3G Alternator Swap Issue

[RedOctober94]

So I went ahead and did a 2g-3g alternator swap on my 87 mgm. I did this while in the midst of having the upper intake off and changing the fuel injectors and valve covers. Now the car has sat for about a couple weeks since the swap and sat for a couple months before that. I ran all new wires including a 4ga to a 175 amp fuse and then to the starter solenoid. I finally got the car back together today and tried to start it. Upon trying to crank it, there was a spark from the starter solenoid and everything went completely dead. There is no power to anything what so ever and it will not crank. I haven't done any testing yet as it but I assume I blew the solenoid. Any advice as to what I should do or where I went wrong. Thank you in advance.

[gadget73]

More likely its loose or just crusty. The solenoid failing won't cause this. Make sure the battery cable is clean and tight at the battery, if thats good, pull the stack of junk off the solenoid and make sure all of those ring terminals are clean.


if you find that some stuff doesn't work you may need to connect that other cable. Some cars seem to tap other things off the same fuse link that feeds to the original alternator charge cables. My 86 did not but I know some people have had the headlights not work without that connected.

[RedOctober94]

More likely its loose or just crusty. The solenoid failing won't cause this. Make sure the battery cable is clean and tight at the battery, if thats good, pull the stack of junk off the solenoid and make sure all of those ring terminals are clean.


if you find that some stuff doesn't work you may need to connect that other cable. Some cars seem to tap other things off the same fuse link that feeds to the original alternator charge cables. My 86 did not but I know some people have had the headlights not work without that connected.

Thank you for your advice. I went through and checked everything. The issue ended up being the the battery ground to the block was bad. Once I replaced it it started and had no issue.

I'm having another problem now though. Everything is hooked up and secured. When I start the car and let it run, the alternator will get extremely hot and even start smoking. The belt will get extremely hot too. When I put the belt back on (I have not changed the belt) it was snug but had play in it. Once the car runs, the belt gets scalding hot it gets very tight and will stay tight even after the car has cooled.

I figure maybe the belt is contributing to the alternator smoking because of added tension on the bearings inside but I am not sure what is causing it. Bad Alternator? Do I need a longer belt? Any help would be much appreciated. I'm lost on it.

[gadget73]

sounds like its working too hard for some reason. Not too much to the wiring so unless its dead shorted to ground and has a blown fuse at the battery end thats preventing it from smoking the cable, a bum alt seems most likely. Shorted internal winding or bad rectifier maybe. If the voltage is like 18v or so, thats a regulator problem.

on the subject of regulator problems, whats the yellow wire on the alternator plug connected to? if its going to the original harness and that isn't connected at the battery end, you will have no battery sense and it will try and charge at full go. Should be able to measure battery voltage from the yellow lead to ground. If not, thats why. Can jump the yellow wire to the charge stud, but a slightly better method is to get it from the starter relay so it compensates for the small bit of loss in the cable. Mine has been jumped to that stud for probably 18 years so its not exactly that big of a deal.

[RedOctober94]

sounds like its working too hard for some reason. Not too much to the wiring so unless its dead shorted to ground and has a blown fuse at the battery end thats preventing it from smoking the cable, a bum alt seems most likely. Shorted internal winding or bad rectifier maybe. If the voltage is like 18v or so, thats a regulator problem.

on the subject of regulator problems, whats the yellow wire on the alternator plug connected to? if its going to the original harness and that isn't connected at the battery end, you will have no battery sense and it will try and charge at full go. Should be able to measure battery voltage from the yellow lead to ground. If not, thats why. Can jump the yellow wire to the charge stud, but a slightly better method is to get it from the starter relay so it compensates for the small bit of loss in the cable. Mine has been jumped to that stud for probably 18 years so its not exactly that big of a deal.

I have the yellow wire routed into the original harness along with the two black/ orange wires which connect to this black plug and then runs to the starter solenoid. They were attached to the original 2g alt harness. Per your advice, I ended up reattaching them and hooking them up to the back of the new Alt. I did this before I fixed the battery to block ground. Everything seemed to be working as it should except for the smoke and the weird issue with the belt. I haven't had the chance to really dig into testing the wire yet. I've gone ahead and bought a new Alt. I'll be hooking it up and doing some test in the next few days. Thanks again for your help.

I'm also having an issue with my tps that I want to run by you soon as well. Would really appreciate your input.. I've had this issue before I even started the Alt conversion.

[gadget73]

so just to confirm you can measure battery voltage at the yellow wire?

I would not hook up the black and orange wires to the new alternator. Having multiple fuses in the system makes it fused for a completely unknown number of amps. If that harness happens to have other things connected to it at the battery end it needs to be hooked up on that side only. Some do, some don't, seems to depend on the specific vehicle. I cut the black wires at the driver's side fender on mine a long time ago, the only cable is the heavy one to the 150 amp breaker I'm using to protect it.