CWerner

Guys, I need some help figuring out an electrical issue on my car.

As you may remember I completed my LS3 conversion on my 72 about 2 yrs ago(LS3/480 with GM harness, Keisler 5 spd, external fuel pump). I've got about 3500 miles on it and it's been running great. Two weeks ago I had it out and after a little while I noticed the volt meter was sitting on 11 v. It normally is 13-14. I know I have a small drain on the battery when it sits so I decided to put it on the trickle charger when I got home. 6 hrs later it was charging in storage mode. The next time I drove it the volts were like normal so I didn't give it another thought.

Yesterday I took it out for an hour ride. About 45 minutes into the ride as I was running about 45mph, the car stalled. It was a blink of the eye stall and restart. I saw the engine light flick on and then off as soon as the car restarted.

I glanced over to the volt meter and it was about 11.5 v. :About one minute later it jumped up to 14, and then in a blink of an eye it went right back to 11.5. As I slowed down it would drift down to 11. By the time I got home it had dipped below 11, maybe 10.8 v.

I have no idea where to start other than looking for obvious wiring issues. I'll check the emc box and fuel pump wiring first.

I'm curious to understand what are potential causes of the voltage diving down like it has and what are some diagnostic things to go do?

Cheers,

Chris

MelWff

have you checked the wiring connections to the alternator, not sure what you have but if you have the two wire connector in addition to the battery output they can fail right where the wire meets the connector and the failure can be hidden by the insulation.

mrvette

First thing, you need a DVM to get accurate readings....and you need measure the voltage at the alt out stud directly.....after that we get more involved.....

need know more about your wiring also....like does the alt out stud go to the horn relay on the firewall?? is it routed the same/stock route to get there?? are your sure your engine is grounded to the chassis/frame with a good solid battery cable??

PM me to make sure I don't miss this thread, if necessary....

CWerner

All of the wiring from the engine goes to the gauges via the GM LS wiring harness. The gauges are the new Dakota Digital corvette gauges.

Engine ground is good.

zwede

How did you connect the LS harness to the car wiring? Any crimp connectors? Everything needs to either be soldered or use good crimp connectors and they must be crimped with the proper tool.

CWerner

Zwede, I did the proper crimp connections with the right tools( no solder ). With that said, I won't rule out that I made a bad connection.

Gordonm

The PO on my Cobra use daily a lot of crimp connections. When I bought it the car was running but he only put 300 miles on it. One of the cooling fans was not working. While I had the body off I went through the wiring. It all looked good until I started pulling off the tape. One of the butt connectors was fried totally. These fans draw a few amps and the connection was bad. I have since redid all the butt connectors I could find with a soldered joint. Time consuming and tedious but I think it is worth it. I would start checking some of the alternator connections.

CWerner

So I've found a couple issues. First off, my negative battery post connection wasn't tight.

The second thing I found was a hot wire with the coating melting off. The wire is in the engine bay next to the wiper motor. My DocRebuild diagram shows it's in a fusible link connection. One end of the wire is connected to the 10g horn relay battery connection. The other "red end is the 12g solenoid "B". These wires are tied to a fusible link which has a 14g wire running to the "AA ignition harness to firewall connection ". Part of this wiring connection is a 2" long 20g orange wire between 2 rubber fusible links. That is the wire that's gotten hot and melted the wire coating.

I have no clue if this is an old or new problem. I want to say I remember seeing this wire before (during my LS conversion) and it was already melted. ??????

I'm guessing my voltage issue was the battery terminal. Does that make since?

What the thoughts on this fusible link issue? I wrapped the wire with electrical tape as part of the wire was exposed.

zwede

There is a fusible link that attaches to the horn relay and then goes to the field winding of the alternator. If it's broken the alternator will not charge. There's a second fusible link on the horn relay for the ammeter.

If the insulation was melting it means the wire itself has degraded and has too much resistance. Maybe strands have broken and its pushing all the current through just a few intact wire strands.

You should replace the fusible link. They are available from the usual suppliers.

CWerner

I took the car out today to see if I had fixed the voltage drop issue. Within 2 minutes of driving I started seeing the voltage drop from 13+ to 12.8, then 12.6...... I pulled into O'Reily's to get the alternator tested and it failed. At that point I was dropping down close to 11 v. When I got home (5 miles) I noticed the voltage had bounced back to 14.

I hope it's the alternator. I bought a new one so we will see.

Antz81

the orange wire is the fusible link. The black rubber covers where it is spliced to the other wires. As zwede said you need to replace it. Better to do it now while it's showing signs of failure than to wait for to totally fail at the most inconvenient time.

personally I would try this before swapping the altenator

CWerner

New alternator installed.

Volt meter reads 13v.

Battery reads 14.3 v

Alt reads 14.4 v


?????

CWerner

This morning I tested the voltage at the fuse block. I got 14.4 on both the battery and the ignition terminals.

Went for a ride. Started with 13 v. As I accelerated the voltage dropped. When I let off the gas, or put the clutch in, the voltage would rise. Went all the way down to 11v.

I went inside the grocery store, came back out and drove 10 miles to O'Reily's to retest the alternator. On the way there, it pegged at 13 and never moved.

They tested the alternator and it read 13.2v but failed the voltage regulator test (same as the prior alternator).

Now What?

CWerner

My wiring is very straight forward. .


1) I have the one power line to the alternator. It is the original C3 wire from the car's wiring harness.
2) I have the plug-in connector from the ECM wiring harness. My setup is an LS1 108 amp alternator which has a 4 wire plug connector. There is only one wire coming from the harness that goes into the factory plug connector (using the wiring harness's factory connection plug).


When I installed the LS3, I ensured that all of the existing harness connections were replicated. So on the Starter, the same exact wires from before are connected to the new starter. There is one new power line coming off the starter and it goes straight to the ECM.


When I connected the interface between the new wiring harness and the old, there were very few direct connections from the new harness. Per the GM instructions:
1) Orange wire, circuit 40F....10A Fused 12V battery power - this is a power output supply and is always enabled.
2) Pink wire, circuit 5292......15A Fused 12V Ignition On Power - This a power output supply and is enabled only when the ignition is on.
3) Black wire, circuit 50B......Ground - this is used as the low reference ground for completion of the MAP, TPS, and oil pressure output circuits. It can also be used for modules conned to either of the fused 12 V outputs.


I also had to connect other items directly to the ECM such as fuel pump, and electric fans.


I've checked all of my battery connections again, the engine and frame grounds, and the starter connections. all are solid.


What confuses me is that the alternator is managed by the ECM and has nothing to do with my old wiring system....other than the power wire that feeds it. Maybe I should run a jumper wire between the starter and the alternator and see what that does.

zwede

I have an aftermarket fuel injection system (Holley HP) and the instruction were very specific that ECM power & ground must be connected directly to the battery.

71 Vert LS1

I don't know if this helps but I've always run a heavier cable (6 gauge?) from the starter solenoid stud to the stud on the back of the alternator.

I'm wondering if some where in the old Corvette harness wire that supplies power to the alternator there are maybe wire breaks or corrosion?

I've done about a dozen LS swaps that way without an issue. I've used Autometer gauges exclusively and the volt meter is always at 14 volts.

You don't need to wire the 12 volt constant to the battery if you connect it to the stud on the starter solenoid.

I too only have the one wire that plugs into the alternator from the LS harness.

Gordonm

This is the way I wired my LS3 swap also. I don't have to many miles on it but I figured over kill on the wiring is better than not enough.

Solid LT1

My 72 LT-1 had intermittent high speed miss....that finally exposed itself as the factory harness rubbing against the cast iron steering column bracket behind the drivers side dash. It would blow the fuseable link at the horn relay, I would search for shorts and do changes to my Accel 300+ mutispark box...finally wired a separate relay to feed +12V from a dedicated feed to the CDI box....took the wife on a nice 200 mile trip....car didn't miss a beat the entire trip....the following weekend....while warming up the Vette to make an AutoX event...started missing and a wisp of smoke came out of LH dash.....Time to take things apart and found the problem...96K miles on the Vette....Intermittants are very hard to diagnose....I had put the next size bigger fuse link in and then....found the problem....

CWerner

Okay, so I put a 10g jumper between the starter and the alternator. I used 10g because it's what I had available. I had it tested this morning at O'Reily's and it passed...... 6 times. The odd thing is that the tester was reading 13.8 and my gauge was 12.5. My Dakota Digital gauge might also have an issue.

For some reason, I don't think this chapter is closed yet.

71 Vert LS1

I would buy cheap volt gauge and wire it in to see what it reads compared to the digital gauge.