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Old 05 May 2010, 11:08 am
jim beam
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Default civic "coolant problem" redux

ok, i'm not repeating past mistakes by declaring a definitive fix, but
this time i /think/ i've finally solved my 89 civic's
coolant/transmission lockup problem.

engine takes forever to warm up before lockup comes on - sometimes 20+
minutes. even in warmer weather.

there are several potential causes, including:

1. faulty ecu
2. faulty transmission
3. faulty lockup solenoid
4. faulty throttle cable adjustment
5. faulty thermostat
6. faulty tw sensor
7. faulty tw sensor wiring

as a quick read online will quickly tell you, and as a cruise of the
average junkyard will also tell you judging by the way the coolant
sensors in hondas are /always/ harvested, the ecu needs to know the
engine coolant is at the correct temperature before it will signal for
the lockup clutch to come on - this is to rev the engine higher while
cold and warm it up quicker. i myself have spent quite some time
chasing this symptom swapping out tw sensors three times, thermostats
three times, etc., all to no avail. so, testing the ecu by wiring in a
led to see when the lockup solenoid was getting signal was informative -
every time it did, the transmission locked up perfectly.

so, what was preventing the ecu from giving the signal? was it the
sensor/wiring? the sensor is a low resistance type and is only 100Ω -
200Ω when the coolant is at full temp. a small resistance on the wiring
is a relatively large percentage of the sensor resistance, and thus a
potentially significant potential source of problems. and apparently i
was not the only one chasing up this blind alley. the other weekend i
was at a junkyard and saw a civic where the sensor had been re-wired
with 14ga!!! - someone thinking they could reduce the resistance read by
the ecu!

but, on further investigation, and wiring in a dvm to the tw sensor
circuit to read it in operation, it was clear that the ecu was getting
good signal - it could "see" the coolant warming up quickly and
correctly. swapping ecu's made no difference, so that wasn't the issue

with all the above eventually eliminated, and testing done, but with the
problem remaining, what else could it be??? going back to the wiring
block diagram, something has to be telling the ecu to not give signal.
what? one of the ecu inputs is the transmission selector switch - it
tells the ecu what gear it's in. in theory, if this switch was faulty,
the ecu wouldn't "know" its gear and thus the transmission would be in
pure mechanical mode, no computer control. i tapped the leads to this
switch and found that the electrical contactor for "d4" was both
intermittent and electrically noisy. so, on pulling the switch and
looking inside, what i find is a contact set glued in place by what used
to be grease, but after 20+ years, now more closely resembled sticky
brown contact cement.

this is post clean-up, but you get the point:

and the circuit diagram:

de-greasing, and de-oxidizing the electrical contactors, re-lubing with
high quality silicone dielectric grease, and making sure they could
properly float in their holder and were no longer glued in by crap,
appears to have solved the problem.

the cold sticky grease was not allowing the contactor to float until the
transmission tunnel was warmed and had softened the grease sufficiently
to be soft. and added to that, oxidized contactor surfaces were making
the electrical connection noisy.

thorough cleaning and re-lube of the d4 electrical contact appears to
have eliminated the bad connection and thus, the transmission lockup

nomina rutrum rutrum
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