TC-33 electrical nightmare ENDS ! HAPPILY !

[jinman]

So, that would explain why dirty safety interlock switches could cause the starter solenoid to blow a fuse.

I think that is very close to correct if not completely correct. I would never argue with that statement Steve. The important thing in my mind is saying it's the starter solenoid blowing the fuse instead of saying it is the dirty switch. The dirty switch can cause the solenoid to malfuntion and draw more current than normal.

Guy and I both just had a problem with a statement that increased resistance could lead to increased current. That statement is what caused us to respond as we did. There are several ways a circuit can malfunction, but increased resistance always results in decreased current at the point where the resistance is increased.

 

[SteveInMD]

Okay, thanks. I just wanted to be sure I understood. So, the increased resistance is increasing the period of high current draw while the solenoid fires. The increased duration of high current draw is blowing the fuse. Right?

 

[jinman]

Okay, thanks. I just wanted to be sure I understood. So, the increased resistance is increasing the period of high current draw while the solenoid fires. The increased duration of high current draw is blowing the fuse. Right?

Yes, or the solenoid is being repeatedly energized and de-energized at a very high rate due to a dirty contact making intermittent contact within the switch or relay. Think of it like a light bulb in a room. Is that bulb going to last long if you stand at the switch and rapidly switch it on and off? Chattering relays and dirty switches with contact bounce can cause problems, especially in circuits where the protection (fuse) is not rated much higher than the maximum normal load.

I suspect that when the engineer(s) designed the TC33 ignition, he (they) did not expect contact failure or degradation to be a problem. In hindsight, they did not design the circuit properly to account for degradation. That's just my opinion because I would have to do extensive testing of lots of tractors to prove my theory. Sometimes it's easier to come up with a stable workaround solution than to figure exactly all the relay timing and the static and dynamic loads within a circuit. Not only that, but what if a relay has sticking contacts that don't release as they should to switch from normally-closed to energized-open? The mind boggles at the possibilities...

 

[Superduper]

Perhaps another way of stating ohms law that might make it easier to understand for some (the way I learned it in school) is that:

(1) volt can "push" (1) amp through (1) Ohm.

Therefore, 12 volts can push 1 amp through 12 ohms (or)
12 volts can push 12 amps through 1 ohm.

Taking this a little bit further back to the post where one user said lowering voltage will increase current consumption: if we added resistance to a circuit which reduces available voltage, and the load (ohms of lamp) stayed constant, then it stands to reason based upon the "law" above that current consumption should drop commensurably.

Ohms law can be expressed in several ways depending upon what you are solving for. Jim was 100% correct. The correct formula to apply here is: I=V/R where I = amps, V = voltage and R = resistance. With this formula, you solve for I (amps). As you can see lowering the voltage or increasing the resistance will reduce the amount of current (amps) consumed.

Hopefully this will help clarify the effects of increased resistance in a circuit. As for other posters who've had experience with bad grounds and fuse issues, it has already been mentioned that resistance in a circuit is not 100% constant. Motors in particular have so little resistance when initially started that testing them with an ohmmeter would lead you to believe that they consume massive amounts current, far more than they are rated for. In fact, large motors can sometimes even appear to be shorted if testing their windings at idle. For the most part, I believe that circuits are generally designed and protected with enough leeway to prevent fuses from blowing if the load (number of lamps intended for that circuit) remained unchanged, even with a poor ground situation (motor circuits aside).

But going back to the original post, the existence of poor battery terminal connections: I'm thinking that the owner would probably notice difficulty in starting or other electrical problems before blowing the lighting circuit unless the lighting circuit was already stressed to the limit. Perhaps there were more lamps connected to that circuit than it was designed for?

 

[RickB]

But going back to the original post, the existence of poor battery terminal connections: I'm thinking that the owner would probably notice difficulty in starting or other electrical problems before blowing the lighting circuit unless the lighting circuit was already stressed.

That was my point two weeks ago; before the engineers arrived.

 

[jinman]

That was my point two weeks ago; before the engineers arrived.

It's hard to argue with that logic, Rick. Of course, the "engineers" were trying to describe what might be causing the problem rather than what was not causing it.

 

[John_Mc]

If you have a lighting circuit that pulls current at almost the fuses limit when hot, what do you think will happen if you repeatedly make and break contact? What happens is the average surge current through the lamp's filament exceeds the fuses maximum rating and it finally blows. This is most often seen as a delayed reaction.

Repeatedly making and breaking a contact? Sounds a lot like what can happen when you get a bad ground (or other bad connection)... the loose/poor connection can cause a lot of brief cycles. Lots of surges, maybe some arcing at the poor connection. If you've got any sort of an inductive load in the circuit, weird things can happen to voltage and current. Both can spike pretty high as the induction tries to "resist" the changes. Not sure if a light filament is enough to be a significant inductive load, but I'd bet not... A motor winding (alternator or starter) would.

John Mc

 

[Superduper]

Repeatedly making and breaking a contact? Sounds a lot like what can happen when you get a bad ground (or other bad connection)... the loose/poor connection can cause a lot of brief cycles. Lots of surges, maybe some arcing at the poor connection. If you've got any sort of an inductive load in the circuit, weird things can happen to voltage and current. Both can spike pretty high as the induction tries to "resist" the changes. Not sure if a light filament is enough to be a significant inductive load, but I'd bet not... A motor winding (alternator or starter) would.

John Mc

In an automotive circuit, fuel injectors work exactly like that: Computer senses amount of fuel required and cycles on/off the injector to meter the amount of fuel as required. The cycling uses a specific on/off ratio (amount of time on vs off) to establish the dwell. In use, injectors consume the most current when they are injecting the most fuel (max on). Using the cycling of the lamp filament as an analogy, the circuit should consume more current when maximum on as compared to cycling such as in a simple bell circuit. I would venture to guess that the windings of an injector would have a greater tendency to arc due to the windings compared to the filament of a lamp. If arcing were to occur due to the cycling function, these computers wouldn't be very reliable because the oscillating circuits are solid state as opposed to relay operated which wouldn't work fast or reliably enough.

Sorry, but I'm still not buying the corroded connection causing blown fuse syndrome. Perhaps someone here would like to take that theory to the test? An experiment of some sort like quickly switching lamps on/off in rapid succession in an attempt to disintegrate a fuse?

 

[ericinmich]

I've got a different theory on why a bad battery ground could cause a blown fuse.

First, my background. 25yrs of ECM SW/HW/Calibration on engine controls (and associated wiring) for GM.

In almost all situations, there is more than one ground path and point on a vehicle. Ideally, all grounds are at an equal voltage (no ground offests). If a ground is lost, then any voltage differental will seek a different path (if available through wiring, or through arcing). If a battery ground is opened, then there could be current flowing in directions/places that the system was not designed for, and when ground is reconnected, the initial arc will be at a higher voltage, and then any built up current (inductive or capacitive in nature) would seek the ground. In the end, more current could be flowing through the fuse that what the system was designed to have.

Probably not a well written explaination, but I've seen ground issues blow fuses over the years...

 

[guyw]

I've got a different theory on why a bad battery ground could cause a blown fuse.

First, my background. 25yrs of ECM SW/HW/Calibration on engine controls (and associated wiring) for GM.

In almost all situations, there is more than one ground path and point on a vehicle. Ideally, all grounds are at an equal voltage (no ground offests). If a ground is lost, then any voltage differental will seek a different path (if available through wiring, or through arcing). If a battery ground is opened, then there could be current flowing in directions/places that the system was not designed for, and when ground is reconnected, the initial arc will be at a higher voltage, and then any built up current (inductive or capacitive in nature) would seek the ground. In the end, more current could be flowing through the fuse that what the system was designed to have.

Probably not a well written explaination, but I've seen ground issues blow fuses over the years...

Now you are getting into inductances, capacitances and impedences which is anything but basic. I agree that there could be all sorts of ground paths and other complications. But, the ultimate high current/energy source is the battery, and the return path in the absence of a real earth ground is still the negative terminal of the battery. The fuse is in the path from the positive terminal to the load - not the negative terminal. The most likely low resistance path(s) back to the negative terminal would probably not be through the fuse.

Guy (Electrical Engineer, ASIC and system design, 27 years ;-) ).