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Fan Clutch High Speed Relay Test
I had my fan and clutch assembly out to take care of some surface rust on the steel braces and brackets at the radiator. This gave me a perfect opportunity to check the smoothness of the fan clutch as well as a look at the turbo to make sure the shaft was free and smooth turning without a lot of play.
After getting it all back together, decided to break off the #85 tab on the R78 relay to get the fan to run high on idle to be sure my fan alignment in the shroud was good. In a earlier post at “Ask Prevost” the Prevost Tech made this comment:
When you have the tab off the relay and you simply turn on the ignition switch(don’t start the engine!), go to the rear and grab the fan blade. It should not turn as it should be locked in electrically. If it still turns but with resistance then I would look to the fan clutch as the problem.
Mine does not lock up, but you can feel a resistance when trying to spin the fan. Once the engine was running, the fan was spinning on high as intended. After reading the above, this suggests that my clutch fan may not be working properly because it’s suppose to lock the fan. I also tried it on R79 (speed 2) with the same results.
The bus has never run hot and I know the high speed works as I can see it on the gauge when it cycles and now wondering if the Prevost information above may be incorrect.
Anyone else tried this lock up test with the ignition on and relay modified, or willing to give it a try? Sure don’t want to buy a clutch if not needed.
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6 Replies
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Tom,
It sounds like it is working based on you stating that it was running on high speed.
I have used this relay bandage before and it worked fine. What you need to check is whether voltage is actually getting to the fan with the ignition on and the engine not running. This is easy to do at the clutch connection. FWIW, R99 controls voltage to the clutch. It’s possible that Prevost didn’t want the fan to engage when the engine is started, even though that’s unlikely. What they may have done is put a delay in the circuit energizing R99. You could also turn on the ignition and not start the engine and wait 5 or so minutes and see if the clutch then engages. The timer, if present, is likely not timed for more than 5 minutes.
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Gil,
Since you used this before, do you recall if the fan was hard locked with the ignition on, engine not running? The fan is getting power, hence the resistance when turning by hand with ignition on.
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Tom,
I’m pretty sure I could still turn the fan by hand. It was more difficult to move in high. I can try it again tomorrow if you’d like.
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Gil,
No need to try it, thanks, I did and here’s the outcome:
Relay 78 with 85 tab off and ignition on, motor not running – Resistence felt when spinning the fan.
Relay 79 with 85 tab off and ignition on, motor not running – Fan locked up, couldn’t move by hand.
Bill Jensen explained it in further detail and confirmed my clutch is working properly.
Tom,
Here is the setup for the two vehicle types. For the XL and XL2 we have R78 that controls the LOW speed or speed 1 and R79 that controls the HIGH speed or speed 2.
For the H3 series we have R84 for the LOW speed and R85 for the HIGH speed.
So if you break the tab (85) off of R78 you will be in low speed all the time except when the coolant temperature rises enough to get the high speed to come on. So turning on the ignition and not starting the vehicle you would find that the fan blade can be turned with just a little resistance. If you used R79 then the fan is locked on high and it moves with engine rpm all the time. It would be then locked when you did the ignition on/no start test you mentioned.
Ideally you should see if the fan will go into low speed normally but not high speed. Then you could use the modified relay in the high speed position to get better cooling all the time.
Keeping the fan locked in electrically this way is not good for long periods as this was a means to get you back home or to your destination and have the problem fixed.
On the extreme end of this issue would be that the fan has failed electrically. Then you need to use the bolts supplied on the fan clutch and you bolt the fan into fixed mechanical drive. This also is a temporary fix as you can eventually break off the tabs and then you have a free-wheeling fan with no control at all.
I hope this explains this a little better.
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Tom,
Glad to hear all is well. It’s weird that you didn’t feel it lock up the first time you have the relay in #79.
FWIW, I chased a fan clutch problem a couple times. It worked well some times not so well others. At the suggestion of others I replaced R79. I’m now confident it was always good. The culprit was probably R78. At 3 bucks a piece and when in doubt, replace them all, including R99. I should have thought the issue out more as I never had the problem with dash air on and it engages the high speed relay.
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It was my fault on R79 Gil the first time, the 85 tab still had just a little left after I broke it off and it made contact. This time I didn’t push the relay in all the way so 85 would not make any contact.
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