I have a TC 40 D tractor that after the tractor has been running for awhile the cold start light comes on. When this happens the tractors rpm drop down. Sometimes the light flashes on and if running at low speed nothing happens. Any suggestions.
By "cold start light" do you mean the glow plug light? If so, you need to replace your glow plug timer. You can buy one from New Holland for $70 or buy this one on eBay for around $13 with shipping. The box says Kubota, but it will work just fine in your New Holland. You'll have to remove the cowling on the left side under the steering wheel to get to the relays and the timer.
Ouch, that's ridiculous. However, I did mention that this one is available and is a nearly exact duplicate. You paid the xtra money for New Holland's part number. Parts for our Shibaura built tractors have become so high they can only be called price gouging.