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keeps blowing main fuse

CBHONDA750

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i have a 1977 honda cb750a hondamatic. currently not running. i was seeing if the back brake lights were working and after i pushed down on the rear brake. the brake lights lite up then all lights shut off. i checked the fuses and the 15 amp main fuse was blown. replaced the fuse and turned on the key everything came on for a few seconds then the main fuse blew again. any ideas on what the can be?
 
Blowing that quickly is usually a short somewhere. Fuses that take longer to blow are often a result of corroded connections that contribute to a higher amp draw. Electrical gremlins can be maddening. Check each circuit for shorts, look for corroded connectors, clean the fuse holders until they are bright and shiny. Finding electrical problems requires a systematic approach and often involves finding out what it's not.

Here is some of the methods I have used with good results that I had posted on a different forum:

You can buy just the connector blocks (the plastic part) if the old ones are melted or damaged, and clean up the connectors (the brass parts). I have rehabilitated the entire harness of more than one CB750. Takes about two weeks working through the weekends and almost every evening. Manually cleaned EVERY connector:
Wiring1.jpg


Some of the worst connectors looked like this:
Wiring2.jpg


Vintage connections makes a tool that helps getting connectors apart:
Wiring3.jpg


Keep a notepad handy for those multi connector blocks:
Wiring4.jpg


YUCK!
Wiring5.jpg


A Dremel with a wire wheel is a POWERFUL tool!
Wiring6.jpg


Steel wool does a good job of cleaning rattlecan overspray off the wire insulation that some goob PO got all over EVERYTHING:
Wiring7.jpg


Nice!
Wiring8.jpg


Don't forget to bend the retaining tabs back into place:
Wiring9.jpg


Voila'! (as opposed to "Viola" which everyone knows is a musical instrument)
Wiring10.jpg


I then gave all the newly cleaned contacts a shot of tuner cleaner/lubricant: http://www.radioshack.com/search/index.jsp?kwCatId=&kw=tuner%20cleaner&origkw=tuner+cleaner&sr=1

For those connectors that were too far gone were replaced with new connectors from vintage connections: http://vintageconnections.com/
Be sure to use the smaller bullet connectors to match the stock ones (there are two sizes), and splurge for the crimp tool. Looks like a factory crimp when done properly.

Purple Power (or "That Purple Stuff") does a bang up job of removing paint from plastic without damaging the plastic. Use gloves when removing parts from solution and wash parts thoroughly. It does not appear to harm plastic, but it's a little rough on the skin.

The female connectors (both bullet and spade) are more difficult to clean (insert favorite chauvanist remark here) but the same principles apply. For the female spades I used the dremel w/wire wheel for the outer part of the connector (just to remove any oxidation that might promote more oxidation) and then used a narrow strip of tin with a narrow strip of 320grit sandpaper folded over it to clean the inside of the spade connection. A pinch with the pliers prior to reassembly to assure a good, tight connection finishes the spade connectors (don't forget the locking tab on these too). The male bullets clean up the same as the male spades, while the female bullets require either a small diameter wire bottle brush (.177 cal bore brush is a popular choice) or an assortment of small diameter diamond stones for the Dremel (my choice). Again, a little pinch with the pliers to assure a tight connection, a little shot of tuner cleaner/lubricant, and, if you did everything right, you're good for another 30 years.

While I was at it I pulled apart the switches and cleaned them too. Here is the rear brake switch at the rear brake foot pedal:

Before:
Wiring11.jpg


After:
Wiring12.jpg


If you don't want to tackle the whole harness at once (quite daunting) then start with the primary circuits starting at the fuse box. Clean all the fuse holder connectors and harness connections to the fuse box. Then inspect/clean all the problem connectors, especially those connections to the alternator. I did this more targeted approach (as opposed to the whole harness) with my daily driver a couple years ago after it started blowing the main fuse. Still trouble free after this form of maintenance. This process WILL try your patience.

Oh yeah, DISCONNECT THE BATTERY!
 
If you think the rear brakelight had something to do with it then disconnect all brake wiring connectors and try again. Past that you'll need to weed the location down, that involves again, disconnecting major sections of the harness to then check for fuse blowing again, it will stop when you have the proper wiring undone and then you can detail that harness in particular.
 
I recommend saving yourself some time and trouble by inspecting the entire wiring harness now. Check every connector, every switch, and every bulb socket. Also check the harness for any spots that may be rubbed through.
 
When you run out of places then look close at the flexpoint where harness has to flex as the steering head turns, the wires can flaw inside the harness covers to not show a thing yet be bad, oh so bad. Make sure your grounds are solid, power surges from bad grounds can pop fuses sometimes.
 
When you run out of places then look close at the flexpoint where harness has to flex as the steering head turns, the wires can flaw inside the harness covers to not show a thing yet be bad, oh so bad.

Oh yeah, forgot about that one. Had this happen in my daughters Honda (car). All the lights in the trunk lid went dead. Some idiot (From the factory) put a hard metal wire connector right in the middle of the flex loop inside the harness. It eventually broke in half.
 
Practically every Ford Focus on the planet does it at the trunk or tailgate hinge flex point as well, the plastic insulation is biodegradeable and they took that to the Nth degree, it begins before the car is even junked. Repairing one of mine there right now. Why so many of the cars catch fire now, between them trying to save a penny on each car here and there with less and less wire gauge size (to decrease weight for mileage they say) or length and the insulation cracking into pieces on 3 year old cars.
 
General Motors vehicles have made me more money in the automotive repair field then any other manufacturer out there.
 
ALL brands pretty much have their major flaws and much more major now that they are trying to make them lighter and lighter. What disturbs me most about the Fords though is a steady insidious plan that has them using the engineers to now re-design to make parts break much more on cue to increase parts throughput sales. Thought I was going crazy there for a while but now it's obvious I'm not and they are definitely doing it. I bought pairs of same car there for a few years to minimize issues with not being able to diagnose problems but I always bought a year or two later with each one. You can clearly see the changes that occur during the maturity of each model and how they cheapen them, not just to make cheaper to make on the assembly line, but rather to increase parts sales too. Ford sent engineers through all the parts books to 'unitize' all the parts they could, they combine all loose fasteners, clips, and various 'small parts' needed on things to be part of much bigger sub-assemblies to make the buyer commonly buy a much more expensive assembly now to get the smaller part that was designed to break early. An effort to make sure Ford 'has an absolute minimum $50 sale on every part sold' as one Ford parts guy explained to me some 15 years ago. You have fits finding single specialty bolts now on most things, they are NLA. They won't tell you but many parts are NLA at 6 years now too. Just try buying ATX or MTX parts, they only sell the entire transmission commonly. Wiring being copper and heavy has them making most harnesses too short now and way smaller in gauge too for most circuits, why they burn up parts so much now. Very common now to melt all sorts of parts at the connectors since the electrical cross-sectional area is too small now for the amps going across it. Then they help you along by picking one very expensive part there to design it with a smaller interface than the rest and that one part then melts over and over to make you have trouble with it and the new part after it. The wire insulation is more 'green' now, in fact it biodegrades early enough to crack and break off all over the cars, buying entire engine harnesses now is a way of life if you have car longer than 7 years. The shorter wiring breaks at stress points all over the cars as well, pulled too tight. Intentional re-design has a part made of metal, then plastic later, then it gets holes punched in the plastic, and then commonly the hole locations get moved around or made bigger to make the part easier to break.

I personally think it's because the engines and transmissions now last so long they have gone to other ways of convincing owners to buy another new car by making them break so many 'nuisance' items that owner gets tired of it to talk himself into a new car. I also found that a bit of creative thinking can often shortchange Ford of the part they hoped to sell you, I often re-design the broken part to have it go back in service to last longer than when it was made by them. That's pretty damn funny to me. Doing it to a cheap plastic window clip today to save buying a $130 window regulator, like the 4th one I've done on two crap Focus cars. The regulators break if you so much as look at them cross-eyed. Like the door latch assemblies, my cost to fix like $5 on each part, of course the time involved is mine as well.

Take the Focus, the early ones are unbreakable as far as engine, it lasts 300K in the hands of somebody who changes oil but fitted with cooling system electrics that it takes an electrical engineer to understand, the electrics are what commonly put the cars in the scrapyards from overheating fails. That wasn't bad enough, they then complicated it even more 4 years later to fail even faster. One engine type used in the cars for 10 years routinely dropped valve seats to retire them and Ford never touched that issue whatsoever, it even plagued the Escorts with same engine yet the fix was easy and involved little money. Now it's the 'automatic that's not an automatic' DCT trans that gives them the utter fits they deserve, people are fleeing those in such large numbers now they are losing long term customers by the thousands. The 2 disc clutch and electronics now have been re-designed so many times you cannot keep count. They got too cheap and made the clutch dry instead of wet and the PCM cannot recalculate what the clutch needs as a dry clutch changes so much with heat input no calibration on the planet can account for that. Add more insult to that, a cheap trans seal that leaks oil all over the discs to make your day worse. Re-design it 5 times and it STILL leaks.

I finally gave up and went Nissan, best move I ever made, the car has had zero issues in the last 6 years now, the last 5 Fords I had in no way can say that, the issues got much worse with the newer cars. All had 2-3 things go wrong and the last two cars had like 5-6 by that length of time.
 
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When you work parts you realize all the American car companies are turning into crap, even some of the Euro brands are too. Beemer for one has its' share of cheaper crap car problems, I sold way more parts for them than I though I would. Dodge Ram trucks of any type? Keep them as far away from me as possible, those trucks were utter garbage to me. Saturns sucked far worse than the conventional wisdom ever realized too. VW? Junk with a big price tag.
 
We have an 05 Honda Civic. 320,000+ miles on it. Runs like new because it is serviced as it should be.

Outside of normal I've replaced the crank sensor and water temp sensor.

Doesn't handle real well because it still has the factory struts on it but it rides fine, no bounce. Factory Honda shocks are the 8th wonder.
 
When you work parts you realize all the American car companies are turning into crap, even some of the Euro brands are too. Beemer for one has its' share of cheaper crap car problems, I sold way more parts for them than I though I would. Dodge Ram trucks of any type? Keep them as far away from me as possible, those trucks were utter garbage to me. Saturns sucked far worse than the conventional wisdom ever realized too. VW? Junk with a big price tag.

Great idea amc49!
 
This is the same problem I had about a year ago. I have a 1978 Honda cb750k and it blew the main 15 amp fuse and I put a new one in and it blew. After tracing and cleaning all connectors on main wire harness I happened to unplugged the Horn and left it out and installed a new fuse and it did not blow. So just unplug your horn and I bet you it will solve your problem, just buy a new horn.

Steve Colville
 
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