1990 k5 blazer, I have a kill switch hooked up to my altinator, runs good with out the alt but runs like crap when alt is ingauged, I have had this truck in with 3 different mechanics and they have all said the same thing, can't figure it out and if you do let us know. You name it I have replaced it, from fuel tank to computer

 

What voltage is the alt putting out when engaged?

 

13.4, it charges the battery, but runs like crap

 

That is a strange one. I assume you have cleaned and made sure there is good contact at the battery terminals and all the grounds. Does it throw any codes after running rough? Also, you changed the computer, did you re-use the PROM chip from the old computer? Have you checked for AC voltage on the ALT output, real common for the diodes to go bad and leak AC into the system. Whacks out all sorts of sensor readings.

 

Have changed computer and prom, mechanics that worked on the truck said it doesn't throw any codes out. Someone suggested new wiring harness, but not sure that will fix it but I don't think I have a choice at this point.

 

Did you clean and check all the grounds and check for AC on the output? Randomly replacing parts in the hope you stumble upon the solution can get expensive fast.

A 1990 should still have the internal voltage reg, have you tried running the switched activation wire directly to the battery to eliminate the vehicle harness from the equation?

 

Did check all grounds and no on the ac part, now you have gone above what I know about this truck. But hopefully the guys I took it did what you asked, but now I'm at a stand still and yes it does get expensive replacing parts but on the other hand it's nice knowing that a lot of these parts are new now

 

13.4 is too low, should be 14.2 volts. Might be a problem with the alternator, regulator, or battery, but there is a problem.


Up until some time in the 1960's, cars had generators, and they generated DC, (Direct Current). Since then, cars use alternators, and they generate AC, (Alternating Current). There is a rectifier bridge, (diode trio) in the alternator that changes it back to DC. As mentioned, if a diode in the bridge is shorted out, AC current will "leak" into the system. In 40 some odd years working on vehicles, I found what you are experiencing one time. Once it hit a certain RPM, it ran like crap! Replaced the alternator and all was fine. A problem in the rectifier could easily cause low charging voltage, and cause the engine to run "like crap".

 

Find someone with a Fluke brand multimeter, set on AC the output of the ALT when turned on should read 0 volts. If you get a several volt reading, the diodes are shot.

 

I have a 1990 blazer 350 5.7ltr tbi.
Truck runs good without the alt plugged in. Plug in the alt and it falls on its face, bogs and won't run right. I have had three different mechanics look at it. This is what they replaced. New gas tank. New sending unit along with fuel pump, new fuel filter. New computer new prom chip. New injectors new alt. New map sensor
I'm again at a wtf moment. Oh and new distributor and coil. I have a toggle switch i hook up to the wiring of alt so i can give my battery some juice when I come to a stop light, has anyone ever had something like this happen?