Not Getting Enough Voltage
 

I've run into a problem on my '70 GMC k2500 that is really boggling me. I swapped out the original 350 with points style distributor to a brand new crate motor modeled after a '78 with the HEI distributor. I currently have the Holley Msd Street fire distributor in it and I have bypassed the original ballast wire that was factory on the '70 trucks. I have a miss that is really only noticeable at around 2500 rpm that is random in how it misses. I also have a new alternator, new battery, and new voltage regulator but I'm only getting 12.5 volts at the battery when the truck is running. Everything under the hood is pretty much new so I'm confused as to why it's running so poorly. Any suggestions?

Re: Not Getting Enough Voltage
 

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Re: Not Getting Enough Voltage
 

Sounds like the alternator isn't charging. You're reading battery voltage.
Pull the plug on the alternator and check the white wire for 12 volts with the key on and zero with the key off.


Attachment 1795411



This is the exciter wire that turns on the alternator. The blue wire should have 12 volts all the time with the battery connected.

If you don't get the readings then check the regulator plug for the same readings on the brown wire 12 on/ 0/off and the red wire for 12/12 volts.

If you get these readings then the alternator is most likely bad.


The MSD distributors don't like the voltage surges that the external regulator points generate. You would be better off with a SI or CS style alternator for more amperage at the lower RPMs. There are tons of conversion threads on the net and here in the forums if you're interested.

Re: Not Getting Enough Voltage
 

That's a good idea. I work at the oreillys by my house and I have put 2 different factory 55 amp alternators on there plus a new regulator and every time I check it it says the regulator failed and it's only pulling the 12.5 volts. I had the factory capacitor on the alternator which is likely bad but I don't know enough about those things to know if that would affect my voltage. I don't know what to look for for an si alternator since I'm not familiar with that swap. What can I do in the mean time to up my voltage and get the charging system up to par? My dad had almost the identical setup in his '68 when he was a kid and never had any of the problems I'm experiencing

Re: Not Getting Enough Voltage
 

See the black wire in the picture above. It is a ground wire from the regulator to the alternator and they must be connected plus the regulator has to be grounded to the radiator shroud. The regulator is mounted in rubber nuts so it needs to be grounded.

The capacitor is not part of the charging system, it helps prevent the static in the radio but I think it is mounted on the regulator.

The internally regulated alternators started in 1973 or 74 and the better ones came out in the late eighties on camaros firebirds buicks and chevy trucks. The 12 SIs are the best. Most GM vehicles used them.
You can tell them by the larger vents in the rear of the body.

Attachment 1795624

The wiring conversion is easy to do, if you decide to get one just post up and well show you how to do it.



You can jump the regulator and full field the alternator just to see if it will charge. by jumping the red and white wires in the regulator plug and the alternator will go to full charge but don't do it for more than 30 seconds because the alternator will get very warm and could over charge the system.
It could also blow some lights if they are on. I';ll double check the jumping procedure to be sure I told you the right wires.

Re: Not Getting Enough Voltage
 

Interesting about the rubber nuts on the regulator. When I had gone to replace it it was all held in by regular flathead machine screws with nuts on them through the radiator support. I would think that would give it better ground since I've done all sorts of extra ground wires all over the truck to ensure that body and frame are sufficiently grounded. And then it's metal on metal for better connection. That may be my whole problem as to why I'm not getting the ≈14.2 volts when running, just another ground problem. I still think I'm going to do a 12si swap since I plan on building a heavier front bumper that will have a winch as well as a roll bar with lights so I'll need the extra amperage anyway so I'm sure you will all hear about my beginning to the swap and I'll be needing some helpful hands to walk me through and I'd definitely appreciate the advice.