help solve cooling fan puzzle?

[Dr. Herb Tokyo]

86 accord DX...carb
New radiator with no damage on installation.
New genuine Honda thermostat (180*)
New genuine Honda thermosensor in tank (stamped 90M as they all are).
Honda blue coolant pristine.
New Aisin water pump with excellent flow.
Everything is sealed up right, no leaks
Head gasket is fine up & down that checklist....just the fans don't come at temperature.

Both radiator hoses heat up appropriately, when thermostat opens.
I've been through 7 junkyard switches, 7 aftermarket switches & the beck arnley switch, all 90*C


Purged air from every hose through bleeder bolt with heater on high til I couldn't get a single bubble, while parked on a hill with the nose up higher.

COOLING FANS COME ON WHEN JUMPED WITH A WIRE AT THE CONNECTOR, ALSO WITH THE AC ON.

Relays are good and fuses too, which I guess is obvious if jumper and ac override work.
DX model with a carburetor, no box under passenger seat for timer, just the switch.

I'm stumped and I could use some advise on the best path forward, thank you.

[Dr. Herb Tokyo]

78,000 miles with regular service records going back to 1990, needed a lot of work and it's been a journey with this one, but it purrs, got it all adjusted sweet,I raced some ugly KIA supercharged turbo thing the other day 0-80 and smoked him.
I guess it's not the car that wins the race too though

I just bought this thermosensor 21 minutes ago at the dealership for $75, it does not turn the fans on even at 3/4 Temp.
TEC 90M(C)
part number: 37773-PH2-004 same part as 37773-PH1-621

[Dr. Herb Tokyo]

I've also tried a known working original part thermosensor, that one turned fans on at 3/4 temp....I put it back in later and it never turned fans on at all.

[Dr. Herb Tokyo]

The thermosensor has a big dot of thick black paint on the tip, should I sand this off and try the sensor again? figured it was there to protect the probe from corrosion.

I'd hoped to find the correct solution for this, unfortunately this part design needs to re-engineered.

I guess I will try an aftermarket fan controller with adjustable temperature, after this genuine honda sensor failed to turn on fans, I now believe this thermoresistor is a gamble no matter where it comes from.

[Dr. Herb Tokyo]

Is it rude to say "Bump"?....I ain't too familiar with forum etiquette.


Should I reinstall the Motorad (yuck!) 195* thermostat? I forgot to mention I have the genuine thermostat gasket as well.

Also, should I clean out the original radiator and reinstall that with the genuine thermo switch I just purchased?

The original radiator had a brass fitting threaded for the sensor, aftermarket has plastic fitting...is this the missing piece of puzzle? The brass sensor needs brass fitting?

[Oldblueaccord]

Traffic on this sight is a little slow it may take a few days for a suggestion to a tough problem.

My question I think is IS the car overheating or do you suspect it is overheating?

My fans just dont normally come on without the car running the AC.

[Dr. Herb Tokyo]

Ya know, that is the strange thing....just now I was messin with stuff in the engine while it idled, the fans came on so I ran to check the temp gauge 4/5>H, but the car was not overheating. (Steaming, rough running etc....)
So the factory thermosensor turns the fans on at 4/5 on the gauge, could this be have anything to do with the 180* factory thermostat?
Possibly, the sensor needs the factory 192* thermostat to send a burst of hotter coolant to turn the fans on right when the thermostat is fully opened at (212*?)....then, radiator cools that, thermostat closes and gets ready to start the cycle again?
Then again if the temp gauge is at 3/4, the thermosensor isn't doing it's job. Period.

Is it possible the sensor for the gauge needs to be cleaned and gives a false reading?

You would think the car would run rough at 3/4 temp, seems to run better at that temp

[Dr. Herb Tokyo]

I will replace the temp sensor for gauge with a known working one, flush the system with water a few more times, put in a Motorad 192* thermostat and use a fresh bottle of Pentofrost Blue coolant, then bleed the system and report back....thank you for your reply.
If I understand you correctly, your fans come on rarely because your cooling system keeps dial in the middle of gauge?

On cool temp days here I've only needed fans a few times, lately it's been hot and muggy, stale air.

I still feel there is something fishy about honda discontinuing the original 192* thermostat that came with the car and redirecting the part number to the current 180*...I scratched my head at the dealership when I saw that on his computer....my gut told me "something won't work right with this thermostat, hold out for the genuine 192*"

At one point, I pulled an original part thermostat from a junkyard 3G just to find out, the car did run a lot better and did hold temp at 1/2 more often, that thermostat stuck closed 1,000 miles later ;(

Beck Arnley 192* stuck closed @ 200 miles.
Beck Arnley 195* stuck closed @ 300 miles.
Motorad 180* stuck closed right away
Motorad 195* stuck closed right away
Lots of others stuck closed, a few stuck open too.

[Oldblueaccord]

I will replace the temp sensor for gauge with a known working one, flush the system with water a few more times, put in a Motorad 192* thermostat and use a fresh bottle of Pentofrost Blue coolant, then bleed the system and report back....thank you for your reply.
If I understand you correctly, your fans come on rarely because your cooling system keeps dial in the middle of gauge?

On cool temp days here I've only needed fans a few times, lately it's been hot and muggy, stale air.

I still feel there is something fishy about honda discontinuing the original 192* thermostat that came with the car and redirecting the part number to the current 180*...I scratched my head at the dealership when I saw that on his computer....my gut told me "something won't work right with this thermostat, hold out for the genuine 192*"

At one point, I pulled an original part thermostat from a junkyard 3G just to find out, the car did run a lot better and did hold temp at 1/2 more often, that thermostat stuck closed 1,000 miles later ;(

Beck Arnley 192* stuck closed @ 200 miles.
Beck Arnley 195* stuck closed @ 300 miles.
Motorad 180* stuck closed right away
Motorad 195* stuck closed right away
Lots of others stuck closed, a few stuck open too.

Yeah my stock gauge never goes past half way. Lately it has been riding close but everything is very old on it related to the coolant system on my car.

The stuck t stats is a wierd one. your are using the gasket that goes with it,not just permatex? Mixing water with the coolant 50% or more? Bleed hole up on the t stat?

I can run mine mixed at about 10 degrees above zero all year in my climate if i dont run 100% water in it quite a bit.

Assuming you tried to run it without a T stats I would guess it should run dead cold.

[Dr. Herb Tokyo]

Dead cold with no thermostat.
Genuine honda thermostat gasket.
60% water 40% pentofrost blue..
Bled through bleeder bolt on tstat housing with clear vacuum hose to watch bubbles.

Is it possible a smaller thermostat housing > intake manifold hose could give the gauge temp sensor a hotter reading due to insufficient flow?

I've also only heard the switch turn the fans on twice with the thin aftermarket radiator, could swapping in a Denso original part radiator in good shape make any difference? The Denso radiator has the brass fitting for the switch and stores way
more coolant.

Today I will swap in the clean Denso radiator, new coolant hoses, flush it all, maybe install a 195* thermostat.

[Dr. Herb Tokyo]

Maybe before I do all that I'll grab an infrared thermometer and start getting some actual numbers on the different parts of the engine, especially the sensor... My gut is telling me the gauge is giving a false reading and the fan is actually kicking on at 1/2.

Thanks again for your help oldblueaccord, I've found very valuable information in your posts on this site, I thank you for that as well.

[ryan427]

Replace the wire running between the thermosensor and relay. Or, peel back all of the insulation and you'll likely find a weak spot/spot that is just about to break. Less resistance when jumping it than when it is connected to the thermosensor. Trust me, I went through this same fiasco on my LX only to find an almost-broken-piece of wire about 6 inches from the harness that plugs in to the thermosensor. Easy way to find out is to take a good piece of wire and connect it from the thermosensor to the underside of the fuse box fan relay. Or use a test light. I really overthought this whole issue only to find a very simple problem. Best of luck sir.


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[Dr. Herb Tokyo]

Ok thank you, I think I'm gonna do that right away...I considered running a new wire at one point, but the puzzle plays tricks, I x'd that one off list when jumper/ac turned the fans on...
It makes a lot of sense now, never failed to kick on through the ac circuit

[ryan427]

No problem. Hoping that solves your problem. My wire was easily repairable. It lit up the test light but it was dim. The weak spot made it to where it would not carry a proper current. If you don't find a break that is easy to spot, it's easiest just to run a new wire. It was 99 degrees here today so I am glad my fans are kicking on frequently to keep the engine cool.


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[Dr. Herb Tokyo]

I'm gonna run that wire now, one wire that splits into two at the connector right?

Im a mechanic, not an electrician.. @ 13 yrs old I bought a 1981 accord HB with 300,00+ miles and about every electrical issue imaginable...big stacks of alternator cores ;/ I vaguely remember having this issue, I had to rewire almost everything on that car.

[Dr. Herb Tokyo]

Ran a new wire from small blue wire at fusebox to sensor, grounded black wire at connector to frame...no fans, I guess I was suppose to tie into the ac wire too as ac override doesn't work now, I just hear a click from relay box.
I guess a 195* thermostat, the Denso radiator and new hoses are my last shot at getting these fans to run.

Thanks for the advise y'all.

[Dr_Snooz]

If I'm reading the thread correctly, the fans do turn on if you wait long enough. The gauge, however, reads hotter than normal. Have you checked any temps with an infrared thermometer yet? I would do that first. You need to find out if you truly have an overheating issue, or just a faulty gauge/sender. Those are very different problems, requiring very different solutions. Find out at what temp your thermostat opens. Find out at what temp the fan switch turns on at. Find out what temp shows 1/2 on your dash gauge, etc. etc. etc. A diagnosis should emerge quickly.

The constant thermostat problems are a strange one. You mention that the car was neglected for a long time and it's not uncommon to have a lot of ongoing cooling system problems henceforth. I wonder if there is a lot of residual detritus in the system getting lodged in your thermostats, making them stick. If you haven't done a thorough cooling system flush, consider doing so.

[Dr. Herb Tokyo]

I've flushed the system pretty thoroughly and the car was no more neglected than any car on the road, it had some rusty in the system, but I got that out...
I definitely suspect the gauge sensor, aftermarket radiator, lower temp thermostat and tiny thermo>intake hose.
If a good Denso radiator, a known working gauge sensor, new hoses and a 195* thermostat don't start the fans I will grab an infrared thermometer and get some readings.

Back when I built custom coffee roasters, I had a few $1000 worth of fluke temp meters,.....I guess a cheapo infrared at hardware store is calling my name

I have an entire graveyard of stuck thermostats from all kinds of cars, none say Honda, Toyota, Koyo, Tama, Nitoma, FEW, etc.... All of them say Motorad, stant, beck arnely, murray, etc.... So I won't worry much about that, the genuine thermo I picked up seems to keep things a bit cool sometimes, but it's working well after 1500 miles.

Thank you for the advise mr snooze, bringing me back to reality a bit sir.....


Could I join the ac (thick blue wire?) and thermoswitch wire under fusebox to get the ac override working again?

[Dr_Snooz]

If a good Denso radiator, a known working gauge sensor, new hoses and a 195* thermostat don't start the fans I will grab an infrared thermometer and get some readings.

I'd say you have the steps reversed, but it's your car. You can get a decent infrared thermometer from Harbor Freight for $20. If you don't trust it, verify it with a pot of boiling water and a glass of ice water. Then you can spend higher sums where needed, based on the data you gather. If you'd rather spend $100 on a bunch of stuff you may or may not need first, you can. It wouldn't be my way to do it, but you have to do what works for you.

I'm not sure what you mean by "ac override." I think the fans are coming on with the A/C now and they also come on if the temp gets high enough? As near as I can tell from the thread, everything is working correctly, albeit at a higher temperature than what would seem right. Thus why I say get some accurate temps so we know what we're really dealing with. I mean, if you have a 80F temp drop from one side of the thermostat to the other, you know that thermostat is stuck closed. If you have a 100F difference between the left side of the radiator and the right side, you know there's a blockage in the radiator. If the thermoswitch is at 230F but the fans aren't on, then you know you have a sensor/wiring problem. Those are all very different situations. The thermometer can tell you everything you need to know so you don't have to throw money at the car and hope. Just last week, my truck was doing the exact same thing your car is doing, running hot without the fan turning on. I spent all of 5 minutes under the hood with my thermometer and diagnosed a stuck thermostat. Thirty minutes later the problem was fixed for under $5. Get the thermometer and know.

The only other thing I can think of with regard to the failing thermostats is that you're using Honda's fancy blue coolant. I know that the blue coolant is just the green coolant with coloring added, but to my knowledge, these cars would have used green coolant. Consider using the cheapo green coolant from the parts store for a while. Cooling system metallurgy/chemistry can do strange things sometimes.

Keep us posted on what you discover.

[Dr. Herb Tokyo]

Thank you for the detailed response...
I will be firing $6.99 on a new 192*, I have a clean Denso radiator & a known working gauge sensor on another accord.
I have new hose (1/2'&5/8') and plumbing tube/clamps.
I know the thermometer would reveal the answer, but it's 30 miles down a traffic infested highway into unfriendly territory and all of the parts mentioned are within 2 city blocks for a total cost of $6.99.
I'll grab the thermometer for sure if it doesn't work.

I ran a new wire last night from the small blue thermoswitch wire below the fusebox to the switch, then grounded the other wire to the frame, now the fans don't run at all.

So I wondered if the large blue wire (ac?) and small blue wire are connected as the new wire would've bypasses that.

Yes, as of yesterday the fans did come on at 3/4 temp, also with the ac switched on.

[Dr. Herb Tokyo]

Ok, Home Depots got an infrared, I'll take your advise and do that first...I really would like to know some definitive answers. Thanks snoozer & thank you for this too....

[ryan427]

I am 100% in agreement with everything Snooz said. And we are conservatives! We just watched Cowspiracy. Highly recommend. I know, it's a Honda forum. But man, one of the reasons I am keeping this damn 89 Accord carb'd running is for when the shit hits the fan. EMP proof and whatnot.


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[Dr. Herb Tokyo]

Classic, the shit kinda did hit the fan when China decided no more fake currency for debt,....the fans kick on right before 1/2, bring it to 1/3, then repeat....the cooling system is working perfectly now, fans cycle correctly.
I bought the thermometer, never got a reading higher than 190* so I returned it because it was cheap Asian crap.
I drained the coolant at the car wash and got the pressure washer into the smaller hoses, deep flush (never really saw anything but water pour out) made a new , thicker intake>TH hose, new coolant, bled it for an hour....temp was reading 4/5
So, I snuck in the darkness to my other accord, spun the known working temp gauge sensor out, spun it into the other one and everything was right as rain.
Two years I've owned this car I have suspected just a few times the gauge was getting the wrong reading.

Thanks everyone for solving this puzzle.

[Farmay]

I've also tried a known working original part thermosensor, that one turned fans on at 3/4 temp....I put it back in later and it never turned fans on at all.
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Yeah the thermosensor is working good at my end. I need some help here with the other parts of it.

[ryan427]

I realized I misread this string when I originally replied. I somehow didn't pick up on the fact that the fans were working even without the AC. My bad! But I am glad you solved your problem.


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