Electrical Question
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Joined: May 2011
Posts: 60
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From: Wenatchee WA
Electrical Question
Is it normal on a 85 vette with stock 17si alternator to have the idle dip about 100 rpms when I turn on the headlights. The idle does recover immediately though.
I am thinking my alternator is weak as I get a flicker in dash and interior lights at idle. Idle set to 550rpm in drive and at operating temps.
Voltage output is good. 14.3 with no accessories on.
I am thinking my alternator is weak as I get a flicker in dash and interior lights at idle. Idle set to 550rpm in drive and at operating temps.
Voltage output is good. 14.3 with no accessories on.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
Its not your alt....it IS your grounds..the harness grounds.
You have 2 real battery grounds. One cable and a small strap by the oil filter. Those get tired and barely make contact and things start to flicker and act up. You can add a ground from battery to ?? Your call. I added one from battery to block and another battery to frame near the PS shaft. I also added ground wires from other items to frame or some common location. Because these cars are managed by the grounds and not the power wires, having good solid grounding is essential for good ecm stability and other system controls.
You also need to clean and repair the harness grounds on the trans/block bolt.
As far as the dip in RPM...its a matter of the load placed on the system. If there is a sudden high load then yes, the increased load on the alt is enough to drag the engine down. The alt uses 3 or 4 hp just to turn, and at idle the engine is making its min hp. The alt drag might be as much as 10% of what the engine is able to produce, so it lugs the engine down for a second. Mine does that if I bottom out the power windows...
You have 2 real battery grounds. One cable and a small strap by the oil filter. Those get tired and barely make contact and things start to flicker and act up. You can add a ground from battery to ?? Your call. I added one from battery to block and another battery to frame near the PS shaft. I also added ground wires from other items to frame or some common location. Because these cars are managed by the grounds and not the power wires, having good solid grounding is essential for good ecm stability and other system controls.
You also need to clean and repair the harness grounds on the trans/block bolt.
As far as the dip in RPM...its a matter of the load placed on the system. If there is a sudden high load then yes, the increased load on the alt is enough to drag the engine down. The alt uses 3 or 4 hp just to turn, and at idle the engine is making its min hp. The alt drag might be as much as 10% of what the engine is able to produce, so it lugs the engine down for a second. Mine does that if I bottom out the power windows...
Intermediate
Joined: Feb 2013
Posts: 41
Likes: 0
From: Spring Texas
Its not your alt....it IS your grounds..the harness grounds.
You have 2 real battery grounds. One cable and a small strap by the oil filter. Those get tired and barely make contact and things start to flicker and act up. You can add a ground from battery to ?? Your call. I added one from battery to block and another battery to frame near the PS shaft. I also added ground wires from other items to frame or some common location. Because these cars are managed by the grounds and not the power wires, having good solid grounding is essential for good ecm stability and other system controls.
You also need to clean and repair the harness grounds on the trans/block bolt.
As far as the dip in RPM...its a matter of the load placed on the system. If there is a sudden high load then yes, the increased load on the alt is enough to drag the engine down. The alt uses 3 or 4 hp just to turn, and at idle the engine is making its min hp. The alt drag might be as much as 10% of what the engine is able to produce, so it lugs the engine down for a second. Mine does that if I bottom out the power windows...
You have 2 real battery grounds. One cable and a small strap by the oil filter. Those get tired and barely make contact and things start to flicker and act up. You can add a ground from battery to ?? Your call. I added one from battery to block and another battery to frame near the PS shaft. I also added ground wires from other items to frame or some common location. Because these cars are managed by the grounds and not the power wires, having good solid grounding is essential for good ecm stability and other system controls.
You also need to clean and repair the harness grounds on the trans/block bolt.
As far as the dip in RPM...its a matter of the load placed on the system. If there is a sudden high load then yes, the increased load on the alt is enough to drag the engine down. The alt uses 3 or 4 hp just to turn, and at idle the engine is making its min hp. The alt drag might be as much as 10% of what the engine is able to produce, so it lugs the engine down for a second. Mine does that if I bottom out the power windows...
What you are describing here, would this be my most likely problem on my '86 convertible (pertaining to the grounds)? What I have going on is when I start the car, it acts like my battery is about dead. Also, the problem gets worse after I have driven the car and it's hot. The dash practically goes out, the engine barely turns over but does manage to start. Also, after driving it, the dash and lights have a flicker to them and I get a low voltage warning at 11.3 volts. I eliminated the battery by jump starting it with my truck and it does the same thing. I have yet to get under my car but I plan to in the near future. Go easy on me, I'm a complete noob to Vettes!
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
The electrical connections on the battery, alt, starter and ALL the harness grounds are critical.
The power connections on the jumper post also cause these symptoms.
These cars are electric. Run by a computer. Yes, all this matters...a LOT.
The power connections on the jumper post also cause these symptoms.
These cars are electric. Run by a computer. Yes, all this matters...a LOT.
Intermediate
Joined: Feb 2013
Posts: 41
Likes: 0
From: Spring Texas
LOL, thanks. I did forget to mention that the connections on the battery and alternator are tight. I just have to find time to get under the car and check the grounds. I guess I will learn real fast how electric they are! I've been doing a lot of reading and the one thing I've gotten drilled into my head so far is that grounds are extremely critical!
Safety Car
Joined: Aug 2009
Posts: 3,751
Likes: 276
From: Melbourne, Victoria, Australia
Is it normal on a 85 vette with stock 17si alternator to have the idle dip about 100 rpms when I turn on the headlights. The idle does recover immediately though.
I am thinking my alternator is weak as I get a flicker in dash and interior lights at idle. Idle set to 550rpm in drive and at operating temps.
Voltage output is good. 14.3 with no accessories on.
I am thinking my alternator is weak as I get a flicker in dash and interior lights at idle. Idle set to 550rpm in drive and at operating temps.
Voltage output is good. 14.3 with no accessories on.
The alternator draws quite a load from the engine when charging, the ECM is doing it's job adjusting the IAC valve to maintain the idle.
The idle is usually around 600 rpm cold but in Summer on hot days with a/c, fans, lights etc it does idle lower and you will see around 550rpm but mostly will be around 600 rpm.
The places to check for wiring issues if you are concerned and see dash lights flicker/surge are at the battery check the fuse wires, check the starter 12 volt supply from battery is tight there should be a few cables connected there.
Check the ground wires at the driver side above oil filter, there is also a flat braided cable there that goes to the frame. Check the bolt is tight that secures all the ground wires to the engine block.
