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Old 09-11-2020, 10:13 AM   #1
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Onan 7.5kw Temperature Sender

I was getting the code 33 (over-temp) on my Onan 7.5kw HDKAJ-H. I was convinced I had a temperature sender problem as the fluids were all correct. I pulled the unit (had to remove front cap to do so). I replaced temp sender, water pump, thermostat, and belt. Of course replaced all fluids and filters while I had it out. I flushed out the radiator with water and took a quick glance at it. In retrospect I wish I would have looked more closely but was working under a time pressure. I am still getting the code 33 over-temp if I try to run both ACs in the hot Florida weather. Sometimes it will run for almost an hour before it shuts down. If I only run 1 AC it seems like it will run continually. I didn't replace the temp-sender holder, it did seem like there was some crud around the sender interior hole which I did seem to clean fairly well with a round wire brush. My plan (input welcome) is to run it again until it shuts down and immediately drain the coolant and measure the temperature to determine whether it is truly over-heated or whether I am getting a false reading. I guess my question is: If this is viable plan what temperature should the hot coolant be? Thanks
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Old 09-11-2020, 07:59 PM   #2
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If it's overheated for real, the system will be pressurized. Opening the drain (or even the cap) could be hazardous. A guy I used to work for, a long time ago when I turned wrenches for money, got severely burned from fingers to elbow by opening a cap without taking precautions on an overheated engine. He was a very experienced, competent mechanic who was careless just that once. He owned the shop but worked one-handed for a few weeks.

I'd use an IR thermometer on a radiator tank instead. At a guess, anything very far north of 190*F is suspect. The service manual says 160-220*F.

I'd also examine the wiring harness. I don't know how that specific sensor works, but many change resistance to ground and will indicate high if shorted to ground.

The manual says, for code 33:

Corrective Action:
1. Check the engine coolant level and add coolant and repair leaks as necessary (p. 9-1, 12-1).
2. Check for and remove any objects blocking the air inlet or outlet openings in the bottom of the genset.
3. Reduce the number of appliances connected at the same time. (Note that high altitude and high ambient
temperature decrease engine cooling capacity.)
4. Look for a broken or worn coolant pump drive belt (p. 12-3) and replace as necessary (p. 12-1). (Visible
through the maintenance access door in the skid-base.)
5. Remove the air intake grille in the skid-base and check to see that the cooling blower (p. 11-1) is
secure and that the blades are not fouled.
6. Check for air leaks in the housing—missing or loose housing panels and access covers, including
the access covers for the fuses and AC terminals.
7. Clean and flush the cooling system to remove coolant passage fouling.
8. Remove the top housing panel (p. 6-4). If loose, reconnect engine harness spade terminal connector
E3-1 to the coolant temperature sender (p. 12-1, 19-1). (If the lead is loose and touching ground, the
controller will sense a high engine temperature.)
9. While the top housing panel is off (Step 8), check for fouling of the radiator air passages and clear
out dust and debris with compressed air directed from below (reverse of normal air flow).
10. Replace the coolant thermostat (p. 12-1).
11. If the genset overheats only underway, see the coach manufacture regarding air baffles or other
means to direct cooling air into the genset.
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Old 09-14-2020, 04:15 PM   #3
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Thanks for the response and warning. I did lightly burn myself as a teenager trying to pull a hot radiator cap so I certainly intended to be as cautious as possible. Of all the tools I have surprisingly an IR thermometer is not one of them however I have borrowed one. Thanks for enclosing the corrective actions, most I feel like I have covered already which kind of brings me back to having to determine whether it is actually over-heating or a sensor problem. As soon as I get a chance I intend to let it runs till it shuts down and measure the temp with the IR and then if there as any question I intend to pull the cap to release pressure followed by immediately draining and measuring coolant temp in a bucket. Thanks again.
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Old 09-14-2020, 05:14 PM   #4
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Deposit build up

Quote:
Originally Posted by mdc99 View Post
I was getting the code 33 (over-temp) on my Onan 7.5kw HDKAJ-H. I was convinced I had a temperature sender problem as the fluids were all correct. I pulled the unit (had to remove front cap to do so). I replaced temp sender, water pump, thermostat, and belt. Of course replaced all fluids and filters while I had it out. I flushed out the radiator with water and took a quick glance at it. In retrospect I wish I would have looked more closely but was working under a time pressure. I am still getting the code 33 over-temp if I try to run both ACs in the hot Florida weather. Sometimes it will run for almost an hour before it shuts down. If I only run 1 AC it seems like it will run continually. I didn't replace the temp-sender holder, it did seem like there was some crud around the sender interior hole which I did seem to clean fairly well with a round wire brush. My plan (input welcome) is to run it again until it shuts down and immediately drain the coolant and measure the temperature to determine whether it is truly over-heated or whether I am getting a false reading. I guess my question is: If this is viable plan what temperature should the hot coolant be? Thanks
I had a similar problem in April. My generator would run for almost an hour with the a/c on and then throw a code 33. My guess was thermostat. Pulling it out of the housing I noticed some deposit build up inside the cavity and on the stat that could be scraped off with my fingernail. The thermostat checked out on my stove in hot water with a thermometer and opened right at tempurature. With that I flushed the block with a descaler. The eruption foaming liquide that came out of the thermostat cavity when dumping in the descaler was like a geyser. That confirmed to me a blockage in the cooling system. I flushed the whole system with the descaler and several times with clean water. reassembled, added coolant and haven't had a problem since.

My rig is an 04 Phaeton and I have only owned it about a year so have no history. The generator only has about 600hrs on it. Getting ready to go on a big trip it is the first time the generator ran that long. For all I know it had the problem when I purchased it.
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Old 09-15-2020, 01:35 PM   #5
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Just want to make sure you're getting an actual code 33. You're getting a 3 for the first level fault code, then hitting stop/prime and flashing out 33. Right? I know it's confusing. I don't know why they assigned a fault to the 33 position. It's just confusing.
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Old 09-16-2020, 10:32 AM   #6
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Yes it is code 33. I ran it yesterday for an hour at 18amps with no problems, I then bumped it up to 40amps and it shut down within a couple of minutes with the code 33. I immediately drained the coolant into a bucket (carefully releasing the pressure via the drain cap was not to difficult). I couldn't get to the radiator to get a reading off of it. However I did measure the temp of the drained coolant with the IR thermometer and the highest reading I could get was 175. Obviously that is within operating range. I am now flushing the system with Thermocare. I guess my hope is that scale is somehow affecting the temperature sender, although I thought it was clean around the opening. I figured it was worth the $20. gamble before pulling the unit again. Thermocare recommends running the engine for 4-5hrs before draining so it has been running at approx. 15 amps for a couple of hrs now. Will report back. Thanks for any suggestions.
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Old 09-21-2020, 10:32 AM   #7
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So flushing with Thermocare has seemed to resolve the issue. Of course I had to flush with water 4-5 times afterwards before refilling with coolant. If I had time to waste I would drain the hot coolant and re-test temperature. However for the time being i am just happy it is running fine.
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