roasted amp meter and alterantor after jump

[Macgyverjr]

Yesterday, silly me left the lights on my truck. (79 j10 360) Got a buddy to jump it with his truck. After i got it started, smoke came from under the dash. amp meter was reading way over to the + side and the red wire was burning itself up on the back of the meter. Thought i could make the ten minute drive back to my house.... nay nay. Drove a minute or so and then a large blue flash from under the truck. lost lights and gauges, then the engine died a few seconds later. coasted into a parking lot and left it there for the night.

While there, i inspected things to get a head start for today. (i WAS planning on going hiking today... ) the red wire on the amp meter roasted the ring connector right off the post and burned 1/2' of insulation off the wire. Alternator smells of electrical hazard, death, and most definitely deep fried and long gone. reading about fusible links and bad ground connections this morning, ill start there. BTW, brand new yellow top and starter solenoid. ground occasionally disconnects on the solenoid. little wiggle and its right as rain for a while. PO was terrible with wiring and birds nested everything under the dash. hope i don't have to do anything with that.

Any help from y'all would be most helpful!

[nograin]

I'll tell you what I think happened.
Then you can start to figure out what steps to take.

Nesting material tends to have a low ignition temperature -

Yellow top is an AGM battery. Because the liquid is held in the glass mats, it can't dissipate heat quickly. Because of this AGM batteries need to be charged slowly. The hotter a battery gets, the more it resists recharging.

When the engine started, the voltage regulator responded to the low voltage of the drained battery by signalling the alternator to provide maximum output.

Unfortunately the jeep's charging system is not setup to handle high current charging for any length of time. The wiring got very hot, especially at the connections and the ammeter's shunt. Driving made it worse because the alternator was putting out even more current as the rpms went up.

If you ever have to do it again. Let it idle for a while, then shut off and let the battery cool. Turning on the headlights would have diverted some of the current as well. See the simplified diagrams of the charging and light system here

http://www.heritech.org/wagoneer/mggw5.html

Follow the wire from the alternator output to the battery. A dead battery is the worst (highest load) situation for the ammeter and charging circuit wiring. The next worst scenario, dead alternator, is not as bad because the current out of the battery will be limited to just the ignition and any accessories that are turned on. Even headlights on is not going to pull more than 15 amps, so maybe a total of 20-25 amps driving home with lights on. When the alternator is working and the battery is good, as you can see not much power goes through ammeter because the connections are all on the alternator side.



Deep cycle batteries may be worse in this situation as they are designed to be drawn down low and then recharged and therefore accept a big charge when dead. Someone else, or a manufacturer, should be able to confirm or dispute that.

[Macgyverjr]

thank you nograin! makes perfect sense to me. however, ive killed this battery once before and this has not happened to me. dumb luck?

the jeep is home. fuse link and alternator replaced and the amp meter bypassed.

[PlasticBoob]

Always sucks to read posts like this. This exact situation happened to me in a friend's very clean J-10 with a wet cell battery while we were driving - one of four FSJs I've seen with ammeters that melt down or cause fires, which is why I and many others always recommend bypassing it. Sometimes they just blow the fusible link, sometimes they melt down and turn every wire behind the dash into a ball of spaghetti, and on one occasion I saw one start a fire from inside the cabin.

The ammeter situation on these FSJs kind of reminds me of a nuclear reactor with a positive void coefficient (Chernobyl) - everything works just fine until one part of the delicate system goes out of whack (or there's an operator error) and it runs away to meltdown.

That blue flash you saw was the fusible link - either that or you had a criticality accident.

Thinking about putting up a sticky to warn/inform new owners of '85 and older rigs. Nograin, would you like to contribute to and manage that thread?

[Mr Carts]

Always turn the dead battery vehicles headlights on when getting a jump.

It sounds strange to do but it keeps from having a voltage surge.

[serehill]

Wow Just wow

The are tons of threads on this same issue. It's not rocket science or surges. It's simple terrible design & being in the wrong place at the wrong time. The manufacturers of these systems Chrysler & jeep were always adamant about not charging dead batteries with the alternator through the system because it simply can not take the long term amperage load. There are tons of threads about how to fix it permanently . Too many Naysayers that say it doesn't happen has created more victims.Another compound issue you mention is loose connections also contribute to the issue. the blown Fuse link indicates you need to check your wiring. The loose connection at the ammeter will not blow the fuse. A short or overload will. Repair the ammeter all you want its still a bad design.
The great thing about these circuits is the ammeter employees nylon collars on the stude which melt when hot. You are fortunate you didn't have a Car B Que & the fusible link did it's job.
A couple of options are is the voltmeter bypass or a shunt ammeter install.

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

[delta 18]

well.... this thread was different than I thought it would be. I was looking for pictures of a jeep waist deep in mud/water (thus roasting the electrical) after a "hold my beer"/dukes of hazzard moment trying to clear a water crossing on road that used to have a bridge.

[nograin]

I've thought about this for a few days. And seeing Rick's post, I basically agree with him, there's no need for a special thread for the reasons he's given and more. We're not there, where ever there is, and prescriptive solutions can go wrong. Also, there are many things about older vehicles that have limitations and weaknesses that many new owners are unaware of. This wiring arrangement for the charging system is not unusual. In fact, every Chrysler, Dodge, Plymouth car and truck from the 60s into the 80s came this way because even the base models got full instrumentation.
The important thing is for owners new and old to to develop their own skills to recognize, assess and solve. It's along the lines of "teach a person how to fish.."

[babywag]

Agreed, this isn't a Jeep thing. It's an old car/old wiring thing.

Anybody who owns an old car should evaluate the wiring/entire electrical system. ALL of it, and fix anything suspect, then enjoy the vehicle.

[Macgyverjr]

ok so new problem. no lights. No head lamps, gauge lights, park lights and left turn signal.. I've checked the fuses, wires to the head lamps and the head lamp switch. any help is great. no lights are not good.

[nograin]

Time for your trusty voltmeter

Assuming its the same arrangement as my '85, the headlight switch gets power direct from the alternator (as shown above). Inside the switch is a 20 amp circuit breaker (self resetting). See if its getting power from the alternator. You'll have to look up the color and or terminal. It will either B1 or B2. The other terminal gets power for the parking lights and instrument panel through the fuse block.

With one blinker hanging up, its possibly a bad ground in one of the turn signals or turn signal circuit.

PS. You can make that diagram into one for your vehicle. Go to my homepage -> wiring diagram page and download the GIF. Open in MS paint or similar. Then change the colors, wiring, whatever you need to match your year and whatever modifications you've made.

[PlasticBoob]

Well I should have made it obvious that my idea is to have ONE master sticky thread that contains links to the best ammeter threads we've had over the past 15+ years. A central repository so new people can easily see and be made aware of the issue without searching or randomly stumbling across it.

Obviously we still have many new owners who don't know a thing about the problem, and continue to get burned (in some cases, literally) by it. It's not common knowledge if you're a guy/gal in your twenties just getting into old cars. Heck I had already rebuilt my first engine before I was even aware of the ammeter issue on certain older vehicles. You can probably still find my thread on it here if you look for it.

[Macgyverjr]

So i still have no lights. Power is going to, and coming out of the light switch. no lights at all. switch does work. No power as the wires come out of the fire wall harness. Power to and out of the high beam foot switch. Looking at the schematic in the service manual, there is nothing else in between the foot switch and the headlight. So a short grounding it out? I'm running out of ideas. No running lights either....

[nograin]

Maybe I'm missing something but sounds like something at the bulkhead connector is loose. Don't think its grounding out 'cause the circuit breaker would open and the parking light fuses blow.

If its like the '85, the bulkhead connector is secured from the engine side by a large screw in the center.

[Tanker]

If you need me to come down an help I can. Let me know.