My 10KW HDCAA Genset is cured. I wanted to post this follow up with some reference info in case someone else has to diagnose an overheat issue. When the generator shut down it had what felt like decent air exhaust flow out the bottom fan outlet and no abnormal noise from a slipping belt. Generator was running fine then just a quick shut down and a blinking one code at 350 hours of use. The genset had been pulled before I bought the coach and the temp sensor had been changed for the same shut down issue. I spoke with Onan tech support and got the sensor data and then from the parts department learned how many sensors, belts, thermostats, and water pumps have been sold for this genset. This is a good way to narrow down the possibilities. It turns out thet they sell a lot of sensors, next comes belts, then a handful of thermostats (5) and just two water pumps over the last 3 years. So belts or sensors are what we are likely to have to deal with.
The problem is that due to the cabinet design we have no access to the sensor or the belt area without a major hassle of pulling the cabinet top, front, and end covers. Gerald warned me that it is not a lot of fun given the access issues and he is absolutely right. Getting the cover bolts on and off is the problem. Pulling the fan assembly and changing the belt is relatively easy. I found that using a mini 10 inch woodworking bar plamp between the end of the slotted alternator bracket and the socket on the adjustment bolt was an easy way to move the alternator to adjust the belt tension before tightening. There was a lot of belt debris on the fan and pullies that needed to be cleaned out.
What was learned is that my belt was 1/8 inch narrower than a new belt due to wear slippage and had stretched to have very little tension, yet there was no sign that this was going on until an overheat shut down. The concern is that the generator itself is cooled by the fan air flow and that was running about 40 degrees hotter (generator housing temp) with the old belt at only 30% load and an ambient of 70 degrees. I'm not sure what the winding temp was but it must be much hotter. The engine over temp shut down will protect the engine from self destructing but this belt may slip for a long time without reaching the engine shut down limit and I'm not sure what temp the generator windings would reach under load. Obviously good cooling airflow would make all the components last longer and knowing if you have a slipping belt and getting it changed as a maintenance item when needed, before being shut down in use on the road, has its advantages. One side note...Onan/Cummins will not be able to provide the right belt without knowing your pulley width. The serial number vs belt PN/pulley type info they have is incorrect. For reference the over temp shut down system limit (blinking one fail) is set at 239degrees for 10 seconds which corresponds to some sensor value that is less than 40 ohms.
I have two recommendations; if your belt is 4 years old or older (Onan maintenance schedule would have you change it at 5 years) get an IR gun ($30-$50) and take some measurements on the radiator face under the left side of the genset to get a reference and see where your temps are running. The second is at some point if have the covers off you might want to remove the lead from the temp sensor, extend it with a wire loop to a point where a spade lug could be uncoupled by the access door so you can measure the resistance to ground without 3 hours of disassembly.
With the temp numbers I have posted you should have the information needed to diagnose the condition of your fan belt/cooling system and watch it as your belt ages. With the slipping belt the radiator temp kept climbing. With the new belt the temp stabilized in about 20 minutes and remained stable beyond that point. (see values on chart) The radiator measurements were taken from the bottom with the genset running and all others were taken through the access door right after shut down. The generator should not be run without all the covers on due to the fact that the cooling relies on a sealed cabinet air path. Included are some sensor vs temp measurements that will help determine if that is your problem area. Thanks again to Gerald, if he had not gone there first, I'm not sure I would have tackled this job. Hope this helps.
Later Ed
