Scan tool not not reading voltage correctly

joeb1300

Member
Can someone help me . I have been battling alternator issues for some time. Recently bought a alternator from highway stars (ac delco). Car was running 14.4 volts when out in the car. 8 months later I decided to take car out for a drive and it started over charging for up to 19 volts or so the scan tool said. Killed my red top optima battery . Replaced battery and had new alternator rebuilt my a local shop. Scan tool when car is started shows 11 volt to 10.9. Volt light is off . Meter shows 14 on alternator and battery . Please help me understand this. After about 10 to 15 minutes scan tool shows 13.9 which is still low but closer the 10.9
 

TurboBob

Resident mad scientist
Staff member
Scantools generally don't measure voltage, they show what the ECM thinks the voltage is. The stock ECM measures voltage on the wire from the ignition/injectors circuit. you should measure the voltage in that circuit and look for issues in the fusebox/bulkhead.

A bad ignition module can pull this voltage down also.

Bob
 

joeb1300

Member
Scantools generally don't measure voltage, they show what the ECM thinks the voltage is. The stock ECM measures voltage on the wire from the ignition/injectors circuit. you should measure the voltage in that circuit and look for issues in the fusebox/bulkhead.

A bad ignition module can pull this voltage down also.

Bob
Thanks will look into this.
 

GN86Rome

Member
Question my original alternator is rated at 120amp. Is that enough to run my car and support hot wire kit and direct voltage for my injectors?
 

TurboBob

Resident mad scientist
Staff member
They would be wrong, and the 200A alternators tend to not charge well at idle.

(how many amps do they think this stuff draws anyway....?)

Bob
 

GN86Rome

Member
Thank you very well said I want
to ask you another question please. Can I eliminate the two small water hoses that go to the throttle body? Is their purpose to warm the air entering the air Plenum ?
 

TurboBob

Resident mad scientist
Staff member
everyone removes those hoses, I'm not aware of anyone having any issues.
I believe the original purpose is more to prevent the throttle body from icing up on cold/wet days.

Bob
 

dynoman

Well-Known Member
Thank you very well said I want
to ask you another question please. Can I eliminate the two small water hoses that go to the throttle body? Is their purpose to warm the air entering the air Plenum ?
Yes , you can loop them or block them off . I had my TIG welded shut .
 

GN86Rome

Member
Question eye’ve updated my value springs, installed new injector harness, installed fuel pump 255 LPH to include hot wire kit, installed 60lbs injectors, fuel regulator, RJC racing air flow plate, 3”up pipe, 3” air pipe w/ 3.25 mass air flow. With K&N filter Stock turbo and inner cooler. The plan is to install 10” 2800 stall converter. Oh yeah ordering a scanmaster. Hopefully she should run no spray. Do you think im in the low 12’s?
 

joeb1300

Member
Thanks will look into this.
Thought I had this figured out but looks like I don’t. Added a 4 gauge ground wire and cleaned up some frame grounds.. Drove the car this morning and had normal voltage until the drive home from work and dam back down to 12-10.9 volts. Have three different volt patterns. First one is normal 13,5-14.1 on scan master. Second pattern is low all the time 10.9-12.0 third pattern is low voltage at idle then climbs up as rpm’s increases then goes back down with rpm’s. Where is that ignition module? I have a Casper’s light bulb. Next thing is to change this. Don’t think this is my problem.
 

joeb1300

Member
Gotcha. Thanks
After a whole day Sunday I found that my instrument cluster is the problem. Apparently the rib ion strip that connects the light bulb or the plug that cluster makes contact with isn’t working like it should. Pushing on cluster and moving it around effect the charging. . Just ordered a Casper’s charging fix. Hope this helps . Will keep things posted as I find out . Thanks for everyone jumping on here to help.
 
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