Can I install a higher output alternator?

[Buzz]

My Cobra has a Pro Design wiring harness and a 60 amp ammeter. My battery goes weak and dies after extended driving and I suspect my alternator - a 60 amp unit - is not working correctly.

Now, I have a new in box 100 amp alternator sitting in my garage. Can I bolt it on without fear of overloading my electrical system?

Even with lights on and fans running, I have never seen my ammeter read anywhere close to a 60 amp draw. Will the additional output capacity of the 100 amp unit ever come into play and if so what are the risks? Can I use it temporarily until I can import a new 60 amp unit?

BTW, I can't find a website for Pro Design to pose the question directly to them.

[rustyBob]

Buzz.....is there any of getting your battery checked for a load test just maybe its the battery and not the alternator........and yes you can use the 100 amp alternator.

[Buzz]

Thanks Bob - brand new battery fully charged before hooking it up. Holds its charge fine when the car is sitting, but after three or four short night drives with lights on and one 45 minute drive using the fans, it was just barely able to turn the engine over. I delivered the car to the paintshop yesterday for a new paint job and when I go to check on it today, I bet it won't be able to start.

The car ran beautifully when the new battery was just installed, but as battery output dropped, preformance began to suffer noticeably. I suspect I would not have made it much further than the paintshop before she died completely on me.

[mickmate]

You should be able to run a bigger alternator no worries but I wouldn't run all that amperage through my dash board and an ammeter. Those darn things have caused more fires in these cars than anything else I believe. I change them out for voltmeters.

[Buzz]

Hi Nick. By all that amperage - do you mean the additional 40 amps or just full alternator output in general? IOW - would the 60 amps be safe with the ammeter but not the 100?

Is there a simple way to lose the ammeter and hook up a voltmeter, or does that require more extensive wiring harness modification?

[rustyBob]

did you check the alternator to see if it was putting out 12 volts ? that 60 amper should be able to handle your power dray if you trips are short like you say.I have a 1 wire ford 75 amper that will run everything and keep the battery charged no problem.

[Barnsnake]

Some things to consider:
If you change to the 100 amp alternator you will need to make some other changes.
1. You should NOT run a 100 amp alternator through an original style ammeter. Ammeters are nostalgic, but just about every other aspect of them is a fire hazard. A voltmeter is passive and tells you what you need to know abut the charging system.
2. The wire from your current alternator-to-the-ammeter-to-the-battery cable may not be large enough to handle 100 amps. You may need to run a larger (or additional) wire to handle the additional amperage.

As noted previously, getting your current alternator checked/repaired would be the "easiest" solution to your problem.

Don't ignore the fact that hot-cranking problems are often due to starters, cables, and grounding. As a diagnostic, get it hot and in a location you can work on it. Verify that it cranks poorly or not at all. Let it cool and try it again. If it still cranks poorly, pull the alternator and get it checked.

Keep us posted on what you find.

[mickmate]

Take a look at the ammeter, they are what 50 or 60 amps? You don't want 100 amps going through that thing. The ammeters I have replaced I disconnect the feed wire to them and replace it with a suitably rated wire to feed the ignition and other electricals and make sure it's well insulated at connections. The voltmeter connects to anything that is live with the ignition on, it's really that easy. Some good points already mentioned on diagnosing your current set up, start there. I think the alternator should be putting out more like 13.8 volts if you're testing it, does your ammeter give you any indication of what's going on?

[Buzz]

The ammeter appears to function normally - showing a positive flow after startup and gradually moving to zero as the drive progresses. When I turn on the lights and fan, while the engine is running,though, there is only a very small deflection in the gauge. Before this problem started, there would be a more noticeable deflection with a gradual return towards zero.

The car starts, runs and restarts perfectly hot or cold as long as the battery is fully charged. The battery retains its charge if the car sits unused but it discharges in a few days of normal driving and use - while the car is running.

Here is a partial schematic of my wiring harness I drew up trying to sort out a previous issue:

QUESTION: Can I bypass the ammeter by simply connecting up the heavy feed wire (27) to the 25, 39 and 41 wires?

[Maurice Johnson]

Buzz,

If you are not a fanatic about authenticity, and are not familiar with shunt type meters, go to a marine site such as Balmar and see how they solve this problem. I can tell you, as a marine electrician, that your problem is an everyday event on cruising sailboats. You got some of the best problem solvers in the world right there on the island with you.

[Buzz]

I came across the shunt approach today while researching my problem on the web. Basically, a shunt of known resistance bypasses the ammeter, thereby reducing the current running through it. The gauge is then calibrated based on the calculated current flow that it will measure. This was used on Detroit products in years gone by. One fellow said his shunt wire broke and the ammeter was fried by the increase in current, causing a fire under his dash.

You are right about the marine guys, too. They find ways to make things work under very adverse conditions for any electrical system.

[rustyBob]

Buzz do a load test on that battery, i know its new but it sounds like it wount handle the load of the electrical system and check the 60 amper to see if its putting out the proper amps..............

[Gunner]

This whole thread is an illustration of how useless ammeters are in modern (or modern electricals) cars. They had something useful to tell in the generator days, but not now.

The basic instrument for monitoring a car's electricals is a voltmeter. It will tell you more useful information about your electrical system's health than any other single instrument. Voltmeters also do not have the downside of needing to route 40-50-100 amps through the wiring to the gauge.

Ideally, the voltmeter should be wired straight to unswitched battery voltage - it draws only a few microamperes, not enough to drain a car battery in months. Connecting it to unswitched voltage lets you monitor resting and cranking voltage, both important.

A simple guide to automotive voltages:

• At rest (engine and all accessories off): 11.5-12.5 volts. Any lower and you have a charging problem. It will sometimes drop to about 11 volts in very cold weather.
• Engine running (at any speed): 13-14 volts. Any less, and your alternator or voltage regulator is faulty. Any higher and your voltage regulator is faulty. This voltage should stay fairly stable even when you switch on and off every accessory including headlights and fans. If all of your accessories on pull the voltage down below 13 volts, you probably need a larger alternator.
• Cranking: Varies quite a bit with engine size etc. but should go no lower than 7.5-8 volts while cranking. Any lower and you've got a failing battery, a charging problem or (rarely) a starter problem. If it doesn't drop at least a volt or two, the starter may not be engaging properly or may have other problems.

Alternator, VR, battery, starter - that's the whole electrical system and this gauge is telling you the health of each item. Ammeters... may as well be an idiot light.

[Frank Messina]

I think your problem lies in the fact that you don't have ( or at least it's not on the diagram) an "exciter" circuit to start the alternator. Normally this would be the light on the dash but a 500 Ohm resistor does the same thing and tells the alternator to get to work. I'd switch out the ammeter for a voltmeter too.
Frank

[CowtownCobra]

By all means go to a 100 amp alternater. I did this and it helped solved an overheating issue.....really. I found with the 60 amp the elec fan would slow down at idle in stop and go traffic. But with the 100 the fan keeps its speed and the engine runs cooler on a warm day in stop and go traffic, the hardest condition I've found to keep a hipo motor cool.

[Buzz]

Thanks guys. I have decided to bolt on the 100 amp alternator and go with the voltmeter, but I still need to know if - based on my diagram - I can simply bypass the ammeter by connecting and insulating the wires going through it. That is; connect the 25, 39 and 41 wires to 27, heavily insulate and wrap, and leave them behind the dash.

[KSW64]

Just as a reference, a 100 amp alternator, under max. output, takes about 9-10 horsepower to turn, similar to an AC Compressor. Most vehicles with an alternator rated over 75 amps are usually equipped with either 2 belts or a serpentine setup. I have a 105amp MSD single-wire alternator with a single v-belt. If the belt slips just once, it will play havic on your electrical system. Symptoms includes battery not charging, problems mimicking a bad ground, irregular voltmeter reading. Ask me how I know.

One new battery, 2 flatbed rides home, one trip to the alternator shop, and finally a $6 belt. I bought two.

Happy charging!
Ken

[mickmate]

Hey Buzz, nice diagram. Yes connect 27 and 41 securely together and insulate them.

[Ronbo]

I can't really agree about the comments concerning ampmeters, idealy both it and a voltmeter give you a true look at the vehicle electrical system.

Voltmeters cannot accurately indicate alternator output or system loading, period. If your concerned about high current levels under the dash, worry about that Lucas ignition switch.

As far as a 100A swap out, upgrade your ampmeter and verify your lines between the alt - meter - battery are at least 6ga fine strand wire. Obviously someone who actually understands vehicle electrics is your best insurance against dash fires. Your average mechanic who thinks he understands it is the #1 cause of mishaps.

I've got almost 30yrs as a electronic technican so I'm well aware of car mechanics electrical work, not to mention what the car mfg's sometimes do (especially Ford).

Actually I should mention why the fires occur, a condition called "thermal runaway" occurs in overloaded electrical components: over current causes heat, the heat raises the resistance of the component causing even more power to be dissipated resulting in a cycle which eventually reaches the flash point in the insulating materials - fire. Think of super conductors, they have to be cooled to sub-zero tempatures to work. Scientists are still striving for the ellusive room temp superconductor.