70 C20 Won't run with battery unhooked

[70C20Longhorn]

I've always understood that an engine should run without a battery. It could be jumped and then run til it's out of fuel. My truck starts and drives but as soon as I unhook the battery it shuts off. I did the external to internal alternator conversion using an alternator from a 1980 K20. Now I checked for power at the alternator (truck not running) key off I have 12 volts and key on 12 volts. As I understand I shouldn't have power at the alternator with the key off. At first I didn't have my dash plugged in but now I do and same results. Some wiring diagrams show a horn relay, not sure if that would have these symptoms if it's not there. Any guesses...

[custom10nut]

Check your alternator connection, and all your body to frame grounds.

[prodjay10]

I've read on this forum that the newer style alternators won't kick on until you rev the engine. I'm not 100% on that. Maybe someone with more experience will chime in.

[LongBox]

I tried this on my 69 when I first went to check it out. It had been put away with no battery. I jumped it with my truck, took off the cables, and it continued to run just fine.

I believe yours should do the same, and the connections are the first place to start, as pointed out above.

[Mike C]

I'd be wary of running the truck without the battery. It is an integral part of the electrical systems feedback that controls voltage output and current. Without somewhere to dump the output the voltage goes into other components that can cause their failure.

Haven't proven it on a car since I learned that the hard way on a riding mower.

[VetteVet]

Originally Posted by 70C20Longhorn

I've always understood that an engine should run without a battery. It could be jumped and then run til it's out of fuel. My truck starts and drives but as soon as I unhook the battery it shuts off. I did the external to internal alternator conversion using an alternator from a 1980 K20. Now I checked for power at the alternator (truck not running) key off I have 12 volts and key on 12 volts. As I understand I shouldn't have power at the alternator with the key off. At first I didn't have my dash plugged in but now I do and same results. Some wiring diagrams show a horn relay, not sure if that would have these symptoms if it's not there. Any guesses...

Your alternator is The SI series internally regulated, and you should have power on the large lug, key on or key off. It should be connected to a terminal junction with the battery charge wire. The truck should continue to run with the battery disconnected because the alternator if it is charging,, is self sustaining by generating it's own field current. As said above, this is not a good idea because the battery sets the voltage limit for the alternator at 14.5 +or- a few 10ths via the sensing circuit and the internal regulator. If you disconnect the battery the alternator will default to a full charge condition, and will go as high as 18 volts output, which may damage other components, especially lights on the older vehicles, and computers and PTMs on the newer models.

You need to recheck the conversion wiring, and especially the wire on terminal 1 on the alternator. It is the alternator field exciting terminal, and it must have 12 volts on it with the key on, and no voltage with the key off. The wire on terminal 2 is the sensing wire, and it should have the same voltage as the large terminal on the charging stud. Key on, or key off.

The conversion wiring is relatively simple. All it does is wire around the external voltage regulator. The brown wire on terminal 4 of the EVR is the exciter wire from the ignition, and it can be moved directly to the no. 1 terminal on the alternator, or it can be jumped to the no. 2 White wire on the EVR plug. Then all that's left is to run a battery hot wire, to terminal 2. Either directly via the no.3 red wire on the EVR plug or jumping the no. 3 red wire on the plug to the F terminal (blue) that goes to the alternator. The plug in for the SI alternator is different, as you know, so you will nave to change that.

Here is the diagram for the jumpered plug on the EVR.



Here is the simpler way I do it, which uses less wiring and eliminates the EVR harness.




Some guys will just loop the no.2 terminal directly to the large wire on the alternator stud. This will work, but it decreases the alternator's ability to remote voltage sense the the wiring circuit downstream from the alternator, and it will not allow the alternator to compensate for voltage drop away from the alternator. This can amount to as much as 2 volts on some circuits.


The first check you will want to do is to measure for 12 volts on the wire going to terminal 1 on the alternator 12 volts key on and zero key off. Then measure 12 volts on terminal 2 both ways, key on and key off.
If you get those measurements then you should measure for 14 volts at the output stud of the alternator. You may get 13,5 at idle but definitely more than battery voltage.

The alternator exciter wire for the alternator brown/white originates at the key switch and runs through a resistor (10 ohms to the firewall plug to the 16 gauge brown wire which runs to the EVR plug no. 4.

If you have the charging light dash cluster you'll also have a parallel wire from the ignition which runs to the light, which also has a resistance of 10 ohms.
Then the wire from the light, also brown, joins the resistance wire from the key switch and the 16 gauge wire from the firewall block to the EVR plug.
The factory used the resistance wire as a backup in case the charging light bulb blows out. The gauge cluster does not have the charging light, so it just uses the resistance wire. This means the cluster does not have to be plugged in to get the alternator to charge.

Hope this helps.