A scan tool won't help. You've already located the problem - an apparent
short on the power source for several critical devices. The cam and crank
position sensors are what tells the ecm when to fire which coil. You need
find what's blowing the fuse. The likely place to start is to find C42 /
C142 (you don't say which engine), and open up the pins leaving it, and
at which device has the short on it's power supply lead. Use a meter, don't
keep putting fuses in it. You might also look at the resistance to ground
the downstream side of the fuse. Start disconnecting things like the TPS,
sensors, etc., and see what makes the short go away.
it is the 2.7L v6 engine.
I have a multimeter and can do the basics, but i am not the advanced in
The fuse pops about 3-4 seconds after i turn the ignition key to on
I can easily unplug all of the sesors except maybe the canister close valve.
Can i just plug them back in one at a time until it pops?
I don't know what the schematics show for this circuit Bob, but he'd likely
be ok at least trying this approach - assuming that the circuit does really
consist of the components the OP first indicated.
You're right that 3-4 seconds implies a short with a resistance and not a
direct short., and that kind of troubleshooting with a meter can get
confusing for folks who are not a little experienced. Depending upon how
the meter is set, it can potentially either look like a direct short
(leading to replacing the wrong component in vain), or not even show the
short (leading to the obvious oversight).
I think in this case - again, assuming the circuit is as described, just go
ahead as the OP suggests and disconnect the loads and selectively re-connect
them. The caution with this technique is to make sure to inspect the wiring
as well as the components.
Over his head pretty quickly??? Hell - every time I get into
troubleshooting automobile wiring/electronics I make the same statement - "I
hate working on car wiring". But... I do it, and I've been doing it a long
time, so I understand this stuff pretty well, but I still hate it!
What concerns me is given that there's a delay before the fuse blows, and
wiring at 12 volts typically can't fail in a way that gives a high
resistance short that's hard enough that it blows a fuse. If it were a high
resistance short, he'd be dissipating about 240 watts (~12 volts at ~20 amps
to blow a 10 amp fuse in a short but not instant time) at that high
resistance point - much smoke would be present. The other possibility is a
motion induced hard short to ground. If he in fact has something that is
driven - a valve, etc - that has a short, and there's a delay before that
device is actuated, he runs the possibility of damaging the driver. I mean,
how many times can an output take a short before failing. To try to explain
to use a current limiting device in place of a fuse for troubleshooting - a
lamp, etc.- and what the results should be is really difficult.
I disagree. If it's a 10A circuit the normal load would be
~12/10=1.2 ohm. It's going to look like a short to anything but a
specialty resistance measuring circuit. I doubt it's really that low
and it is not that low per sensor so measuring may locate the bad
sensor. I would expect something like a 50% safety factor on the fuse
so I'd expect an agregate load of ~2.4 ohms. Each individual load
will be over that. Possibly well over that. If the circuit wiring is
properly sized it will not smoke with that load. I have seen
connectors with an ohm or more in a bad connection.
Polarity probably will also be an issue. You are going to need to use
the positive voltage resistance lead on the power lead to the sensor.
If you are not sure what one that is use a second meter set to measure
voltage and measure the voltage accross the probes of your resistance
Once You know the polarity probe the leads to the sensors with any
ground lead connected to the chassis. I bet several will be over 100
ohms so you can discount them for a first pass. If one is clearly
below 2 or 3 ohms it is probably the problem. If all of them look OK
then it is probably a function of the sensor activating and drawing
too much current. The easiest way to find that will be to disconnect
all of them then reconnect one at a time. You can either wait for the
fuse to blow or connect an ammeter accross a blown fuse. Monitor the
current draw as each sensor is connected. If I was doing this much
that is what I would start with. ;-)
If the fuse is blowing with the key on but not cranking the engine I
bet you will find the answer fairly easily. You probaly have a blown
semiconductor junction. It still has resistance, as does the
packaging leads, but it's well below what it should be.
I'm not talking about burning the wire. A properly sized fuse will open
prior to the wiring heating significantly. What I am concerned with - if it
is a wiring issue - is that the point where the wire is being grounded, and
whatever the path to ground is is dropping the entire battery voltage over
what is likely a small area. According to the fusing tables, you need 15
amps of current to blow a 10 amp fuse in 4 seconds. So, the entire
dissipation over the wire and the short to ground is 12v * 15a = 180 watts.
15 amps would be a total resistance to ground of .8 ohms. If he looks at O2
sensors, they start out as fairly low resistance, so that may be a little
If he's got a digital meter, most of them will not forward bias any diodes
present in things like position sensors, so he won't even see them. They are
not likely culprits anyway.
If he puts an ammeter (like the one in his meter) across the open fuse,
he'll either cook his meter, or blow the current fuse in it (if there is
Something that works well - if you don't have access to a current limited
power supply - is to use a high current lamp - like one or both filaments in
a 60 watt headlamp - in place of the fuse. Low current flow = no heating of
filament, so little drop across it. Short to ground = light lights, and
current is limited to about 5 amps for one filament, or about 10 amps for 2
in parallel. In either case, you can start disconnecting (or connecting)
things and watch continuously for what changes.
took this out of the other thread because it's less relevant to his
I think he indicated the problem circuit does not include the O2
sensor. I'm not too worried about the power draw as long as he does
not jumper the fuse with a large wire. As you pointed out, and he
measured, the current is limited to around 12 - 15 A so On times of 10
seconds or so won't do more than run down the battery or blow the bad
sensor open. Even melting something is not all bad as it points out
the problem. The only thing I would feel bad about there is one wire
in a large bundle and it took out the bundle.
I was thinking more about a reverse polarity diode accross the sensor.
I've seen them on a lot of automotive comm gear as the power from the
alternator is pretty dirty. Most meters will detect that. That's how
we check transistors. ;-)
Not really. It's already taking several seconds to blow the fuse so
we know the current is limited. A 20 or 50 A current meter like he
needs is pretty rugged. What I think I advised is that he pull all
the sensor wires and put them back as he watched the meter. I assumed
he would turn off the power or pull off the lead if he pegged the
meter. Perhaps I assume too much?
Your way is more elegant, just takes more fussing around. If I was
doing much trouble shooting I would probably build a lamp bank. I was
giving him a one shot quicky approach.
Which is my concern. At 15 amps, something is getting hot. I would prefer
not to torch something in the harness.
It really depends on the meter. I typically use Fluke 73s. In ohms mode,
they don't "see" diodes real well. That is done intentionally. They do have
a mode for looking at semiconductor junctions.
We really don't know what kind of meter he has. The less expensive meters
have a fuse on the low current path, and nothing but a current shunt on the
high current path. Cat. III / IV meters like the ones I use are actually
fused at relatively low values - 440 ma, and 11 amps for the high current
port. For high current DC I use either an external shunt, or a DC clamp on.
For AC, I normally use a clamp on. The DC systems I work on can have
available source currents as high as 40,000 amps at voltages from 12 on up
to 480 volts. I also work on switching equipment and control systems for
voltages as high as 230,000 volts at lots of current.
So, maybe you can understand why I'm not real big on the keep feeding it
In automotive, it might work without major issues, but it may also
eventually damage something besides what is already broken. feeding a line
with a current limited source - either a fancy power supply, or a lamp as a
ballast resistor - is a pretty effective technique. When you've isolated it
to one line, and it's a wiring issue, you can feed that line from either
end, and determine which end the fault to ground is closer to, and maybe
even estimate relative distance. I work on a wide variety of DC powered
devices that communicate with serial data, tones, voltages, and currents.
The things I recommended against are just things I cannot do because I have
to be very careful not to cause additional damage, as repeatedly re-fusing a
known failed circuit - even in a car - can do.
I'm disputing that your suggestion will work. It just makes me cringe when
you suggested shorting across a blown fuse with an ammeter when we are not
sure whether that is fused, or will just melt. Also, the assumption that
he'll get his meter off it fast enough is another variable.
I'm just getting caught up on this thread and have read the conversations
between you and northmark. Good stuff. I have to say - I have used the
technique of throwing a bulb in the circuit as a current limiter. It's a
great technique for more than one reason. Besides providing the current
limiting benefit, it also gives a pretty visible, easy to "read" indication,
which is often times a big plus when working on car wiring. I wish I had
thought a little further down the troubleshooting process to have
recommended that Bob.
Ok, I unplugged all the sensors except the the canister close valve by the
fuel tank and canister purge valve under the throttle body. I turned on the
key and measured the current without the fuse in and it read 12.8
Then as soon as i put the fuse in it popped. I should mention the the car
was hit from behind recently enought to damage the rear bumper and
Have to leave town tomorrow but will try some of the things suggested.
Thanks for all your help.
I am thinking pinched wire around the purge valve only because of the
accident. It looks like you have about 1 ohm so it could be a bad
connection limiting the current. If you are getting 12.8 A I would be
tempted to put in a 20 A fuse and see where things were gettting warm.
I'm not sure that is a good answer to do alone as I would be concerned
about burning something up. If you can have someone turn the key on
for 10 or 15 seconds while you hold the wires to the sensors in
question you may find one heating noticeably. Just watch for wires
getting hot enough to melt insulation.
The other thing that might happen is that you blow the sensor open and
stop drawing current. If it was a bad power transistor the initial
failure would be the reverse biased junction shorting so the total
wiring resistance becomes the current limit. The internal leads in
the package are usually the next thing to fuse but they usually blow
open. An open circuit should let the diagnostic routines find the bad
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.