Ideas, theories, and wild speculation: '84 300zx electrical issues


I’m plastering the web looking for ideas and pointers as I prepare to investigate my car’s electrical issues. If you have any theories, please, post them! So here’s the story…

On Friday, I decided to move my old 300zx out of the garage so I could do some cleaning. I haven’t driven it in a week, since it’s been snowing and icing and freezing for the last few days. I turn the key…and nothing happens. Not a single click. The sound of silence. I notice that the headlight switch is turned on…must have drained the battery…

So, I proceed to jump it. Connect the cables in the right order with both cars off, turn on my “electricity donor”, get in my “recipient”, turn the key and…I see a small amount of smoke come from near, or under, the battery. I also heard the power antenna motor engage when the key was in the accessory position (didn’t raise it like normal, the stereo didn’t turn on either…maybe just enough voltage to engage the motor, but not enough to engage anything else?), but no dash lights, no cranking, no other signs of life. Turn off the car, disconnect everything, investigate. I didn’t find any evidence of burnt wires or other damage. First instinct was that I mixed up the cables, but I verified that they were +/+ and -/- while disconnecting. To be honest, I then thought it was corrosion on the battery terminal and jaws burning up.

I still wanted to charge the battery, so I disconnected the negative terminal from it, and charged the battery up with my other car for 15 mins. No smoke from the battery. To see if any life could be seen, I then reconnected the negative terminal, got in, turned the key…and smelled electrical burning. Again, the ONLY thing that engaged was the power antenna motor. No lights, no cranking. Of course, I disconnected the battery again, and began tracking down the source of the smoke. Traced it to the main bundle of wires on the passenger side foot well, below the battery and behind the ECU.

I haven’t traced it any further or started pulling out the ECU and wiring bundles. I also haven’t had a chance to check all the fuses and fusable links yet…please forgive me, it’s been -5F out since this happened, and I don’t have a heated garage!

So, has anyone ever seen symptoms like this? No signs of life at all before jumping. None after, EXCEPT for a motor on the accessory bus…and smoke coming under the passenger dash. Smoke started when jumping car and continued UNTIL battery was disconnected.

My plan of attack:
-Is the battery good?
-Check fuses and fusable links
-Check battery cables and ground links
-Remove and test ECU
-Check cables and connections behind ECU/footwell area- my best bet it that a 30 year old connection shorted out
-Check stereo wiring- I recently installed a new head unit and speakers…maybe I screwed something up. Strange that it would work for a month though

Any ideas? I’ll post here with updates…electrical work isn’t my forte, so I may need pointers if you can offer them! I’m also concerned about damage to other electrical components of the car.


I didn’t have time to read all your ideas and symptoms, but what struck me as a potential problem was when you attempted to jump it with the battery stone dead. That puts a lot of load on the electrical system of both cars. It is preferable to at least partially charge a stone dead battery before attempting a jump. I’m assuming this didn’t do any damage to the car doing the jumping, which is a good thing, but the high current flow, especially at the first connect, could have damaged something important in your car…

I’m assuming at this point the car still doesn’t even crank. Here’s what I’d do. Charge the battery to 100% full charge over the course of two days, with a battery charger on the low current mode. Then have the battery load tested. If it tests ok, recharge as before, then measure the voltage on the two starter motor terminals during attempted cranking. If both are above 10.5 volts, the starter motor is in need of replacement. If one or the other is below 10.5 volts, work backwards to find out why.

I’m no expert but I agree the first thing would be making sure the battery is ok and fully charged with a battery charger. You may have a stuck relay, bad electronic component, or mice chewing on the wires causing a short?

I agree…work back from battery. It was probably stone cold dead. No damage to the other car. Hadn’t thought about potential stress from not charging battery first though.

As for the starter, it never cranked at all. In fact, nothing electrical in the car turned on before, during, or after jumping, except for the power antenna motor when he key was in the accessory position, which sounded “different” from normal and didn’t raise the antenna.

I’ll look into the battery state though, and I’ll let you know what I find.


+1 for @Bing and I might add that problems like these are incalculable.

When you stated you saw a puff of smoke near the battery I was thinking you blew out a main fusible link but then you stated the smoke continued as long as the cables were connected. If that is a different area than where the battery is at then you need to find out where that smoke came from, find the short and eliminate the problem before reconnecting power again. You could use an ohmmeter at the battery connections to check for a short condition as you make the hunt for the trouble.

I like your plan of attack procedure. It is a logical way to find the problem. Usually power is tied to a fuse/distribution panel under the hood which may use fusible links for the protection so be aware of that. Usually a smaller wire comes from the battery to supply power to the panel under the hood and that may be a fusible link. The fuses in the panel under the hood are usually always hot to the battery. If connection is good to that point then move on to the ignition switch and dash fuse panel. Most or all of the dash fuses are provided power through the ignition switch. Power usually passes through the fuse panel under the hood and then to the ignition switch.

One other thing to be cautious of is the main alternator output wire connection. That wire is hot to the battery at all times so disconnect the battery first before working on that lead. Also, don’t bother removing the ECU until you first get power restored to things as removing it may not be necessary.

Thanks for the write up Cougar. The next step is finding my multimeter and starting proper testing…might need to buy a new one.

Here is what I have discovered thus far-

  • The ECU/ECM, which is located below and behind the battery, in the side wall of the passenger footwell, is dead. My guess is that smoke came from it…upon inspection it smelled strongly of electrical burning. I opened it up and found multiple burns and some corrosion on one of the PCBs.
  • NO fuses or fusable links were blown. The stereo fuse is also fine.
  • VISUAL inspection of connections and wires between battery and main cable harness/ECU didn’t show any signs of burning or shorting out. Wiring insulation actually seems to be in pretty good shape for 30yo car. No signs of moisture penetration in wiring duct of ECU cradle.

Again, I’ve gotta find my multimeter and see if that uncovers other faults.

I have yet to get the battery tested, as I can’t get it to charge. My charger will not connect to it, even though I jumped it yesterday. Does this mean that it’s too depleted for the charger to connect, or that a cell is dead?

So, theoretically, is it possible that I fried a 30 year old ECU by jumping a totally dead battery? To be honest, I think I may have smelled electrical burning faintly as soon as I turned on the other car (before I even turned the key on the 300) but I wasn’t perturbed since I was jumping the cars.

One way or another, it fried itself when I jumped it…I just really hope that it was due to a bad battery or electrical stress on a 30yo computer and not some deeper wiring issue. I’ll gladly pay $250 for a new ECU if it means avoiding tearing out the wiring harnesses…I guess I really need that multimeter!



Thanks for the picture

What does the other side of the circuit board look like?

You’ve got nothing to lose at this point. Perhaps you could try to carefully resolder those solder joints and carefully clean the burn marks.

I’m thinking you won’t be able to get your hands on a brand new $250 ECU. I doubt there are any.
You may be able to get your hands on a rebuilt module, or pay somebody to fix yours.

I’ll go a little off-topic . . .

Awhile back, my battery charger wouldn’t turn on at all. Stone cold dead. Before giving up on it, I decided to take it apart and have a look. I found a few bad solder joints and a fried resistor. I carefully resoldered the bad solder joints and soldered in a new radio shack resistor. AFter putting it back together, it worked perfectly again.

A really dead battery will appear as a short circuit to most battery chargers and that will activate a protection circuit. The usual trick to bypass this is to hook up two batteries in parallel, one fully charged and the dead battery and then hook the charger to the pair. That will not appear as a dead battery anymore and the charger should start working. After a couple of hours, remove the good battery.

If the charger still won’t work, the battery is toast and you may have to charge the good battery as it will probably be depleted by the dead battery.

You’re right…not brand new…but a reman’d one is still new to me. :wink:

While the other side is fine, the burns are pretty deep and the thing stinks terribly of burnt electronics. Made the whole garage smell like a fire was burning after while. Even if it can be fixed, it’s never going in my car again!!!

The charger is in working order- will connect to and charge a known good battery.

I have ordered the re-manufactured ECU. Less than $200 when you ignore the core charge.

To recap-
The Good
-Fusible links have continuity when removed, no fuses are blown
-Plastic wire pin connections and pins to and around ECU do not show any signs of burns or overheating
-Couldn’t find any wires with visible damage to insulation

The Bad
-Haven’t yet tested continuity of wiring (started to rain, and car is stuck outside!)
-Haven’t tried reconnecting the battery when ECU out to see what gets power…or just what happens!

The Ugly
-No idea if battery is okay- won’t take a charge, and I can’t get it out on my own to get it inspected (accident with suspension spring last year left my right hand very weak…can’t grip heavy things)

So, assuming that I don’t find any other electrical faults, should I be afraid of installing the “new” ECU? I don’t want to fry it too.

That’s a big assumption though…so any ideas where should I look for continuity or proper voltage before I put it back in?

If the battery will not take a charge, replace it. You could swap it out with a known good one once the new ECU comes in. I would not trust that dead battery n the next attempt.

If the battery has been sitting dead flat for more than a couple of days stick a fork in it, it’s done.

Looking at the picture of the ECU board I think you were wise in not trying to fix it. It had multiple signs of damage to it. You definately need to also replace the battery. The other concern I have is the alternator. If there is excessive AC or DC voltage coming from it that may damage the replacement ECU. If there is an external voltage regulator for it that needs to be checked also. A bad battery shouldn’t cause damage to the ECU but the charging system could if it has a problem. It may be a good thing to remove those items from the car and have them tested externally from it. Be sure to have the AC voltage checked along with the DC. There should be less than .1 volt of AC ripple voltage when tied to a battery and no more than 15 volts of DC.

If you what to see if things will work and you are able to start the engine when you replace the battery you can disable the alternator by disconnecting the wiring on the back side of it. If there is a problem with it you won’t hurt anything by having it disabled. You can at least verify that other electrical systems are working and can start the engine.