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Old 12-19-2019, 06:31 AM   #141
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Originally Posted by ziaptrn View Post
As best I can tell after lots of research - next is call to Horton....
1. The Cummins engine sends either "on" or "off" to the fan for operation. The engine itself achieves a set temp and then triggers the fan.
2. The fan clutch on our rigs is controlled electronically via "Pulse Width Modulation" which is managed by the harness that runs from the engine to the fan clutch. The harness speeds up/slows down the fan (supposedly) as needed. I wonder now if the harness is the true culprit?



I also installed a switch that disconnects the 5amp fuse (or reconnects depending on position) and mounted the switch in the dash. What I can tell you is that the switch does work, but it takes a little time i.e. it isn't instantaneous while the engine is running. It is easy to see when it does engage if you watch temps and transmission temp especially. I was surprised at the length of time required, but found that if I wait maybe 60-90s, the fan will lock and engage and the reverse when the switch is put back in 'normal'.


Like you, the issue is also resolved by pulling over to the side of the road, turning off the engine, counting to 4, and then re-starting. This works every time and is immediate.


The other odd thing is that the error is not reliably reproduced. Furthermore, it seems to be worse in cold weather versus warm i.e. doesn't happen when the ambient temps are in the 60-70 degree range to start, but when it is cold out, it requires intervention more reliably.


No one can find anything wrong with the engine or fan clutch - new radiator, cleaned precooler, new thermostat, new radiator cap, installed 'Guard Dog' (made by Horton for this specific fan clutch)



For some reason all shops insist on having it happen while in the shop and won't believe the photographs that I show (over-temp light, transmission in 200s, engine in 200s followed by resumption of normal temps after flip the switch or re-start). No one has "ever heard of such a problem) at Cummins, Freightliner (including Gaffney), or Winnebago.


Just to validate your experiences if nothing else....
Did you ask them specifically to find and use Freightliner Service Solution 3737? This walks them through the process of diagnosing the fan clutch and will reliably identify the fault. They can't possibly say they won't believe if they run through the Service Solution and it identifies the fault.

You've unnecessarily been subjected to the shops that like to just swap things (at your expense) without determining the actual cause of the problem before performing maintenance action. We see it ALL the time - throughout the maintenance industry.
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Old 12-19-2019, 05:57 PM   #142
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Service Code 3737

Yep. I told all of them about the 3737 advisory - including Gaffney. Still "we've never seen anything like this and can't reproduce it". I've shown them all postings about the 3737 advisory as well.


Last call to Freightliner had them tell me it "sounds like a Guard Dog problem - take it to Tolleson AZ for further workup".


I'm a little stymied; no need to fund further 'workup' at another service center as you state. I should note that the rig is on its 3rd fan clutch (I didn't have any replaced, but PO did). Currently at 66k miles, and when the clutch operates properly, it does just that....it works fine. There are no leaks, fluid stains, no dragging, and it was (supposedly) tested to see if it was weak and/or slipping and they say no - it is functioning properly.
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Old 12-20-2019, 12:13 AM   #143
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Botewcc --

Welcome to the forum! There are many good members who are willing to help anytime!

For your overheating issue, when was the last time that you cleaned your charge air cooler (CAC) and the outside of your radiator? A dirty CAC would cause overheating regardless of fan speed as a dirty CAC restricts air flow through the CAC and radiator.

When was the last time you had your coolant changed, radiator flushed and thermostat replaced? A bad or "sticky" thermostat can caused overheating by not opening properly.

Just a couple of zero / low cost thoughts before spending big money on fan clutch replacement or finding ways to keep the fan in high speed at all times.
I have power washed the CAC and radiator and visually checked with camera on the engine side. I have changed the engine coolant temperature sensor and thermostat (as recommended by Cummins) and changed the coolant within the last year (4500 miles). After all those measures, still had the intermittent overheating issues, usually within 10 minutes of first starting up.
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Old 12-20-2019, 12:18 AM   #144
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I see where some have wired a switch to the fuse so they can turn it on and off at will. That might be the best solution. I'd be wary of allowing the fan to run continuously for a number of reasons.

I had my fan clutch replaced and it never gave me a problem for the next 40K miles that I owned the coach. If you plan to keep your coach for a long time, I'd go that route.
Replacing the fan clutch with a new one would seem to be the right answer EXCEPT for so many people having more issues with replaced fan clutches that were not that old. Id hate to have the work done only to have this issue happen again In 20-30k miles
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Old 12-21-2019, 06:35 AM   #145
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Go with the toggle switch to add a little peace of mind. As long as you know everything else has been addressed and is clean. My biggest concern is being in heavy traffic and no place to pull over. Flip the switch and cool it back to normal.
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Old 12-21-2019, 06:50 PM   #146
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Thanks for your feedback....I agree. Before installing the switch, there were a few hair-raising moments looking for somewhere to pull over and re-start, as you noted.
For now, I'm going to just see how it goes :-)
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Old 12-29-2019, 12:01 PM   #147
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5amp toggle switch bypass

I have a 6.7 Cummins in my Freightliner but it is a front-mounted engine. I assume I have the same fan clutch as the rest of you (part # HOR9803007) and I therefore run the risk of having the same overheating issue.

I want to be sure I understand the safeguard of adding a switch to the setup.

The 5amp fuse completes the circuit, right? And the way the system is set up, if the 5amp fuse blows, the fan is switched ON to keep the cooling at maximum until the fuse is replaced. So what the additional switch does is break the circuit the way a blown fuse would and force the fan ON.

Am I understanding this correctly?
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Old 12-29-2019, 12:49 PM   #148
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Yes that is a great summary

As I understand it.
Pulling the fuse opens the circuit which causes the fan to go in to a default "always engaged" or "running nonstop" mode.


The switches that we have installed 'pull the fuse' electrically when activated. When switched back, the circuit is restored and the fan should not run nonstop.
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Old 12-29-2019, 03:07 PM   #149
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The switches that we have installed 'pull the fuse' electrically when activated.
Excellent! Makes sense. Now I gotta just find that fuse.
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Old 12-29-2019, 03:27 PM   #150
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Also - does anyone here have the part# for the fan clutch in question? I'm wondering if I have the same clutch as the rest of you.
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Old 01-02-2020, 07:47 AM   #151
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I would like to no what controls the fan clutch ? Relay/thermos/ or what ?
The reason I ask is it only happens when first startup when engine is cold, after a pull over and shut down 1 or 2 times, it works fine 192* and it doesn't do it every day I start up.
It is my thinking that if it was the fan clutch it would do it all the time..
I installed a switch at the fuse panel and when I initial start up with cold engine I have the switch in off possession until I reach a rest stop maybe a hour or so.
I then go back and turn on switch, and so far everything has been working good.
As I said before, only at first startup when engine is cold Did I have the problem. My thoughts are it is not the fan, it is whatever controls the clutch fan, if it was a bad fan wouldn't it do it ALL the time ?
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Old 02-01-2020, 02:43 PM   #152
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Update for Fan Clutch/Overheat

The part number for the fan clutch in question is:
Fan Clutch: Horton HOR 9805002

The Viscous Speed Controller (aka "Guard Dog") is part number 994671.

The purpose of my posting here is to cite a possible fix.
I'm not sure yet - because I've *thought* it was repaired before only to act up again - BUT....

I spoke with a Cummins rep at length when we were in Quartzsite a few weeks back. He was an engine guy and wasn't aware of the problem (they all say that) BUT, he did have a suggestion.
Based on what I was telling him, and especially the episodic nature of the problem plus the fact that stopping and re-starting seems to temporarily "fix" the problem, he said that it sounds like a sticking relay to him. He said that the relays get old, and also get water inside of them (yes, even in the desert where we live - as I pointed out), but that one idea would be to replace the relay for the fan clutch.

The relay is on the PDM (power distribution module) box that is tucked away in the electric cord bin behind a non-obvious removable panel as discussed previously in this thread.

The Fan relay is #R10 and it is the second relay from the right on the bottom row. My rig has 10 of the identical relay all plugged in to the PDM module.

I replaced the fan clutch relay #R10 and drove several days back from Quartzite without a single hitch and what I would have called totally normal performance. The rig temp rises a bit going up hills and drops going down - but zero overheating and the fan shuttled on and off appropriately.

So. While I'm a bit irritated that perhaps an $11.00 relay (2 for $9.00 on Amazon), I'm hopeful that this may have been the problem all along.

If this doesn't turn out to be the final fix and the problem recurs, I'll post again.


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Old 02-01-2020, 05:53 PM   #153
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Is that relay located in the same box where the 5 amp fuse that controls the fan is located ?
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Old 02-01-2020, 06:10 PM   #154
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Relay location - yes

I realize now that I could/should have posted a pic!
I'm attaching a photo of the box opened. Fuse #15 for the fan clutch has been pulled (now replaced with a switch), and Relay #10 is the one immediately to the left.

Both are circled in red.
Hope this helps.....
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