Rick's Free Auto Repair Advice

Car stalls when turning

GMC Envoy stalls when making left turns

If you have an issue where a car stalls when turning this article is for you. I’ll walk you through how to diagnose this kind of a problem.

In this case, the owner of a 2002 GMC Envoy complained that his car would stall and die but only when making left hand turns. When checking out the vehicle the other symptom was a dim left headlight.

Here’s how to start diagnosis an automotive electrical problem

Since the dim headlight situation isn’t dependent on vehicle movement, that’s the starting point. Most people think that a dim light is caused by corrosion in the power feed portion of the circuit. But that’s rarely the case. Keep in mind that auto electrical wiring often relies on the vehicle’s body as the electrical return path. It’s an incredibly stupid idea because all the wire-to-ground connections are prone to rusting. That corrosion eventually causes the majority of electrical issue. Car makers do this because is saves the cost of running ground wires to every electrical device. They really don’t care about corrosion several years later because the vehicle will be out of warranty.

Corrosion causes resistance to electrical flow. Think of this like a blockage in your home’s pipes. If the water pressure coming into your house was 40-psi but you only got 5-psi when you opened the faucet that would be a significant pressure drop. The same rule applies in the electrical world.

Perform a voltage drop test.

use a DVOM to test for voltage dropThe image I show here is one example of a voltage drop test. Set the meter to 20-volts DC and connect the positive meter lead to the positive battery post. Then connect the negative lead to the negative battery post to get a reading of battery voltage. Next, leave the positive meter lean connected to the positive battery post and touch the negative lead to the ground for the left headlight. If the connection is good, you should see battery voltage (12-volts) or close to it. In the case of this GMC Envoy with the stalling problem, we see 2-volts, which means we have a 10-volt drop. That’s HUGE and means the ground connection is severely corroded, preventing the circuit from offering a good electrical return path. That would explain the dim headlight.

Why does the stall happen on left turns?

To find the answer to that, we have to consult the wiring diagrams. Here we have the wiring diagrams for the left headlight and left turn signal. Both use the same ground, G107. So we have to pull up the ground distribution diagram to see what other electrical accessories use the same ground G107. envoy wiring diagramAh ha, the fuel pump relay uses the same ground G107. Ok, but why does the car run when we’re not making a left turn if the ground is that badly corroded? Simple, when making a left turn and using the turn signal, this ground connection simply can’t carry enough return voltage to complete the circuit for the turn signal and the fuel pump relay. Since the fuel pump relay can’t get a good return path, is shuts off the fuel pump. Once the driver is no longer using the left turn signal, the corroded connection carries just enough current to allow the fuel pump relay to operate.

How to fix a corroded ground connection

Remove the corroded bolt or nut and clean the connection with a wire brush until you’ve removed all the rust. GM recommends installing a new stud and nut that’s properly treated for electrical conductivity and corrosion resistance. If you don’t want to do that, at least treat the clean surfaces with dielectric grease to slow due future corrosion.

©, 2016 Rick Muscoplat

 

 

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Posted on by Rick Muscoplat


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