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Upgrading the Alternator

When not enough current is being produced

by Graham Pring

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This article was first published in 2009.
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The modifications covered in two previous articles (see Chasing Overheating and Chasing Overheating - Again!) had led me to the point where I thought I had fixed the overheating problems I was having with my V8 Cobra replica.

But doing some further testing of the cooling system indicated that perhaps I had another problem, one of a completely different sort.

What happened was that while on-road testing, the engine cut out on me and wouldn’t start. Voltage testing indicated a dead battery. A shortish walk home to pick up a set of jumper leads and a good large battery and the car was back in business.

Keeping an eye on the voltmeter showed that with the large electric radiator fan running, battery voltage was going backwards at an unreasonable rate. This told me that if I was stuck in traffic, the coolant wouldn’t overheat but the battery and engine management would die - and so would I, of embarrassment!

And it would all be even worse if I had the lights on and my foot on the brake...

The Alternator

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The alternator I had used to this point was fairly large with an output of 85 amps. It was a Bosch BXU 1285, or 12 volt 85 amp universal type, which was rated as a “14 Volt 14/85 amp”, with 14 amps being the minimum and 85 the maximum.

It had a single ‘A’ section belt drive and was mounted on a modified Holden V8 bracket. The modified bracket was required as the original mounting position was on the other side of the motor, and that spot was now filled with remote oil filter assembly, lines, oil line pressure switches and a Hobbs switch.

The BMW Alternator

I needed a better alternator, but where to get it? And at what cost? As BMW seemed to have fitted 140 amp alternators to many of their models for a long time, I went to a BMW rubbish bin, the one from which I had previously obtained my ‘new’ large electric radiator fan. Since I had the BMW fan, surely an alternator from the same vehicle would suffice?

I soon found one with a seriously worn-out set of rotor bush contacts.

Checking with a couple of auto electricians revealed that I could get a new set of contacts. However, they had to be genuine, came attached to the rotor and the assembly was just under AUD$600!

Not the amount of money I wanted to spend...

I chased up the Web and found I could get a complete reconditioned unit from the United States for about half that, however the mounting was going to be difficult and the type of pulley/belt type could be problematic.

So what else would fit and do the job when the engine was at idle?

More Alternators

I talked to a local auto electrician about what was available and he said I would probably have to try a few, then handed me the Bosch Starter Motors and Alternators for Passenger Vehicles and Light Commercial Vehicles catalog that he had - and left me to it. That was very nice of him; he could have told me to get lost!

After a brief look through I realised there was no way I could make a decision there and then. I needed to understand a lot better how alternators worked and what I was actually looking for. After all, I had an 85 amp alternator that wouldn’t keep up – why wasn’t that enough power output?

As the Bosch book graphs gave only a speed versus amperage output, I also knew I had to find out what the ratio was between engine crankshaft pulley and alternator pulley.

To get some more information, I looked up my Bosch Automotive Handbook, 4th edition, which is getting on a bit now since it was printed in 1996, however it gave me a couple of things to think about.

According to the Handbook, alternators run ratios of 2:1 to 3:1 with ratios up to 5:1 in large utility vehicles, and they normally have 12 or 16 poles.

The designs are:

  • ‘Claw-pole’ with compact diode assembly and an “external fan providing single-flow axial ventilation”.

  • ‘Compact’ alternator, a variant of the claw-pole with two smaller internal fan elements with advantages of higher operating speeds, less aerodynamic and magnetic noise plus longer brush service life.

  • ‘Salient-pole’ alternator, normally used in touring coaches and a very large unit with an external regulator.

  • ‘Winding-less’ rotor alternators, normally used in long service life equipment like railroad and construction equipment and weighing “somewhat more than a claw-pole”.

The first two are used in cars and the step-up ratios they run are 2.15 to 2.5, so I was not surprised to find that my car had a ratio of about 2.3:1. With ribbed belts, the ratio can be higher due to the shorter bend radii that can be achieved with that type of belt.

These ratios are based on the maximum speed limit of the type of alternator, with the claw-pole having a limit of 15 to 18,000 rpm and the compact style having a limit of 18 to 20,000 rpm.

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Reading this information made sense of the diagrams in the back of the Bosch catalog that showed outputs of alternators based on speeds, however the auto electrician couldn’t tell me if these were engine speeds or alternator speeds - and the catalog didn’t say!

An engine redline of 6500 rpm with a ratio of 2.33:1 equals maximum alternator speed of 15,145, so I was safe with either type of alternator, even if I decided to rev it a bit harder at some stage or put a spacer between the V-belt flanges and so get a higher ratio by ‘sinking’ the belt in the groove.

More questions revealed that the speeds in the catalog were actually alternator rpm, but most of the diagrams stopped at 6000 rpm. When asked “What happens after 6000 rpm?” the answer was “it basically flat-lines from there and uses more engine power to run it”.

Holden VT V8 Alternator

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After re-reading the catalog, I settled on a VT Holden V8 unit which I would have to modify to fit. It was relatively cheap at under AUD$120 from a wrecker, widely available and had a 60 amp minimum output. So off I went and bought one.

I had noticed a Bosch BXH1250 had a ‘V’ pulley while a BXH1253 (14V 60/120A), which I had purchased, had the serpentine, multi-rib unit. However I easily obtained a universal ‘V’ pulley that fitted; it appears that all the Bosch shaft diameters are the same.

When measured, both the serpentine and the ‘V’ type pulleys had the same diameter.

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I took the alternator home and measured it up. In case it wasn’t going to work, I cut off one of the mounting lugs - rather than cut up the mount! I then wired-up the terminals and modified the tensioning bar slightly. The main mounting lug was the same width as the original BXU1285 unit, so that made mounting easier.

The pulley had to be spaced with different washers which came off the old unit and then the engine was able to be started.

But there was no alternator output at all!

I waited, assuming the slip ring contacts (brushes) might not have been brilliant, or making good contact, and sure enough after about three minutes, it started working, intermittently at first then full time.

I checked the alternator output with the battery fully charged and taking only 1 amp, which meant that the alternator was handling the current draw from all items.

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The draw at idle was 14 amps, comprising the load of the fuel pump, injectors, computer, instruments, etc. Turning the headlights on saw another 15 amps being drawn. I then switched on the cooling fan and saw a total draw of 59 amps at 800 rpm idle.

If I put my foot on the brakes, the alternator wouldn’t cope and so the system started drawing current from the battery.

However I am sure that I can manage to live very happily with the current (sorry!) setup as the ‘off’ time of the fan will allow the battery to recharge and run everything without problems.

What was going on

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I drew up some graphs based on engine speed and alternator outputs converted from the data in the back of the Bosch catalog. Going back over this data a few times, I started to understand why the original 85 amp alternator couldn’t cope.

I also saw when I did this that if I had purchased the universal 120 amp alternator (BXU12121A, 14V 2/120A) which has the same maximum output as the BXH1253, it would not have worked for me at idle, which was the main criterion.

I kept thinking what would happen if I was out for a drive on a hot Sunday night on the way back home in a traffic crawl, with my foot on the brake fairly regularly, with the 500 watt sound amplifier in full flight and the air-conditioning auxiliary fan flat out.

I don’t have actually have the amp or the air-conditioning installed; however it gets you thinking.

Most modern cars have ways of conserving power, usually by a dedicated computer which oversees which items can be run without overloading the system and/or increasing idle speed to compensate. They tend to drop items from power supply, so you might find that your seat heaters don’t work anymore or the air-conditioning is turned off without your intervention.

Probably the only way to know is if you have a voltmeter you often look at.

You can run two alternators in parallel if you want to and there are other alternators available up to 200 amps and liquid cooled if you need them.

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When tested, the maximum current my car will draw is just over 78 amps. Checking with the output graph and on-car confirms that at around 900 rpm the output is 80 amps, so in the worst case scenario, I can program the engine ECU to wind the idle speed up by 150 rpm if the battery voltage goes below a set point.


If you have a heavily modified car – and especially if you’re often stuck in traffic – don’t overlook the importance of having an alternator big enough to cope. And remember, in most cases, it’s the output at engine idle which is the really important figure to get right.

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