David Einhorn is going all in on General Motors (GM - Get Report) .

Einhorn's hedge fund, Greenlight Capital, quadrupled its stake in the automaker during the first quarter of the year to more than 54 million shares, according to quarterly holdings filings with the SEC released on Monday. The billionaire activist is undertaking an activist push at GM and per FactSet owns about 3.6% of the company.

Institutional investment managers with over $100 million in qualifying assets are required to file their holdings on a quarterly basis with the SEC on a form 13F. Such forms come with a 45-day delay, meaning that while Greenlight's filing was released on Monday, it signals holdings as of March 31, 2017.

Greenlight on Monday sent a letter to GM shareholders highlighting what it said is a "gap between GM's stagnant share price and its intrinsic value." The fund noted GM completed its IPO at $33 per share in 2010 and today hovers at $34. Greenlight is pushing GM to split its stock into two classes and add three nominees to its board.

"As best we can tell, GM does not recognize its $34 stock price is a problem and has no plan to address the discount to its intrinsic value," Greenlight wrote. "Shareholders should not be as complacent or content as GM's management and its Board of Directors have been in addressing this discount."

GM's board has rejected Einhorn's proposal and is urging shareholders to vote against it in at its June 6 meeting. The automaker in April trimmed CEO Mary Barra's compensation ahead of the shareholder vote.

In his first quarter letter to investors, published by ValueWalk, Einhorn acknowledged GM will be a "tough fight" but held "the math is on our side."

"We believe others recognize that the stock is deeply undervalued," he said.

Beyond GM, Einhorn picked up new positions in Perrigo (PRGO - Get Report) , Conduent (CNDT - Get Report) , Micron Technology (MU - Get Report) , Alere (ALR) , Varex Imaging (VREX - Get Report) and Clipper Realty (CLPR - Get Report) and made sizable additions to his holdings in Consol Energy (CNX - Get Report) and Fred's (FRED - Get Report) .

Greenlight cashed out of Avangrid (AGR - Get Report) and Yelp (YELP - Get Report) , and it significantly reduced its stake in Apple  (AAPL - Get Report) . 

GM is Greenlight's largest holding as of the end of the first quarter; he also has a call option on the company. Greenlight's other top positions are in AerCap Holdings (AER - Get Report) , Apple and Consol Energy.

Einhorn, 48, has a net worth of $1.53 billion according to Forbes.

In his first quarter investor letter Einhorn also discussed his short position in Tesla (TSLA - Get Report) , acknowledging it was one of his fund's biggest losers of the period. "Perhaps as the prospects for tax reform have dimmed, the market has regained enthusiasm for profitless company that aren't at risk of paying taxes," he said, taking a swipe at Elon Musk's brainchild.

He expressed continued confidence in the position. "In due time, we expect these bubbles to pop," he wrote.

At the Sohn Conference in New York last week, Einhorn pitched a short on oil services company Core Labs  (CLB - Get Report) .