If it only has an output voltage of 1.4 volts, it sounds like it has failed internally. Take it to an Invacare service centre or retailer and ask them to check it. My guess is that it need replacing. Here is the manual for it, and it states, 24 volts at 8 amps or 10 amps depending on country of issue. It charges 12 x 2 volt lead acid cells at the same time, cleverly making sure you can't use a standard 12 volt car charger.
https://www.invacare.co.uk/sites/gb/files/csv_migration/product_docs/technical_docs/DTEC012618_1636951~C_OptiCharge_UM.pdf
You can get them on eBay and Amazon
Invacare 1123249 8 Amp Battery Charger; AC Power Line Cord
https://www.amazon.ca > Pronto-Nutron-Series-Battery-...
24V 8A Battery Charger with XLR Connector for Jazzy 1450, invacare tdx 3, Invacare Pronto M51, Wheelchair, Sunrise Medical, Quickie, Drive Medical Electric ...
Rating: 3.7 · 4 reviews
invacare 24 volt battery charger products for sale ' eBay
https://www.ebay.com > invacare-24-volt-battery-charger
Get the best deals on invacare 24 volt battery charger when you shop the largest online selection at eBay.com. Free shipping on many items ' Browse your ...
..