How much resistance is considered too much? I assume any measurable resistance (or, in this case, voltage drop) over a fusible link is too much. I measured the following key on, ignition off:

+12v battery terminal and +12v ignition coil source: 0.200v drop (fusible link H)

I also measured the quality of my battery cables:

+12v battery terminal and +12v junction block: 0.014v drop
Battery ground terminal and block ground: 0.004v drop

I was having trouble getting my meter to give a resistance rating in ohms, hence the above values. I haven't measured the other links yet.

That reading will change depending on how much current is being drawn. Larry

[This message has been edited by trotterlg (edited 10-08-2012).]

Understood, but I believe 0.200v is too much drop - am I correct in this assumption?

What are you trying to fix or accomplish? Are you measuring the + and - at the battery and then measuring the voltage at the + coil and - battery? If you are, than you are not accounting for the ignition switch and all the connections in that circuit. .2v drop will not cause any issues. A fusible link should not have any "real" resistance that can be accurately measured with a typical over the counter Volt Ohm Meter (VOM). If you have power on both ends of a fusible link and the power does not go away when you wiggle the link, it is good.

I have a misfire (or what feels like a misfire) that I am chasing down. All ignition components have been replaced at least once.

I'm going to go ahead and replace the battery cables, they look questionable.

I checked the voltage between the positive battery terminal and the ignition power source with the ignition switch ON - so if all connections were perfect, there shouldn't be any voltage between the two. Looks like the path goes through link A, the ignition switch, C500, and link H.

For the best results.. you need to test, just before the link, and just after.

Since the car runs, I seriously doubt an electrical connection is the source of your problems. Replacing parts is one part of the equation. What type of plugs did you use? (my car ran worse on platinum plugs) Is the symptom the same after you replaced the parts? Is timing set correctly? Is firing order correct? Do you see any sparks at the plug wires with the car running at night and the hood open? Is it worse when it rains? What is the vacuum reading at idle? What is the compression in all cylinders when hot? What RPM do you get the misfire, accelerating or just an occasional miss? Hot or cold? Check engine light on? You could have a burnt valve, weak valve spring, worn cam lobe, vacuum leak or several other issues. Don't worry about a .2 volt voltage drop and start asking questions with data. In the last 20+ years I have never seen a set of battery cables or a fusible link cause a miss, but if the cables look bad, do that too. Just don't expect them to fix the misfire.

I still have some tests to run (mostly compression) but the car has the symptoms hot or cold, rain or shine. Vacuum at idle IIRC was about 18 mmhg.

Would low compression or a bad valve cause an intermittent misfire? Wouldn't this be a very rhythmic misfire?

Not necessarily, depends what the demand is on the engine. It could be anything from a slightly burnt seat to a crack and still be inconsistent. How much slack is in the timing chain and the distributor shaft?

Since it is hot or cold, things like the O2 sensor become less likely candidates for the cause. I assume that was replaced recently as well.

Distributor has some vertical slop in the shaft but very little rotational slop. Timing chain was done about 10,000 miles ago so I assume it's still nice and tight.

O2 sensor was replaced but since it happens in open loop, i haven't been too critical of it.

Let's say there's a slightly burnt valve or valve seat, or even a crack. How would I determine this?

quote Originally posted by masospaghetti: How much resistance is considered too much? I assume any measurable resistance (or, in this case, voltage drop) over a fusible link is too much. I measured the following key on, ignition off: +12v battery terminal and +12v ignition coil source: 0.200v drop (fusible link H) I also measured the quality of my battery cables: +12v battery terminal and +12v junction block: 0.014v drop Battery ground terminal and block ground: 0.004v drop I was having trouble getting my meter to give a resistance rating in ohms, hence the above values. I haven't measured the other links yet.

Did bold test wrong...
This will measure all drops... Ignition switch, all plugs, etc. Result you have no clue what maybe the problem(s).

Test link H then
1. remove cap that covers C500 and back probe link H.
2. back probe power wire on coil
3. measure volts when engine is running.
or measure ohm when engine off

Most ohm meter battery are too weak for wires over 20-18 gauge, IOW useless on battery cables and engine block.
Volt drop test is hard to use on things like link H because coil isn't a constant load. Try Use a HL bulb as load test... See my Cave, Horn

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

I hooked up a H4 headlamp bulb and load tested the circuit. the circuit looked like this:

Battery Positive ---- Junction block --- Fusible Link A --- Ignition switch --- C500 --- Fusible Link H --- H4 headlamp --- Ground

I tested the voltage from the battery positive, to right before the H4 headlamp. 0.700v drop, which at 55W / 4.58A equals 0.152 ohms.

Is this enough to cause problems, or should I ignore it for now?

quote Originally posted by masospaghetti: Is this enough to cause problems, or should I ignore it for now?

<1v likely ignore it.

Did you remove tach filter?

No, I did not remove the tach filter.

Fusible link is either good or bad - no in between, as the name suggests, it is simply a fuse.

Drop in voltage can easily be explained away from any connections in the circuit, and that much of a drop is nothing to worry about - 10%+ drop probably one might start to be concerned.

If your worried about the spark plugs not firing you can always hook up a timing light to the wire you think is out and see if it flashes. I don't highly recommend this but you can also pull the plug out, have a friend crank the engine two or three cycles and see if it flashes (do not hold the plug while cranking the engine! You will get shocked) You could also check the cylinder compression, here's a video i found that explains really well how to do that.

http://www.youtube.com/watch?v=X_tbksFYhl4

bad tach filter will hurt/kill ignition...
See my Cave, HE Ignition

quote Originally posted by masospaghetti: I have a misfire (or what feels like a misfire) that I am chasing down. All ignition components have been replaced at least once. I'm going to go ahead and replace the battery cables, they look questionable. I checked the voltage between the positive battery terminal and the ignition power source with the ignition switch ON - so if all connections were perfect, there shouldn't be any voltage between the two. Looks like the path goes through link A, the ignition switch, C500, and link H.

In my book under trouble shooting is this .

#9 engine misses at idle speed
1. Spark plugs worn or not gapped right .
2. Faulty spark plug wires
3. Vacuum leaks
4. Incorrect ignition timing
5. Uneven or low compression
#10 engine miss through driving range
1. Fuel filter clogged or impurity in fuel system
2. Low fuel output at injectors
3. Faulty or incorrectly gapped plugs
4. Incorrect ignition timing
5. Check distributor cap disconnected distributor wires.
6. Leaking spark plug wires.
7. Faulty emission system components
8. Low or uneven cylinder compression pressures
9. Weak or faulty ignition system
10. Vacuum leak in fuel injection system , intake manifold or vacuum hose.

i would look at the one's i highlighted in red. if a misfire is the probelm and you replaced all the ignition componets. then that leaves fuel systems and compresion of said engine and maybe aa sensort for the emission system. like the #1 of #10 say pluged fuel filter start there.then work you way to a fuel presure gauge see if it is suppling enough fuel wile running? it will studder like a miss fire when starved for fuel. could also be a vaccum leak in the fuel part of it on the fuel presure regulator. etc... or even a charcol canistor etc.... in the emmission system like egr etc....any ways wish you the best. by the way poking around is the best thing for it to find a probelm so is asking questins.

[This message has been edited by blacknotchback86 (edited 10-12-2012).]