation to the wiring they protect and the expected current load. Unlike a fuse they are react slowly to allow for temporary spikes in current close to the max rated current. The battery is the main source of energy capable of lighting up wiring and causing electrical fires. Fusible links are there to protect the larger unfused vehicle wiring from the battery. If you are not planning to increase the size of the charge circuit wiring, and/or increase the total vehicle electrical loads, there will be no need to increase the size of the fusible link. If you planning to by-bass the bulkhead connectors, I would advise you to up the size of the wires to reduce overall resistance in the circuit, especially if that is a high output alternator (75amp or so), stick with the 16ga fusible link.
In your case, warm aftermarket fuse holder & replacement charge wire with the engine running, indicates too high of resistance for the given charging current taking place at the time. A replacement alternator with a higher than stock output rating and a battery that is anything less than at a full state of charge will contribute to heat at any point between the alternator and the battery where there is resistance. Bulkhead connectors in this circuit have been a problem from the beginning. There was a large C-body recall back in the seventies that involved adding a pre-fabricated parallel charge wire between the alternator and the fuse panel bus bar. Same basic design as this.

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