GM 3.1 Engine Cooling Fans inoperable

95 Chevy Lumina w/3.1 overheating. Engine cooling fans are not coming
on, regardless the temperature, or whether the A/C is turned on or not,
(am assuming that at least one of the engine cooling fans should turn
on when the A/C is activated.)
What are the related fuses and or relays and their locations?
What is the circuit description for these fans?

TKS, Jim

I glanced at a diagram for this info because I couldn't remember any
specifics.

First, the ECM controls both fans independently, and each fan has its
own 60a maxi fuse and relay. Both fans are wired to ground at the
"lower left front of engine" according to diagram. All of the related
fuses, fan relays and A/C clutch relay are in the big fuse box under
the hood, I think near the wiper motor. I don't know that the ECM
would turn on the fan(s) with the A/C on -- assuming cold weather - the
cycling pressure switch input would tell the ECM to keep the A/C system
off.

Check the maxi fuses first. Then I would unplug the fans and power
them right to the battery to see if either is bad. Also make sure the
large black ground wires going to each fan are OK. {[Additional step
to verify ground integrity: you could temporarily wire a large load
like a spare headlight to batt positive and ground it at the black
wire. If light turns on then go further and check voltage drop on the
ground side with a DVOM. Look for less than 0.25 volts.]}

Still no luck? The ECM uses coolant temp (ECT) input to turn on the
fans. Find the two wire ECT connector and backprobe the yellow wire
with the engine hot and running. You should see about 0.5 volts hot.
Less if its overheating. If the voltage is high there suspect a bad
ECT sensor or a lifted ground (the black wire).

You can also check / jump the fan relays with all system components
reinstalled - given you know the fans, grounds, and fuses are ok. You
can jump the large wire terminals Red - Blue on one, Red to White on
the other, and pass direct power to the individual fans. Or you can
backprobe the ECM signal wires with ground with the relays installed.
(Loosen the relay block and work from the underside) These wires are
Green/White for one, and Green for the other relay. This will simulate
the ECM trying to turn on the relays.

With all of this info you should be able to trace down the defective
component or wire.

Toyota MDT in MO

Thanks for the very detailed circuit descrtiption and troubleshooting
ideas.
I'm lost without my AllData (local library closed holidays.) Am really
ap-
preciative for sharing your "Black Belt" technique's. Am going to have
a litlle
morning coffee, then brave the cold to go troubleshoot. Will let u know
either way.
Thanks again.

Jim

P.S. How did u come about the name Comboverfish, and are u a manager in
a toyota div.?

> P.S. How did u come about the name Comboverfish, and are u a manager
in
> a toyota div.?

Its such an obscure quote from a show no longer on TV that noone
watched it seems pointless to explain : ) If anyone ever gets it I'll
know they're a fan and thats a more fun prospect.
I'm a dealer mechanic.

Good luck with your Lumina.

Toyota MDT in MO

There are two ecu's (fuse boxes) under the hood. Which one contains the
wiring to jump the fans. Ecu #1 is on RH front, and Ecu #2 is on LH
front.
Also, that ECT sensor, is located where it looks as tho the air duct
assembly, containing the at least the TPS among other assemblies, will
have to be removed from the intake manifold in order to get to the
ECT.It looks fairly simple to take off by simply loosening a couple
c-clamps, although I don't have any experience with removing this assy,
is there any precautions that one might take in order not to disturb
the adjustments on these assy's. TKS Jim

gobroncos wrote:
> There are two ecu's (fuse boxes) under the hood. Which one contains
the
> wiring to jump the fans. Ecu #1 is on RH front, and Ecu #2 is on LH
> front.
> Also, that ECT sensor, is located where it looks as tho the air duct
> assembly, containing the at least the TPS among other assemblies,
will
> have to be removed from the intake manifold in order to get to the
> ECT.It looks fairly simple to take off by simply loosening a couple
> c-clamps, although I don't have any experience with removing this
assy,
> is there any precautions that one might take in order not to disturb
> the adjustments on these assy's. TKS Jim


Please don't take this the wrong way, but you may be in over your head
here. I'm simply basing this from your terminology and observations.
I fear that it's going to be difficult to guide you any further if I
need to go over all of the basics. I'll try to address this post,
though.

A fuse box is different than an ECU (ECM). The fuse box I'm talking
about has a bunch of relays and big fuses in it and should probably
have a black plastic cover on it. The cover should have drawings and
labels for all of the fuses in it. So should your owner's manual
incase the cover is missing. Atleast find this box, locate the cooling
fan relays and the two cooling fan 60a Maxi fuses. That's where your
problem is likely to be. But you still should power the fans as
described before to make sure they work.

The ECT sensor has a small brass body and a black plastic connector.
It threads into a water outlet or into one of the cylinder heads,
probably near the internal "V" of where the two heads meet. I don't
have a 3.1 GM around and can't remember exactly. It will have both a
yellow and a black wire going to it. Test this as previously
described. That's all the ECM uses to turn on fans when the engine
gets too hot, so the ECT sensor or its wiring circuit or the ECM are
all possible causes of your problem.

The TPS is not in any "ductwork", its attached to your throttlebody,
which I'm sure doesn't need to be removed to test the ECT sensor. You
may need to remove the intake boot and aircleaner assly if it helps you
get a test probe to the wiring. You won't be able to keep the engine
running with the snorkle though if there's no air going through the MAF
sensor. So get the engine really good and hot, shut it off, and
procede with ECT sensor test.

Toyota MDT in MO

No offense taken at all. I really appreciate the help.
I'm completely w/you as far as the terminology goes and have been able
to successfully jump the fans from underneath the ECU #1 as u
explained.
Still having a problem accessing the wiring coming out of the ect
sensor to measure the voltage. The wiring comes directly out of the
sensor then under the intake manifold and from there I lose it.
Still not able to access either the sensor or it's wires enough to get
the probes on it to measure the voltage.
Once again, thank-you for your time.
Jim

If you jumped the big wires at each relay and the fans turned on then
you may have a bad relay(s). You atleast know the high current wiring
is sound enough to run the fans.

Your _biggest_ concern right now is "why won't my #1 fan come on when
my temperature gauge is in the red?" The ECM should close the #1 relay
when it sees about 220 degrees F coolant. You know the wiring and both
fans are adequate. --- So --- you could swap the two fan relays to see
if the #1 relay is bad. If the unused #2 relay is good and the #1 is
bad (and that's your only problem) then the #1 fan should come on with
a HOT engine.

Another thought, some GMs will turn on the fan(s) when you simply
unplug the ECT, but if you jumper the yellow and black wires together
it will surely command the fans on. You could test the command ground
signal to the relays this way without overheating the engine. But you
need to clear the resulting trouble codes when you're done. (Pull the
battery negative cable for ~ 20 seconds is the quickest way)
Toyota MDT in MO

Just returned from the salvage yard. I was able to use this car in the
junkyard as a "cadaver", and did some exploring. I managed to obtain
an ECT sensor and some more relays taken out of the same positon as in
my car (however they appear to have a different part # written on
them), nonetheless, they both have the same color wires as mine did
when i jumped the fans( beneath the ECU).
I'll first swap the #1 relay w/the #2 relay (their part #'s are the
same,) and I'll get a voltage reading before and after it heats up.
I'll get back in a few.
TKS

Jim

"Comboverfish" > wrote in message
oups.com...
> > P.S. How did u come about the name Comboverfish, and are u a manager
> in
> > a toyota div.?
>
> Its such an obscure quote from a show no longer on TV that noone
> watched it seems pointless to explain : ) If anyone ever gets it I'll
> know they're a fan and thats a more fun prospect.
> I'm a dealer mechanic.
>
> Good luck with your Lumina.
>
> Toyota MDT in MO
>
>

Sounds like "Red Dwarf" or "Hitchhikers guide TTG" type stuff.




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