How Do You Find a Short?

[diablopony]

I was out breaking in my new 5.0 motor when all the gauges in my dash go out. The car still ran OK and I made it back home with no problems.

My 1st thought was it must be the constant voltage unit. I put in a new one then started the motor. Started right up, but the new unit went up in a puff of smoke. The gagues never moved.

I'm now sure I have a short somewhere, but I have no idea on how to find it. Everything is the engine compartment looks right and the ground to the engine is good. I remember hearing that you can trace back a short with a test light and disconnecting the positive cable, but I'm not sure how that works

Any helpful hints would be great. I spent about 10 hours today on this and got nowhere.

[Rev]

Are there any fuses being blown? I think all circuits are protected by a fuse or a circuit breaker (head lights, horn).

Rev

[diablopony]

All the fuses look fine. I replaced them all just to be sure, but no luck. The wiring diagram I have (Haynes Mustang V8) does not show a circut breaker on the gauges.

What is strange is that everything else is fine. All the dash lights work fine and the tach works. I put a test light on each connection within the 2 wiring harnesses that come into the dash. Every connection that should have power does and every one that should not doesn't.

I guess my next approach is to try a volt meter and see which connection has too much voltage. Does anyone know what this should be?

[Rev]

When the car is running at night with the lights on, do the lights burn really brighter than normal? I'm wondering if the voltage regulator could be off, putting excessively high voltage to the accessories?

Rev

[diablopony]

Yes, the headlights are quite a bit brighter and go brighter when you rev the engine. Do I just replace the regulator alternator assembly or is this a symptom of another problem?

Rev, thanks for your help.

[Rev]

I'm not 100% sure, but if you follow your original plan and just put a volt meter on the main feed terminal at the starter relay, you should see 13-15 volts with the engine running and no accessories (lights, AC, etc). If it's a lot more, you might just replace the voltage regulator with a cheap one from AutoZone ($12-13) and see if it helps any. Just a thought really.

Rev

[diablopony]

Thanks Rev,

Could this have anything to do with the motor install? Everything else seems to be fine.

[Rev]

Only if theres a wire out of the regulator or if there's a mis wiring from the alternator to the regulator or something like that. I'm really just doing a lot of guessing here.

Rev

[diablopony]

I picked up a new regulator from Auto Zone for $10. The volts look OK at around 15v. Now I need to get a new constant voltage unit and see if that fixes the problem.

All my wires look OK. The only thing that may have been a problem was the starter wires were on the headers and melted the plastic coating. I re-routed these and they should be fine now.

Thanks for the advice Rev. I'm hoping that fixes it. I want to get this motor broken in so I can get to the track.

[Rev]

Hope that fixes it. Post how it comes out. I'm interested.

Rev

[diablopony]

Rev, thank you for all your help on this. I installed the new constant voltage unit today and the new voltage regulator did the trick. All gauges are working OK.

My lastest problem is a big vacuum leak somewhere. Gee this is fun having a new motor. Maybe some day I'll get every thing working right so I can actually drive my car.

[PKRWUD]

EXCELLENT!!!

Great job, diablopony, and great advice, Rev!!!

I love a happy ending!

Take care,
-Chris

[Rev]

Glad I was at least some help. I'm really only a fellow hobbiest. Now to the the new problem. If you have a vacuum leak, it can really only be iin some few places:

1.) hoses

2.) carburetor/manifold

3.) Manifold/ head

Check carefully in all those areas using spray on carburetor cleaner at idle around those potential leaks. If the idle improves when spraying , then you may have found the air leak.

The most difficult leak to find is at the intake / lifter valley gasket interface. If an air leak is indicated and no other place can be found, then you may need to pull the intake and look there (last resort).

Rev

[PKRWUD]

Originally posted by Rev
The most difficult leak to find is at the intake / lifter valley gasket interface. If an air leak is indicated and no other place can be found, then you may need to pull the intake and look there (last resort).

Rev

There is a test you can perform to tell if in fact you have an internal vacuum leak, like the one Rev mentioned.

1) Hook up a vacuum gauge to the intake manifold.
2) Fully close the carb throttle plates by backing out the idle screw all the way.
3) Plug up the PCV valve opening.
4) Disconnect and plug any vacuum hoses attached directly to the intake manifold.
5) Disconnect the wire to the positive side of the coil.
6) Crank the engine and observe the vacuum gauge. From 3" to 7" of vacuum (at a minimum of 250 cranking rpm) is normal. A defective intake manifold gasket will give, in most cases, a zero reading.

Take care,
-Chris

[Rev]

I Knew that you (PKRWUD) had a a test for that. Thanks for instructing on that again. I know that I should print that out for prosterity. Like a fool, I didn't do it last time.

Rev

[diablopony]

Thanks again for your expert advice. I have a leaky intake manifold gasket. I torqued to 30 lbs., but I guess the aluminum to aluminum contact needs a re-torque right after the initial start up. All of the intake bolts were very loose. I tried to just re-torque them, but the leak is still there. Looks like a need to pull the intake and start over.

This learning process is not as fun as thought it would be. I just have to focus on how fun it will be after I get everything running right.

[diablopony]

Thanks PKRWUD. I'm printing this one for sure.

I'm pretty sure that it is the intake manifold. I torqued the bolts to 30 lbs., but they seem to have come loose after just a few miles. Looks like I need to be very careful with the aluminum to aluminum contacts. I had the same problem with the water pump bolts. Now I'm thinking that I really need to re-torque everything again just to be sure.

Thanks again for all your help.