HELP NEEDED One to diagnose and fix
 

Okay, my '74 MFI has decided not to want to run. Last weekend at Mid Ohio it ran great.

So what is happening...

-Car will barely accelerate. There is a lot of intake backfires and jerkiness
-Once under way, it pull okay to mid RPM then severe exhaust backfire and jerkiness, have to back off or short shift to maintain acceleration
-The battery is not holding a charge
-The tachometer went to 0 when I applied the brakes while driving
-Turn the key on and the tach reads 5k RPM but will not turn over

and what has changed...I'll list everything eventhough they may not seem related...you never know.

-Installed new front and rear discs.
-Installed new turbo tie rods
-Repacked wheel bearings
-Adjusted sway bar settings
-Ran low on fuel (below R but not out)
-Had a battery kill switch installed at the track on Saturday, ran Saturday and Sunday w/no problems.
-Changed the oil

Heading to Road America on Friday and I will only have Sunday to work on the car.

Thinking fuel filter since I ran it down but that doesn't explain the battery losing it's charge, and the tachometer issues.

Please any and all ideas are welcome.

did you check the voltage with the car running?

No I didn't...I was thinking maybe the regulator or alternator. Since the kill switch shuts down the alternator as well, maybe something shorted it out and the battery had enough of a charge to last the weekend?

How would I check the voltage?

maybe you should check if the battery needs water.. if it does then fill to around 1in. from the top.. then disconnect a battery cable and recharge with a battery recharger. read directions on the battery charger before you start the recharge. then if it starts and runs properly get your hands on a voltmeter. when the engine is idling connect the red volt meter probe to the positive battery post.. the positive post will have a " + " next to it.. then connect the black volt meter probe to the negative battery post.. the negative post will have a " - " close to it.. The " - " battrey cable will connect to the body and should be covered in black. make sure your meter is adjusted to the proper scale... Then read the voltage that registers on your meter.. then come back to your computer and report your findings.

IMPORTANT... the gases that escape from your battery when you are looking at the water/electrolite level are explosive. Don't use a match to look in there, use a flashlight..

My voltage reg went out a month ago and did almost the same thing. I too would check fuel filter and lines. What about points?
Kill switch??

Individually those probs could be lots of things, but collectively they point to a bad ground or possibly a loose 14 pin plug.
You could also check the speed switch is working correctly as that can have some weird effects is it's loose in it's socket.
Jeff

Okay, here are the readings. With the car not running, steady 10 volts. With the car running started at 7.76 V and decreased by 100th of a volt every second or so.

The kill switch is tied in to the battery and the alternator so when it is disengaged, there is no electrical current going through the car. Your basic PCA club race kill switch.

Jeff... this has to be fixed or the car ain't going anywhere.. there is a possibility the new switch is defective, or dirty contacts.. Can you remove it? then leave it out. Then charge the battery and check the voltage with the car running again. re-read my earlier reply. running voltage should be 13.9v - 14.5..........Ron

Battery charged overnight. Started the car and let idle and I got a steady 13.1 v reading. I am letting it idle now to see if that falls off.

I cleaned the switch contacts (though they looked clean) and the battery terminals.

Forgot to mention that there was also an AMB Transponder installed (PCA timing device) that is powered from the battery. Maybe that could be enough of a drain to cause these problems?

After idling for 30 minutes, volts are down to 12.93. Checked the grounds.

The kill switch has a terminal post directly above the battery. When I tested those diodes, the volt readings were .011. Shouldn't those read about the same as the battery?

looks like you have a bad alternator.. your voltage should be higher.. what happens if your watching the meter and you rev the engine to 3k rpm without any thing on.. and again with your lights on? You may have a Marchal alternator?.. which may have a low rpm low voltage .. so it ain't kaput yet............Ron

Okay, after charging some more, v at 13.00. W/lights on at idle they dropped to 12.64. Reved to 3,500 w/lights on and stayed at 12.64

So the alternator is not charging?

Have you verified that your kill switch is not defective? I cannot tell from the posts. I had a similar issue with an old Toyota Celica and it was the brand new, factory part that was causing the short. Replaced the part, no issues.

I haven't verified that the kill switch isn't the culprit. It is a 6 pole switch wired into the alternater and battery. I will try to remove that and see if anything changes.

I have called a couple of parts shops and one of them has a 55 amp, Bosch alternator they say is the correct replacement. Would this unit work or is there something else I have to look out for?

"I haven't verified that the kill switch isn't the culprit."
--- Oh..

"I will try to remove that and see if anything changes."
--- I think that's a good idea.

"and one of them has a 55 amp, Bosch alternator they say is the correct replacement."
--- the stock alt. had an external voltage regulator.. does the replacement use an external regulator? The up-grade is to an internal voltage regulator. Has the up-grade been done already? You will be able to tell if the regulator is removed or disconnected . The reg. is located in the engine compartment, on the fuse pane.

" Would this unit work or is there something else I have to look out for?"
--- I would do the up-grade if it hasn't been done already. It's not any more work.. I would order the Valeo alternator with internal regulator from Pelican Parts .. you definately have a charging problem.. hope it's the kill switch

Robin, and others....you have been a great help. I wasn't comfortable removing the kill switch since it is tied in to the entire electrical system. The car is going to the race shop that installed it tomorrow to do an alignment after my tie rod installation. I think I will have them check the kill switch. If that is okay, I will order the alternator from Wayne and install it at the track this weekend....

OK... Have your battery fully charged. Keep everything off in the car when you're driving it...Good luck..........................Ron