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How to replace the temperature sensor on an Onan 7.5Kw generator model HDKAJ

Time required to do the work: 8 – 10 hours

Challenges: Removing the generator from the RV and accessing the temp sensor housing with a drill

Tools/things required:

  • Set of large SAE wrenches or sockets to remove the generator support brackets. I think the largest bolt/nut was ¾”
  • Set of metric sockets and wrenches. I think you’ll want an open and box end wrench to fit the temp sensor. I don’t recall the exact size but you can verify before starting work
  • Impact wrench. This is really handy for dealing with the 1.5 million bolts holding the side and top shield on the generator
  • Lots of wood blocks
  • Some way of lowering and raising the generator. We used a front end loader with pallet forks. A forklift would be even better. You might be able to do this with some transmission jacks
  • An assistant. This is mostly for raising and lowering the generator. Even with two people this took a long time as the front end loader doesn’t lift straight up or down so there was lots of checking both sides and front/back of the generator as it moved up/down
  • Something to plug the fuel line and return fuel line. A pair of golf tee’s work well. Not being a golfer I used a couple of small bolts
  • Short flat bladed screwdriver to remove the wire from the temp sensor. There’s not enough clearance for a long screwdriver
  • Power drill with drill bit that is slightly larger than the temp sensor body
  • Muititool with a wood/metal/fiberglass cutting blade

When we purchased our 2004 model year 39J in May of 2020 the generator had 45 hours on it. The coach had 11K miles on it and we were the 3rd owners. The PO purchased it back in 2007 and kept it indoors since the purchase, using it once a year or so. We knew there’s be issues due to lack of use vs things being worn out. I think the temp sensor on the generator was one if those lack of use issues.

The symptoms of the failing temp sensor on the generator were hard starting when cold. The glow plug is automatically turned on when the temp sensor detects cold weather. Our glow plugs never came on. We bought the RV in the spring and did all our testing in warm weather. The generator would also turn off occasionally when it had a load on it. Not always but sometimes. The fault code indicated the engine was overheating.

I tried to replace the temp sensor with the generator in-place but it was too tight a fit. I was able to get the cover bolts off but couldn’t remove the side covers. I decided to wait until I had some help to pull the whole generator so we ended up taking a trip that didn’t really require the generator driving from WA to SoCal and back before doing the repairs.

When we returned, I found the cover bolts had come loose. * I realized I should have used Loctite when reassembling the cover. We had run the only a few hours on the trip so the bolts really need Loctite or something to keep them in place.

To remove the generator I built some ramps out of some scrap 8x8, 6x6, and 4x4 lumber, plus a lot of 2x8s. We needed to raise the front of the coach high enough so we could lower the generator and pull it forward under the coach bumper.

We used pallet forks attached to a front-end loader to support the generator from below. Make sure you have some wood blocks to support the generator on the forks. We got the forks/blocks in place then after removing the bolts from the generator support were able to lift the generator an inch or so which allowed us to removed the supports and slowly lower the generator. Of course you have to remove all wiring and fuel lines from the generator prior to removal. Make sure you mark with line is the fuel line and which is the return line. They look the same.

Once out of the coach it was pretty easy to remove all the cover bolts. * Keep track of the long and short cover bolts. They are two lengths that are almost the same. The long ones need to go back in the holes that require long ones. Be careful to not use the long ones on the corner where the coolant recovery tank goes. You must use short ones there.

With the side and top covers off you can access the temp sensor. The temp sensor can be removed with a socket or wrench but there’s a fiberglass heat shield directly above temp sensor that makes accessing the sensor difficult.

When I tried to remove the temp sensor the sensor broke off. Apparently this is not unusual as there’s multiple postings about this issue on various RV message boards. Unfortunately the fiberglass heat shield blocks access from directly above the temp sensor so if you want to drill out the body of the temp sensor you need to either remove the fiberglass heat shield or remove the housing that holds the temp sensor. If your temp sensor comes out in one piece, take a moment to celebrate then simply install the new one. Don’t forget to use plumbers tape of some sort of thread sealant. If your temp sensor broke apart then keep reading.

Don’t pass up the chance to replace the belt and t-stat while you have everything apart!

I realized that it was pretty easy to cut a large hole in the fiberglass heat shield with a multitool which gives you a straight shot to drill out the temp sensor housing. I found this to be very quick and easy compared to disassembling the engine to the point where I could remove the heat shield or remove the temp sensor housing. Before drilling remove the t-stat and stuff a rag down the housing that holds the temp sensor. You can push the rag past the temp sensor port so that when you drill out the temp sensor housing the rag can catch the debris/dust. I don’t recall the drill bit size but if you have the replacement temp sensor you can measure that and go up in size a little bit.

Once the temp sensor is relaced use some fiberglass cloth and epoxy resin to glue to part of the heat shield that you removed back into place.

Don’t forget to use Loctite or some other thread-locking material on the cover bolts when you put the covers back on.

Putting it all back together is the opposite of taking it apart. The majority of the time was spent on the removal and installation of the generator. Not because it was hard work, it just went very slowly as we didn’t want to scratch or bend anything.


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