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Old 21 Mar 2010, 12:39 pm
jim beam
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Default honda coolant temps sensor problem - redux

i've had a persistent low-grade problem on my 89 civic which i now think
i've cured and want to post for the archive. i think it's important
because i had been diagnosing it as a sensor problem, and it would
appear that others have too if the fact that in all my local junkyards,
unless you get to a fresh vehicle very quickly, the sensor has been
removed, presumably by someone chasing the same problem on their own
vehicle. it appears to be an issue with most honda vehicles of late
80's, through to the mid-late 90's.

i noticed the problem because i have a 90 japan-made crx, otherwise
identical to my 89 civic, and it does not have the symptoms. prior to
that, i thought my civic was "just that way" and was simply slow to warm
up and uneconomical because it was aged.

symptoms:
i. poorer fuel economy. it's not awful, but it's 10%-20% lower than normal.
ii. rapid discoloration of engine oil
iii. slow engine warm-up and thus delayed torque converter lock-up on
automatics. this is the most immediately noticeable.

diagnostics:
1. check coolant system. i tried three different thermostats before
deciding for sure this wasn't the problem.
2. check sensor. these can and do fail. when i replaced mine, the
fault disappeared for a while showing i'd scratched the itch, but then
it returned.
3. check the sensor wiring.

analysis:
the thermistor used by honda is much lower resistance than is used by
many other manufacturers. only about 2.5kohm at 25C. [others use 10k -
50k.] that's fine for the electronics, but when the operation temp
climbs, thermistor resistance is in the 100-200 ohm range and any
connection problems or noise start to become a significant percentage of
such a low value.

on checking the connector plug, i found that the inside terminals were
shockingly loose and would grip a test spade hardly at all. static
electrical resistance was ok, but when hot or subject to vibration, i
think electrical noise would become very significant. as the computer
would appear to average readings to mitigate electrical noise on
unshielded leads, it would seem to be using a false higher thermistor
reading in its coolant temp sensor math than would otherwise be the case
with a noise-free connection.

solution:
the problem then, how to remedy? the plug is "tamper proof" and hard to
disassemble. it's also below the notoriously leaky honda distributor
and covered in oil and filth.

upon dissecting several junkyard donor plugs, i concluded that it was
possible to remove the terminals from the plug since they have a snap
plug-in feature like a lot of connector plugs - the problem is finding
out how to push the release tab and pulling the lead so it comes out.
also the rubber bung at the end of the plug is glued in place - great
weather proofing, but a bitch for terminal removal. [also, later model
connectors have the release tabs occluded, to removal of leads only
would be pretty much impossible.] but, with terminals removed finally,
it was possible to crimp the connectors back so they would grip the
terminal spades tightly, reinsert with silicone grease to keep the
weather out, and reconnect.

http://www.flickr.com/photos/38636024@N00/4451349382/
http://www.flickr.com/photos/38636024@N00/4451349388/
http://www.flickr.com/photos/38636024@N00/4451349400/
http://www.flickr.com/photos/38636024@N00/4451402360/

upon reassembly, the connector offered a little slide resistance going
back onto the sensor - just as it should! a quick test drive had the
car warming up just as fast as the crx, with torque converter lockup
just as it should.

conclusion:
you can recondition the connectors the hard way, as above, or you can
just snip the plug and replace with a junkyard donor tested to connect
better. i prefer the former because i'm a pedant, but the latter would
have been MUCH quicker.


--
nomina rutrum rutrum

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