Accelerator Oscillator (AC): The Fourth Bill Williams Tool

The Accelerator Oscillator (AC) is the fourth and final piece of Bill Williams' original trading system, and it sits one layer deeper than the tool it's built from: the Awesome Oscillator. Where AO measures whether short-term momentum is stronger or weaker than the longer-term trend, AC measures whether that momentum itself — as read by AO — is speeding up or slowing down right now, before AO's own histogram has necessarily turned. It's a histogram like AO, it has no adjustable period settings like AO, and it oscillates around a zero line with no fixed ceiling or floor like AO — but the reading it gives answers a genuinely different question: not "is there momentum," but "is that momentum still gaining strength, or is it already running out of gas."

How AC Is Calculated

AC is built directly on top of AO rather than on price. Recall that AO itself is the difference between a fast 5-period simple moving average and a slow 34-period simple moving average of each bar's midpoint — (High + Low) / 2. AC takes that AO value and subtracts a 5-period simple moving average of AO itself: AC = AO − SMA(AO, 5). In plain terms, it's asking whether AO's current reading is running ahead of or behind its own recent average — momentum of momentum, one derivative further removed from price than AO already is. When AC is positive, AO is currently accelerating faster than its own short-term average would suggest; when AC is negative, AO is decelerating relative to that average. This is exactly the kind of layered construction Bill Williams built throughout his system — the Alligator smooths price, AO measures the difference between two smoothed averages of price, and AC measures the difference between AO and a smoothed average of AO — each tool one step more abstracted from the raw candle than the one before it.

Because both the 5-period fast average inside AO and the 5-period average AC subtracts from it are fixed by the original design, there is no settings dialog to configure here — check the parameters of either the MT4 or MT5 version of this indicator and you'll find no period input at all, only alert toggles. Every trader looking at AC on a given chart is looking at the identical calculation, the same way every AO reading is identical across traders, which keeps the pattern language (twin peaks, saucers, the two-bar trigger below) consistent from one chart to the next.

Reading the Bar Colors: Independent of the Zero Line

AC's bars are colored by comparing each bar directly to the one before it — green when the current bar is higher than the previous bar, red when it's lower — and that comparison has nothing to do with whether the bar sits above or below the zero line. This is worth sitting with, because it's easy to assume a histogram's color tracks its position relative to zero the way many other oscillators do. AC doesn't work that way: a bar can be green while still reading negative, if the current negative value is a smaller negative than the previous one — meaning downward momentum is decelerating and AC is rising back toward zero even though it hasn't crossed it yet. Just as easily, a bar can be red while still reading positive, if the current positive value is smaller than the last one — upward momentum decelerating even while AC is still above zero.

This decoupling is precisely why AC is described as measuring acceleration rather than direction. Direction relative to zero tells you whether AO is currently running above or below its own norm; color tells you whether that reading is currently getting stronger or weaker bar to bar, regardless of which side of zero it's sitting on. Glancing only at whether bars are above or below the line and ignoring color, or glancing only at color and ignoring the zero line, each gives you half the picture — reading AC properly means checking both at once.

Entry Conditions

The included custom indicator uses the classic Bill Williams "two-bar" trigger, which asks for two consecutive same-colored bars before treating a signal as real rather than acting on the very first color flip:

• BUY — two consecutive green bars in a row (each bar higher than the one immediately before it).
• SELL — two consecutive red bars in a row (each bar lower than the one immediately before it).

The alert code checks this by comparing three consecutive AC readings rather than just the latest bar against its neighbor: if the most recent bar is higher than the one before it, and that one is in turn higher than the one before that, both of the last two bars qualify as green, and the signal fires. The mirror logic applies to two consecutive red bars for a SELL. Requiring the second confirming bar, rather than alerting on the first color change alone, filters out the single-bar flickers that are common in this indicator given how many derivatives removed from price it already is — a single stray green bar inside a longer red run says much less than two green bars appearing back to back.

Accelerator Oscillator vs Awesome Oscillator

AC and AO share almost everything structurally — both are histograms with no signal line, both have zero period inputs to configure, and both oscillate with no fixed range around a zero line — but they measure different derivatives of the same underlying move. AO tells you whether short-term momentum is currently stronger or weaker than the longer-term trend; AC tells you whether that specific AO reading is currently strengthening or weakening. In practice this makes AC the more responsive, more sensitive of the two: AC's color can flip, and a two-bar signal can fire, a bar or two before AO's own histogram visibly turns, because AC is reacting to small changes inside AO before those changes accumulate into a full trend change in AO itself. The tradeoff is exactly what you'd expect from something more sensitive — AC also flips color more often on noise that never turns into anything, which is why it's read as an early-warning layer on top of AO rather than a replacement for it.

Parameters

Neither the MT4 nor the MT5 version of this indicator exposes a period setting, and that's not an oversight — there's genuinely nothing to configure. The 5-period fast average inside AO and the 5-period average AC subtracts from AO are both fixed by Bill Williams' original design, the same way AO's own 5/34 combination is fixed. Changing either number would stop producing the Accelerator Oscillator and start producing a different, custom-built indicator instead. The only inputs on the included alert code are EnableAlert and EnablePush, which control whether and how you're notified when the two-bar trigger fires — they don't change how AC itself is calculated or colored. This mirrors the same design choice already documented on the Awesome Oscillator and Fractals lessons: a handful of tools in the Bill Williams system are deliberately not tunable, trading flexibility for the guarantee that every trader's chart shows the exact same reading.

Completing the Bill Williams System: Alligator, Fractals, AO, and AC Together

Bill Williams built the Alligator, Fractals, the Awesome Oscillator, and the Accelerator Oscillator as four layers of one system, not four separate indicators to pick from — and AC is the piece that closes the loop. The Alligator answers the first question: is the market even trending, or asleep and not worth trading? Fractals answer the second: where, specifically, is a candidate turning point or breakout level? The Awesome Oscillator answers the third: does the move breaking through that fractal level actually have real momentum behind it, or is it a weak poke through the level? AC answers the fourth and most forward-looking question: is that momentum itself still building, or has it already started to fade — often visible in AC before it shows up as a shape change in AO's own histogram.

Read together, the four tools form a sequence rather than a checklist: wait for the Alligator to wake up and fan out, take a Fractal breakout in the direction of that fan, confirm the breakout with AO reading in the same direction, and use AC's two-bar trigger as the earliest signal that the momentum behind the move is still accelerating rather than already decelerating. Used in isolation, AC only ever answers its own narrow question about acceleration — it says nothing about whether the market is trending, where the relevant swing point sits, or whether there's underlying momentum at all. It's the last and most sensitive layer of a four-part system, meant to sharpen the other three tools' timing, not to replace them.

A Word of Caution

AC is a derivative of a derivative of price — AO is already the difference between two moving averages of the bar's midpoint, and AC is the difference between AO and yet another moving average — which means it inherits noise from two layers of calculation rather than one. Small, ordinary fluctuations in AO's own value can flip AC's color back and forth without any genuine change in the underlying trend, especially in a quiet or choppy market where AO itself is hovering close to its own 5-period average with nothing decisive happening. The two-bar trigger helps filter single-bar flickers, but it doesn't eliminate this — a market that isn't really trending can still produce a run of two or three same-colored bars purely from noise.

The green-while-negative and red-while-positive nuance covered above is also a common source of misreading: a trader who glances at bar color alone and assumes green always means "bullish, buy here" can end up trading a bar that's merely decelerating downward, not actually turning upward. As with the rest of the Bill Williams toolkit, AC is built to be read alongside the Alligator's trend state and AO's momentum reading, not glanced at on its own — treat a lone AC signal with the same caution the Awesome Oscillator lesson gives twin peaks in a choppy market, and weight it more heavily when the broader system already agrees a real trend is underway.

Download the Indicator

This custom indicator plots the Accelerator Oscillator histogram, colored bar-to-bar independent of the zero line, with automatic alerts when the classic two-bar trigger fires. It's available for both MetaTrader 4 and MetaTrader 5 below.

How to Install — MetaTrader 4

1. Download the accelerator-oscillator-alert.mq4 file below.
2. Open MetaTrader 4 → click File → Open Data Folder.
3. Place the file in the MQL4/Indicators folder.
4. Restart MetaTrader 4, then drag the indicator from the Navigator window onto the chart.

How to Install — MetaTrader 5

1. Download the accelerator-oscillator-alert.mq5 file below.
2. Open MetaTrader 5 → click File → Open Data Folder.
3. Place the file in the MQL5/Indicators folder.
4. Restart MetaTrader 5, then drag the indicator from the Navigator window onto the chart.

Both files are source code — open and review the full code before using it, for your own safety.