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Curious coolant temperature cycling
This may just be a case of having too much information but I wonder if other people have observed the same behavior.
Running VMSpc (records data from DDECIII) in log/graph mode I have noticed a curious cycling of my coolant temperature. It cycles from 187 to 198 degrees give or take a couple of degrees. The cycle period is about 15 seconds. It does that regardless whether I pull a steep hill or cruise at 50% load in the flatlands. I don’t have a cooling problem. The temperature never rises above 200 deg F even on the longest/steepest pulls. But even on the longest pull it cycles down to 187 deg F showing a lot of reserve capacity. The analog gauge shows the same cycling but it just changes a needle-width so it is harder to see.
I’m speculating that this oscillation is caused by the thermostats opening too aggressively causing them to close again as the temperature falls too quickly. Essentially the thermostats provide too much gain causing the oscillation. It could also be that the thermostats are optimized for summer conditions and end up overcontrolling during colder seasons. Any other VMSpc users?
Gerald
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10 Replies
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I don’t see that kind of temperature swing that fast,I do see 10-15 degree change but it is over a 2-4 minute time frame.
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There are two devices which cause engine temps to cycle. The normal one is the thermostatically controlled, 2 speed fan. If the fan is locked then the engine will cycle on the engine thermostat. What I can’t figure out however is what Jack said. I cannot imagine an engine capable of raising the coolant temp that many degrees in so few seconds.
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Like Jon pointed out, the rapid temperature rise is really
strange, just doesn’t figure. I don’t think it’s the thermostats. Here’s a
possible cause. It could be that your low speed on the radiator fan is not
working. When the ECM calls for the low speed to activate and it doesn’t, then
the engine gets hot enough for the ECM to activate the high speed, since it’s
not under a great load it only takes a short time period for the engine to cool
down.Does the Silverleaf show if the fan is off, low or high?
I remember there is way to lock the fan on high by breaking
a terminal off a certain relay. Maybe Jon or someone will know a simple way to
lock the fan on low, then you could make a short test run to see if the rapid temperature
oscillation improves.Ken
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Break off the #85 terminal on the R78 relay in the engine electrical box to lock the fan in low speed or the same terminal on R79 to lock the fan in high speed.
Actually for anyone who wants to keep a means of locking a defective fan clutch it would not be a bad idea to keep a relay with the terminal broken off for just that purpose.
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Gerald,
I’ve been down this path before chasing the wrong problem. I would say your readings are perfectly normal. Setup your VMS to also monitor intake air temperature. This will tell you if the swing is due to normal thermostat cycling or the fan kicking on and off. If it’s the thermostat you will not see much change in intake air temperature. If it’s the fan cycling you will see the intake air temeperature swing with the engine temperature change.
If you have Prevost dash air and it’s on the fan will be locked on high speed. and you should then only see the thermostat opening and closing affects on the temperature.
FWIW, the low speed fan relay get’s its butt worked off when dash air is not on. At about $3 a relay I highly recommend you replace these every 75K miles or thereabouts. There are 3 to 5 relays depending on how your coach was equipped. The important 3 are the Fan relay R99 and the low and high speed clutch relays R78 and R79. If you do replace these make sure you get the ones with the blocking diode, not the ones with the protection resistor or no protection. The diode is there to protect the DDEC from voltage spikes that natuarlly occur with coil relays.
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As long as Gil is offering advice on this let’s make sure we are clear on what is happening. You stated originally your COOLANT temp cycles rapidly and the temperature spread and the duration of the temperature swing suggest strongly to at least three of us that it ain’t happening.
The laws of physics are likely to require a substantial amount of heat to be generated by the engine and absorbed by the coolant to accomplish the temperature difference in the allotted time. We just don’t see that happening.
Further if in fact the cycling is due to dash air locking the fan clutch it is the thermostat in the engine cycling, but regardless of whether it is the fan or thermostat controlling the temp the issue is you cannot generator enough heat from the engine to make the coolant temp rise that much in that short a period of time.
The only temperature I have ever seen rise that fast was the coolant temp at the outlet of the transmission retarder. But if your coach does not have a retarder, and if you are not using it while braking or going down a hill then that is not what is being measured.
I cannot understand how intake air temperature creates the wild swings in the coolant temp.
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Agree, there is just too much thermal mass in the system for the fans to cause temp swings in the 15 second range. Minutes yes, seconds no. Nevertheless, that is what the DDEC temp probe sees.
The only way I can explain the rapid cycling is that the thermostats are actively mixing cooler radiator water with warmer engine block water. As the thermostats open up, the temp drops rapidly which causes the thermostats to restrict flow again, which in turn mixes in hotter engine block water and so on. This could just be the way the system works and the only reason I’m seeing the cycling is because of the 1 second resolution of the VMSpc data stream.
I will check the relays to make sure I have low fan speed operation.
Thanks for all the ideas,
Gerald
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Yes, 15 second swing is too fast. I should have said that. However, the swing can be fast from the combined effects of the thermostat opening and low speed fan engaging.
My reference to the correlation between air and coolant temperature has to do with the affect of the fan. Maybe, I wasn’t clear. If the air temp doesn’t move directionally with the coolant temp then the fan is not cycling. Meaning the fan is locked on or not turning on.
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BTW, I went through this with a Detroit engineer. I was being told By someone my engine temp was normal when I didn’t have dash air on. I was pursuing having the DDEC reprogrammed. What ended up with was a low speed problem that was easily corrected.
This was a good lesson in that free information is what you paid for it.
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I don’t know how the DD thermostat is constructed, but those I have seen for other applications usually use some type of bi-metal spring that has the thermostat fully closed at one temperature and fully open at some higher temperature. I am not aware of any that snap open or closed.
Genrally they gradually open as temperature rises and gradually close as temperature falls. As a result on my 97 coach when I had the fan locked (which yours likely is) I saw the temp rise to 195 and fall back to 190, but over a period of a few minutes. The rapid rise and fall in your coach may not be what your engine or dash gauge shows because the engine and the gauge sensor is likely a different reading point or temperature from the sensor used by DDEC which is probably what your computer reads.
I would guess if you found the gauge sensor and the DDEC sensor they are in two different locations.
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