It's time to look under the hood of this clunker and see what we're driving around here.

On Friday morning, Wall Street's biggest banks kicked off earnings with Wells Fargo, but the real action starts Monday when Citigroup reports.

Why?

Because what everyone will be looking at is investment-bank trading revenue — or the lack thereof — and how it affected Wall Street's bottom line in the second quarter.

The warnings have been coming since this spring when JPMorgan forecast a 20% quarter-over-quarter decline in trading revenue. Citigroup CFO John Gerspach said much of the same at a separate conference.

Day after day, traders have watched in horror as volume and volatility withered across asset classes and just dried up.

Interest rates and volatility have remained low, and heads are rolling at fixed-income desks up and down the Street. The carnage already started at Morgan Stanley, and there are reports that the fixed-income trading staff at Goldman Sachs will see the layoffs soon as well.

Since no one needs to execute trades, brokers are suffering too, as you can see from the chart below from BTIG's chief global strategist, Dan Greenhaus.

This is just how diseases spread on Wall Street. Last month, JPMorgan CFO Marianne Lake said "too much capacity" in bond and currency trading could lead to revenue so anemic that layoffs and compensation cuts may have to be made throughout JPM's investment bank — the biggest investment bank in the world.

That means sectors outside the land of trading doldrums may have to take a hit if the ratio between revenue and compensation reached unsavory levels — those levels being something like 35%.

Lake said this was a "cyclical" issue, problem is that no one knows when the cycle will end.

So what we're about to see is how bad "bad" really is.

Here's what analysts are estimating, according to Bloomberg:

• Citigroup (reporting Monday): Q2 revenue of $18.9 billion expected, down from $20.5 billion in Q2 2013.

• Goldman Sachs (reporting Tuesday): Q2 revenue of $8.0 billion expected, down from $8.6 billion in Q2 2013.

• JPMorgan (reporting Tuesday): Q2 revenue of $23.8 billion expected, down from $25.2 billion in Q2 2013.

• Bank of America (reporting Wednesday): Q2 revenue of $21.8 billion expected, down from $22.9 billion in Q2 2013.

• Morgan Stanley (reporting Thursday): Q2 revenue of $8.4 billion expected, down from $8.5 billion in Q2 2013.

Strap in, people — this could get bumpy.