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850 will fire on pull start but won’t run

T
Oct 27, 2023
39
6
8
MT
I could really use some help troubleshooting this issue on my sled that I’ve spent the last 3 months rebuilding (replaced bulkhead, engine, suspension, wiring harness, etc.). On first startup after all of this work, I was able to get it to run for about 30 seconds last week, but now it won’t start. It will fire when I pull start it so it’s getting spark and fuel is combusting, but it immediately is extinguished on the second pull attempt (spark plugs are drenched with fuel).

The weird thing is that last week when I got it to run, I unintentionally had plugged the coolant temperature sensor into the ignition coil harness (same plug style 🤦‍♂️ - I’m not sure how it was even able to fire like this). Now that I have everything seemingly connected properly, I’m having issues.

Will these engines run at all without TPS? I’m suspecting an issue with my sensor as the input voltage drops from 5VDC to 0.5VDC when it it is plugged into the harness (short in sensor?). I did previously have the sensor functioning properly yesterday though so I don’t think this is my only problem. It’s almost like I fried something or have a weird short somewhere. I should also mention that my temperature sensors are all reading erroneously (way, way off). I’ll attach 2 screenshots from EzLynk (1=TPS unplugged, 2=TPS connected).

Any input is greatly appreciated. This was a big project and I’m going to be bummed if I don’t get it out for at least a couple rides this season.

IMG_3244.png IMG_3245.png
 
P

Pitabroed

Member
Jan 16, 2018
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That's not the input voltage, it's the sensor supply voltage what I can see. But if it drops like that (and tps input voltage seems to be changed as well) you probably have a short in the tps sensor. Does it change from 0-100% when you push the throttle?

The engine temp seems to be wrong too, the coil voltage maybe fried it.
 
T
Oct 27, 2023
39
6
8
MT
That's not the input voltage, it's the sensor supply voltage what I can see. But if it drops like that (and tps input voltage seems to be changed as well) you probably have a short in the tps sensor. Does it change from 0-100% when you push the throttle?

The engine temp seems to be wrong too, the coil voltage maybe fried it.
The TPS was working two days ago and I could watch the % change when I modulated the thumb throttle. At that time, I still had the same no start symptoms so I think this is only part of the issue. I now have the TPS completely disconnected and the sled still won’t start (I assume it should be able to idle without TPS).

ECT isn’t the only temperature sensor that is way off. Intake and exhaust are erroneous too.
 

RBalazs

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Strange for sure. I like looking for obvious things first and the darn tether not being connected has bit me at least while troubleshooting a sled…
 
T
Oct 27, 2023
39
6
8
MT
Strange for sure. I like looking for obvious things first and the darn tether not being connected has bit me at least while troubleshooting a sled…
Yeah thanks for this note, I’ve been trying to triple check all of the little things. The tether is definitely satisfied. Something strange is that the overtemp is triggered (assuming that 1=closed/bad in EzLynk). Is there an overtemp switch separate from the coolant thermistor on these engines?

I ordered a new TPS and engine temp thermistor. I had an extra exhaust temperature sensor and installed it which fixed the erroneous reading on exhaust. It’s very strange that all of these sensors went out simultaneously and I hope there isn’t a bigger issue causing them to blow (e.g., bad ground??, connecting the EzLynk tuner?).

IMG_3272.png
 

IDspud

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I’ve had a stator that would fire and run for seconds then drop power.
That’d give all sensors fits.
 

sidehil

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I had to replace a stator in my 501, they do go bad and cause all kinds of problems in your system, you need to read a volte meter, **** is a pain in the Ass to find
 

IDspud

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Mine ran decent enough every pull we wasted four hours pulling thinking it was almost a go.
Just not enough juice to keep running.
 
T
Oct 27, 2023
39
6
8
MT
I replaced the TPS and engine temperature thermistor this morning before work and both are now actively reading accurately in EzLynk and my sensor supply voltage maintains 5 VDC. I guess I'll try starting the engine this weekend, but I'm a little concerned about why all of these sensors simultaneously failed.
 

Teth-Air

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I replaced the TPS and engine temperature thermistor this morning before work and both are now actively reading accurately in EzLynk and my sensor supply voltage maintains 5 VDC. I guess I'll try starting the engine this weekend, but I'm a little concerned about why all of these sensors simultaneously failed.
When you plugged into the coils is there any chance they induced high voltage back into the sensors?
 
T
Oct 27, 2023
39
6
8
MT
When you plugged into the coils is there any chance they induced high voltage back into the sensors?
I think that unfortunately, you’re probably correct. Obviously it was stupid of me to plug the thermistor into the coil, but I can’t believe Polaris used the same plug style right next to eachother on such critical components!

Update: it runs now with the new TPS and temp sensor! I only tried to idle for a few seconds as it looks like the oil pump isn’t pulsing now. I guess I’m going to need to double check everything that is on that 5 VDC supply circuit… I’m not sure how to figure that out.
 
P

Pitabroed

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Jan 16, 2018
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Very bad from Polaris, I did exactly the same mistake on my 2004 ProX 2. The coil and temp resistor shared the same contact, was turning upside down on everything on the sled to find the fault
 
P

Pitabroed

Member
Jan 16, 2018
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The termistor went broken.
But it's not to compare the cdi system on the 2004 against the ecu system on your sled. Way more sensitive components
 
T
Oct 27, 2023
39
6
8
MT
Now that I've physically bolted the "new" (replaced with a USED TPS from eBay) to the TB, it is functioning and the sensor supply voltage is a constant 5 VDC, but the TPS voltage is too high when the throttle is not pulled. This voltage should be 0.86 VDC at 0% throttle, but mine is reading 4.4 VDC. If I unbolt the TPS and rotate it, I can get the voltage to drop and it then changes when I sweep the thumb throttle.

Should I suspect that the "new" TPS is bad, or should I look into something else? How about the regulator/rectifier/PBR? I could purchase another one of these to test, but do then need to be programmed by the dealer? I ask because P/N 4017985 is listed as "unprogramed" on the Polaris parts site: https://www.polaris.com/en-us/shop-parts/4017/4017985/?pfm=Browse
 

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T
Oct 27, 2023
39
6
8
MT
Update: I think I’m in good shape. The sled is running perfectly and all sensors seem to be accurate. The replacement TPS was inaccurate because I mated the new Hall sensor to my old magnet in the TB. These are factory calibrated as a pair. Correct me if I’m wrong, but I believe you must replace the entire TB if the TPS fails. I don’t see any way to physically remove the magnet from the TB or recalibrate the sensor in EzLynk.

Interestingly, my TPS voltage is around 0.8 VDC at idle with the throttle closed. EzLynk reads around 1% throttle at this position. There is a little wiggle room to rotate the sensor due to the slop in the mounting holes, but there is no way that I am going to achieve the factory specification of 0.86 VDC at 0% throttle (and if I did, it would not register anywhere close to 0% throttle). Does anyone have experience with checking TPS voltage on the 850? Should I just call it close enough and run it, or is this critical to get correct?

So far, I just have the new throttle body plugged in and dangling off the side of the sled. It looks like I’ll have to strip the sled down pretty far again to replace it. Bummer because the snow is great right now. I hope I can still get this sled on the hill before summer.
 

BeartoothBaron

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I'm not positive on replacing the throttle body being the only option (maybe somebody's been able to swap sensors successfully), but I don't see the TPS being available from Polaris; just the throttle body (for nearly $600). Bummer on that - here I thought the 850 TPS setup was an improvement, but it might be one step forward, two steps back. The old-style TPS isn't cheap, but still about half the price (just under $300). That kind of makes me glad I have the old setup. Even if it's more likely to cause problems, at least you have a chance of fixing it with a simple adjustment, vs. apparently needing a replacement TB on the 850/900/Boost.

Anyway, if it's running right - starts, goes, and reverse works - I wouldn't worry about it. If you do have some running issues, that's another story. TPS voltage is a part of life for Polaris owners up through the Axys 800, but I haven't heard of anybody even checking it on the 850. Probably means it's usually not a problem at least, even if the fix is more money and work...
 
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