Backfeed?

[TFagan]

I have installed a 1995 LT1 out of Buick Roadmaster in a 1958 Chevy Truck. I am using the alternator that comes with the engine. I am also using a universal wiring harness for the truck from Speedway (20 circuit) With the battery hooked up, the wire that runs from the 2 wire plug is hot. I believe this is powering up the fuses on that buss bar. I believe they should be getting their power from the accessory wire running from the ignition switch. Even the accessory wire from the fuse panel is hot when I hook up the battery. I am afraid to hook it up to the switch because it already has power. I was thinking it might be a grounding issue - would that cause the backfeed? Any ideas will be helpful
thanks

[VetteVet]

Well let's see if you can fill in the blanks for me.
1.So you have any of the wiring from the LT1? ECM, PCM,
2.What fuel feed are you using, EFI, TBI, Carb?
3. What tranny 4l60E turbo 350, 700r4? any wiring for trans?
4.Have you identified all the circuits in the harness?
5. Do you have a separate fuse panel or a junction.
6. Can you elaborate on the buss bar? any pics.
7. Do you have a horn relay mounted on the radiator core support?
8. Do you have a full flow amp meter in the dash cluster?
9. Do you have the points or HEI distributor?
10. Were you running a generator or an alternator conversion.
11. The plug you reference, is it the alternator plug or something else?
12. Does it have the letters S F L P on It or on the alternator?
13. What key switch are you using and does the harness match it.?

The wire in the plug that is hot if it's the alternator plug is supposed to be hot if it is the S terminal wire for the plug.
The wire for the accessories is hot to the fuse panel only with the key on. It should originate at the key switch and
run to the fuse panel.
Your battery hot wire and the alternator output wire should meet at the junction buss or at the fuse panel. Depending on whether you have the old style amp meter or the newer ammeter or a voltage gauge.

I doubt if it's a grounding issue or feedback. probably just wired at the wrong spot.

[TFagan]

I copied your post so I could answer as clear as my skills will allow:

Well let's see if you can fill in the blanks for me.
1.So you have any of the wiring from the LT1? ECM, PCM,
PCM
2.What fuel feed are you using, EFI, TBI, Carb?
TBI
3. What tranny 4l60E turbo 350, 700r4? any wiring for trans? 4L60E
4.Have you identified all the circuits in the harness? Yes - I have a harness from "LT1 Wiring harness" from Denver CO. Engine starts and runs
5. Do you have a separate fuse panel or a junction. there is a fuse panel for the PCM/Engine and one for the truck harness.
6. Can you elaborate on the buss bar? any pics. On the back side of the fuse panel, there is one buss bar that connects several fuses and has 12 volt all of the time. There is a second buss bar that ties all of the fuses that get power on ignition - there is a third buss bar that should power on accessory. This is the where the problem is.
7. Do you have a horn relay mounted on the radiator core support? I plugged the horn relay into the harness but I have not hooked up any horns yet.
8. Do you have a full flow amp meter in the dash cluster? My dash cluster is a dakota digital - all of the modules are wired up but I have not hooked up the panel aas of yet.
9. Do you have the points or HEI distributor?10. Were you running a generator or an alternator conversion. My LT1 has the OPTI-Spark. I just installed a new one. It was shot - now the engine starts and runs.
11. The plug you reference, is it the alternator plug or something else? yes - the 2 wire plug on the back of the factory alternator (1995 Buick Roadnaster alternator)
12. Does it have the letters S F L P on It or on the alternator? SFLP
13. What key switch are you using and does the harness match it.? I am using the stock switch from the 1958 chevy truck (new one)

The wire in the plug that is hot if it's the alternator plug is supposed to be hot if it is the S terminal wire for the plug.
The wire for the accessories is hot to the fuse panel only with the key on. It should originate at the key switch and
run to the fuse panel.
Your battery hot wire and the alternator output wire should meet at the junction buss or at the fuse panel. Depending on whether you have the old style amp meter or the newer ammeter or a voltage gauge.

I doubt if it's a grounding issue or feedback. probably just wired at the wrong spot.

[VetteVet]

This is a great thread because it challenges me and it spans 4 generations of pickups so we have to figure out how to make it all work.

You have power to the buss bar that feeds the PCM and the accessories all the time. You should be getting that power from the accessory side of the key switch otherwise you will be feeding the accessories from the battery power, which will drain the battery unless you can switch off this power. This is where the accessory side of the key switch comes in.

The problem is, I'm not sure that the key switch for the 58 has that capacity.
That's why I asked about the key switch and the harness to match it. If it doesn't then you may have to change it and wire the harness to match it.

The other problem is minor and it depends on whether the PCM needs power to maintain it's memory. ECM's generally do need memory power to keep the settings from deleting when the key is turned off. I'm not sure about the PCM.

Now we can get around to wiring the alternator and that is fairly simple but it also requires a wire from the accessory side of the key switch.
Getting back to the plug for the alternator and the letters S F L P ,

The wire going to S in the alternator is the sensing wire for the alternator fields and should go to the S terminal in the alternator. It can originate from any hot source or even a keyed hot source as long as it connects directly to the battery positive terminal. In your case I think it is the first buss bar.
In some of the later years of alternators the sensing voltage is not necessary since it is derived from the large output wire off the alternator which is connected directly to the battery or the starter solenoid. I think these were after 2000 though.

That leaves the other wire To the L terminal which is the exciter wire for the alternator regulator and it "turns on" the alternator. This wire must originate from a keyed on hot source, usually the accessory side of the key switch or from a charging light in the dash cluster.

For the stock trucks in the 55 to 60 era. This was not required because the charging system was via generator and used a full flow internal shunt amp gauge. In 1963 or thereabouts GM went to the alternator with the external regulator and an external shunt ammeter or they used the charging light.

Now tying this all together means that there has to be some resistance in the wire that goes to L on the alternator. Before the CS style it was provided by the resistance in the charging light ( 10 ohms ) or the ammeter, by a resistance wire, also 10 ohms. This resistance was needed to provide protection to the diodes inside the alternator, and to block a feedback current to the ignition system, which would keep the engine running after the key was switched off.

In the case of your alternator the CS style, you still need this resistance for the same reasons except it has to be a larger resistance. The range is anywhere between 70 and 300 ohms and 1/2 to 5 watts. Painless recommends 85 ohms 5 watts. The fix is easy, just solder the resistor in series with the L wire before it connects to the alternator.

This is what I have interpreted from your description minus the wiring for the
Opti-Spark which I assume is good since the engine runs.

I hope I have been some help. VV

[TFagan]

Thank you - you have been amazing. I will do that when I get back from vacation. I would like to explain the ignition switch and what I know as well as the PCM power wiring.
First the PCM wiring. It does require a hot wire for the memory. I ran this wire to the hot terminal on the starter with a fuseable link. There is i wire that need power at start and run - I ran this wire to the ignition terminal on the switch. With the 1958 ignition switch it only had 12 volts at the start terminal at start/crank and the ign terminal only had power in the run position. I had to run a jumper wire from the start terminal to the ign terminal. This is defined on "Street and Performance" website - great help.
The ignition Switch - there is a terminal for 12 volt power "BAT" / terminal for ignition "IGN" / terminal for starter Solenoid "SOL" / terminal for accessory "ACC". The key has four positions. Off - ACC - Run - Start. I get power at the start and run positions at their respective terminals. I do not get any power at the ACC terminal with the key at the ACC key position. It does show 12 volts when the key is in the rum position. Right not, because of the wiring problem you have been helping me with, I do not have any wires hooked up to the ACC terminal on the switch. If you think the switch is suspect as well, what would your recommend?
Thanks again for your expert help
tom

[VetteVet]

I'm going to dissect your post for clarification.

Originally Posted by TFagan

Thank you - you have been amazing. I will do that when I get back from vacation. I would like to explain the ignition switch and what I know as well as the PCM power wiring.
First the PCM wiring. It does require a hot wire for the memory. I ran this wire to the hot terminal on the starter with a fuseable link. Sounds Ok There is i wire that need power at start and run - I ran this wire to the ignition terminal on the switch. This might be the fuel pump relay power wire. With the 1958 ignition switch it only had 12 volts at the start terminal at start/crank and the ign terminal only had power in the run position. Normally there would be power at both during cranking, but because of the 12 volt jumper to the ignition coil from the starter "I" terminal they may not have powered the start and Ign terminals on these key switches. You must have some method to power the Opti-Spark during cranking.
I had to run a jumper wire from the start terminal to the ign terminal. This is defined on "Street and Performance" website - great help. This doesn't make any sense because in the run position the ignition terminal would be sending power to the start terminal and the solenoid would try to engage Unless you have a diode in the jumper wire .
The ignition Switch - there is a terminal for 12 volt power "BAT" / terminal for ignition "IGN" / terminal for starter Solenoid "SOL" / terminal for accessory "ACC". The key has four positions. Off - ACC - Run - Start. I get power at the start and run positions at their respective terminals. I do not get any power at the ACC terminal with the key at the ACC I think that it should have power in the ACC position otherwise it's nothing but a junction post for power to the accessories when the key is in run. It does show 12 volts when the key is in the rum position.

Right now, because of the wiring problem you have been helping me with, I do not have any wires hooked up to the ACC terminal on the switch. If you think the switch is suspect as well, what would your recommend?

Well you can try another switch that has power to the ACC position when the key is off or you can do without that feature. About the only thing it does is allow you to listen to the radio with the engine off, and send power to the heater, wiper, and turn signal switches, depending on how they are wired. I'd have to look at a 58 wiring diagram to figure that out.

Thanks again for your expert help. Hey this is fun for me it helps the boredom of these cold winter days when I don't feel like going outside.

tom

[TFagan]

Thanks again. I will let you know the progress next week when I get back home

[TFagan]

I just peaked at the alternator plug before we left. The two wires coming from the plug are the F and the L. Is the F and the S mentioned in a previous reply the same ? F Eire should go to constant 12 volt power

[VetteVet]

Originally Posted by TFagan

I just peaked at the alternator plug before we left. The two wires coming from the plug are the F and the L. Is the F and the S mentioned in a previous reply the same ? F Eire should go to constant 12 volt power

The F terminal is for the ECM. Its a trouble code sender. You'll use the S and the L as I described above. You may not need the S wire but it won't hurt anything if you do.

[TFagan]

So if all I have is the F and L wires. No S wire. How does this work?

[VetteVet]

Try it with just the L wire and if it works, fine, if it doesn't you will have to wire in an S wire.
Don't forget the resistor in the L wire. Remember some of the later alternators had an internal resistor but I don't remember what years. I'll see if I can find out when they started using the internal resistors.

[67 Burb]

Originally Posted by TFagan

I just peaked at the alternator plug before we left. The two wires coming from the plug are the F and the L. Is the F and the S mentioned in a previous reply the same ? F Eire should go to constant 12 volt power

I believe that some of these alternators have the S connected internally.

I have a 95 Roadmaster and I have the factory manuals including all the wiring schematics. I will be back home on Saturday and will be able to check my manuals for any wiring questions you have.

[TFagan]

67 Burb - did you have a chance to check this out? Im almost convinced all I need to rum is the "F" wire. I am still not confident. Not having the "S" wire in the plug is confusing

[VetteVet]

You won't need the "F" wire just the "L" wire with the resistor. I believe the resistor is internal on the later series model 144s. The "S" wire may be internal as well but if it is needed then the alternator won't charge correctly until it is connected.

[67 Burb]

Originally Posted by TFagan

67 Burb - did you have a chance to check this out? Im almost convinced all I need to rum is the "F" wire. I am still not confident. Not having the "S" wire in the plug is confusing

TFagan, In the connector, the S and the P are not used, just the F and the L. The F comes right from a 10 A fuse that is hot in RUN, and L comes from a 10 A fuse that is hot in RUN and START and then through an idiot light.

I scanned the page from the manual, but it is too large of a file to post here, so I sent you a PM.

[67 Burb]

I trimmed the image down to a workable size. Here it is.