Warning: Bradt Climbing and Hiking Guide
The route description in the the Bradt Ecuador Climbing and Hiking guide is wrong. It doesn't mention ANYTHING about the difficulties along the crest to the summit. It doesn't say a thing about the chimney climb, the notch before the summit, or the fact that the summit is a tower that requires technical climbing. It gives the impression that it is a little scrambly walk to the summit once you are on the rim.
I am considering the "base of the climb" to be the last camping spots about 10 minutes below the glacier.
This is approached via the Ambato-Guaranda road. From Quito take the Panamericana south to Ambato then take a bus in the direction of Guaranda. There is also direct bus service from Quito. From Ambato it is only about an hour and 10 minutes further. The ride should cost about $1. All the way to Guaranda is $2.
After crossing the Rio Blanco there is a small community and a red and yellow sign on the left for the Escuela Colegio Manuela Cañizares. This point is far enough along that you might begin to feel that you missed it. Chimbo will look quite close and it seems like Carihuairazo is getting behind you.
Get off at the sign and walk up the road toward Carihuairazo. Within the first 20 minutes (of walking) there are a couple of forks on the road. Take a right, then a left. Continue up the road for another 1.5 hours or so.
The road appears easily drivable by a passenger car.
You want to leave the dirt road and cross the páramo near Piedra Negra. There isn't an obvious path though. Use your best judgement. Piedra Negra is the obvious and cliffy hill on the left side of the road. You want to approach it's left side.
After about 40 minutes to an hour you will reach small ponds below the cliffs. This is good camping also, but I recommend hiking another 1-1.5 hours further.
Here begins a decent trail taking you up and around Piedra Negra. Once behind it, the trail crosses a large, rocky expanse with excellent views of the route. Start looking for camping on the other side, partway up the moraine. There are a number of obvious spots. Elevation is around 4650+ meters.
From the last camping spots ascend 10 minutes to the glacier. The traverse across the lower glacier is very low angle: less than 20°. Once in the rockfall area the angle increases. In spots it reaches perhaps almost 40°. There are crevasses throughout the glacier but they are small and easily avoided. Your goal is the highest point on the glacier. You shouldn't need more than an hour to reach it.
From this false highpoint (the glacier extends further beneath the rockfall) climb through loose rock to a gully in the short cliffs below the crater rim. Take the right side of the gully. Once past the cliffs and at some yellowish dirt, climb up and to the ridge there is a little trail there which will take you to the rim.
Follow the rim left to the summit tower. Begin by traversing right of the first small rock outcrop. The second rock outcrop is climbed directly via a 4th or 5th class (YDS) chimney. Rope shouldn't be necessary. Walk along the top to the final notch before the summit tower.
In years past the notch was not the obstacle it is now. Now climbers should rappel into it in order to climb the tower. Abseil 8 meters or so. There aren't very good anchors.
The tower is 10-15 meters of technical rock that is difficult to impossible to protect. Most people stop at the notch.
For the glacier travel ice axe and crampons are nice to have. "Nice" because the lower glacier is very low angle and could just be carefully walked. The upper portion has some steeper parts but those could be avoided by taking a path through the thick rockfall below the cliffs. I do recommend having both an ax and crampons though. You probably won't feel that a rope is necessary.
The technical portion of the climb is difficult to judge. The rock is rotten or unfeatured enough to make placing pro and anchors a problem. One or two big slings would probably be sufficient to rappel into the notch before the summit off some mediocre anchors.
As for the tower, perhaps this is a good candidate for bolting...