Electrical Problem--Wierd!!

Hey guys, everybody have a good 4th?

A while back, the third fuse blew on my '77 B. Everything was kosher for a few weeks, then it blew again. If I wiggled the wiring harness right next to the steering column, the car would die, for just a second & resume running.

Also, with the car running & lights off, the tach light stays on. If I turn on the headlights, it goes off. At the same time, if I turn the dimmer right, the speedo lights come on. And if I turn it left, the tach light comes on.

Do I have some sort of bad ground? It seems like the big fattie (brown wire) going to the ignition seem warm if not hot.

Beyond these problems, the car is an absolute joy to own!

Warm regards,
Stuart Bear

I am new to this but I believe a good source to help you can be found at http://mgbexperience.com/electrical

stuart!
you need to figure out your electrical gremils soon! you shouldn't have any warm wires, especially a brown one, which is hot all the time, and usually unfused! that is a prelude to a fire.

your wiring next to the steering column has a set of white wires, which is the ignition wires. something is loose, either in the connector, or at the key switch. remove your column covers and have a looksie. try to drive around with the column removed.

the dimmer isn't much use, especially after 20+ years. I would by-pass it all together but putting the red and red/white line on the same connector. that might solve the tach/speedo light from coming on at different times. other than that, can't think of why one would work and not the other.

if the Brown wire is getting warm, you have a high current draw indicating a ground or something that the brown wire feeds is drawing a high current and is about to fail. what did you have running when the fuse went the first time

Bad grounds cause a multitude of problems!! Check them all!! AND, Chuck is right!! Your wire is nearing failure!!

Much truth in what these boys say, Kemo Sabe. I would be looking behind the dash, for starters. There should be a group of black wires coming together in one terminal and attached to the body. The other ends of these should each go to an instrument, which provides the ground for the instrument lights. Also make sure your battery terminals are clean and tight. Brown wires are, as mentioned, always hot (electrically, not temperature!) and unfused. The one going to the ignition switch provides the power for almost everything on the car. If all your grounds are good, you may have a poor connection on the brown wire, probably in the connector to the steering column. A bad (resistive) connection will cause heat to build up.

Hey Guys,

What's the best way to start checking for a bad ground? Where are the grounds under the dash. I have no idea how to use a multimeter. (I have one anyways.....another tool purchase...ya'll know how that goes!)

I thought about undoing all the connections & cleaning them.

Stuart

Grounds aren't easy to "test", you just have to make sure yours is good (I just did this on an earlier post, my engine ground wasn't good, due to rock hard paint, but the multimeter showed almost no resistance).

Look behind the dash, like Ken said, there's a group of black wires. You'll also see a black wire screwed into the metal. Undo it, clean both the wire contact, and the body. You may have some rust.

I'd also check your switch. Something tells me that's where you'll find a problem. A brown wire goes into the switch, and white wires come out of it. Bad white wires will cause your car to die. Since the brown wire going into the switch gets hot that's where you may also find a problem.

(but check your ground connections first-all black wires should have good connections)

Somewhere, Wray has a great article on use of the Muli-meter. Something about " How to impress your friends with your Multi-Meter or something. I am not a techie either but it got me to understanding the tool a little better and I diagnosed a few problems with which got me pretty interested in continuing to use it. Seems like it is posted somewhere. Or just do search for "Wray", get his email address and ask him where it is. I have it at home or I could tell you.

Get into the electrical diagram links on this web site for your car year from a Kinko's or someplace that has computer access and high quality color printer and print the diagrams in color as big as you can and can afford...it get's a bit pricey beyond legal size. Also on this site are links to lists of Wires to Components and Components to Wires that shows colors of what goes to what. Print them in color to. Now you are set to diagnose as you won't be guessing NEARLY as much.

I had front headlight that wouldn't work, other headlight that went high beam when the other one went low, dash lights that lit or didn't at wrong times and with wrong switches, map light that came on when I turned on the headlights, blinkers that didn't work, right rear tail light that didn't work and a small melt-down. It's all fixed and works perfectly thanks to the info above and help from this BBS. Couple of things wired wrong, grounds loose, broken wire, zapped bulb, wire brushed fuse block connectors with wire wheel (that's a great place for a lot of c _ _ p to get started), etc.

Good luck. You'll get through it and know more about your system. If I can ...trust me....most anybody can.

Sounds like a ground problem to me - check the dash grounds behind the instruments (probably on the tach or speedo mounting studs) and on the right side up above the wiper motor.

Multimeters are great, but they can give you useless information if you don't understand Ohm's law. High resistance in a low voltage circuit like a car can be very hard to identify using only an ohmmeter. Best way is to check voltage when the light is supposed to be on. Hook the negative lead to a GOOD ground, then with the meter on the 12VDC scale, put the other lead on the ground that you want to check. When current is flowing to the suspect component, any resistance in the ground connection will show up as a voltage. Make any sense? In other words, ALL grounds should read ZERO volts between the good ground and the ground you're testing when the light they supply is on. Volts=current X resistance. More current, more voltage. Ohm's law.

That is a reasonable test, to see if you have a voltage difference between one ground and another.

However, high resistance is still high resistance no matter whether you're measuring it through a pipe, between your hands, salt water, dirt, with the probes up your nose, through a car body, or anything else.

In our aging LBCs, Kirchoff's law has more relevance.

Personally, I always thought the major aging problem with our cars was the state of the dilithium chrystals.

I've never tried the up your nose thing

One more question. How in the heck do you get under the dash? I can access the left side fine, but it would take Houdini to get behind the speedo, temp guage, etc.

Stuart

A Fluke 77 will read the resistance. Try it with and without a runny nose, let us know waht the diff is.

Three ways, none easy.

1) Pull the dash. PITA but you might end up doing it if something burns up.

2) Remove the tach and speedo and try working through the hole. Not as hard as it sounds.

3) Pull the driver's seat out so you can lay down on your back in the floor with your legs sticking out the rear.

I know it will read the resistance thru my body if I accidently get my fingers on the metal parts of the probes

Hi,HO


Find it fix it. Take it from onme who just pais 1000+ because of a fire behind the dash!!! You do not want to go there. Hence the new name Zippo.

greg

Okay guys,

I made some progress! I removed the speedo & tach. Next, I looked for the black ground wires. I found a group of black wires taped to the blue harness. They appeared to be heat-shrinked together & attached to nothing. Should these be grounded?

I do know the P.O. took the dash out, and had his Jaguar Mech son re-wire the system. Then did me a favor by not screwing the dash back-he just stuck it back!!

Please reply.....I'm on a roll!

Warm regards,
Stuart Bear

Bingo!! You probably found your problem!! Just find a nice clean grounding spot thw they can reach and I bet you got it!! Usually these things are pretty simple!! GOOD LUCK!!!!!!

Like Tony the Tiger used to say, Grrrrrrreeeeeaaaaattttttt!

That was pretty easy. Thanks for all ya'lls help!

Stubear

Stuart, on the earlier 70's cars that black wire had a ring terminal on it and was attached to one of the tach's mounting studs. Not the best place for the instrument grounds but that's the way they came. You can whack the ring terminal off, crimp a push-on terminal in it's place, and use the push-on gorund on the tach.

Wray,

I would think grounding it to the frame behind the dash would be better than grounding at the tach. Is the tach a suitable ground? This is my first foray into the world of electrics, so I'm probably wrong.

Thanks again :)

Stuart

I wasn't trying to talk down to anyone. I was only pointing out that at the high current that 12VDC circuits run under, even a very small resistance can result in a signifigant voltage drop if the current is high. Kirchoff is great for sums of voltage drops equals..., currents into and out of a node.... etc, but for voltage drop, I'll tak Mr. Ohm any day. Sorry if I offended.

hell, you can't offend us and you are right, Ohm's law is the best way to check connections. I use voltage to ground because it is there or it isn't

Wray was talking about something different Stuart!! All the instruments should be individually grounded, and he was telling you how to improve the factory method!! You were on the right track with what you were doing!! Have fun!!!

The ground point on the tach provides a common ground path for the instruments and their lights. Putting the ground back on will probably solve your instrument light conundrum. Whenever lights come on and off when they shouldn't is almost always a ground problem. The ground is taking a different path, usually through another light or device, making this other device light up.
Grounds are as important as the hot leads. Kirchoff told us that all the current flowing into a circuit has to flow back out of it, at some point, too.

As to the brown wire to the ignition switch getting warm, that is probably another problem you need to cipher out.

Wray,

You are exactly right. I just grounded the bundle of black wires to a clean spot of the frame. As soon I as I started the car, the 3rd fuse blew again and the brown wire began to get warm if not hot. However, the dash lights now work much better than before.

Next, I'll trouble shoot from the 3rd fuse and down the line. Enough for tonight, time for bed!

Stuart

That means that one wire is going straight to ground!! There will be something funny!! Sorry Wray!! I didn't mean to step on your toes, I forgot, I grounded all my instruments, but for me, it is just an electical fuel gauge and tach, the others are mechanical. So I am only dealing with the lights and that is maybe why my dash lights work fine for me!!

You're on the right track. I bet that the PO pulled the ground leads off to cure the 3rd fuse problem and never put them back on. You need to look over the wiring diagram and find out what's causing the abnormal current through the brown wire before it burns the harness up.