Electrical Voltage drop problem when the car moves, idling ok.

You have a bad or missing ground between the engine and body. Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.


2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.

Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972


Correct negative battery ground cable.


3.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally: it comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

Couldn't this also be a sign that the alternator is starting to fail? Sounds like it's no longer providing enough power to keep things going under a load but has just enough to keep things going at idle.

If what Jrich advised doesn't work I'd take out my alternator and get it checked for free at a local parts store.

FoxMustangLvr said:

Couldn't this also be a sign that the alternator is starting to fail? Sounds like it's no longer providing enough power to keep things going under a load but has just enough to keep things going at idle.

If what Jrich advised doesn't work I'd take out my alternator and get it checked for free at a local parts store.

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You have your thinking cap on - I like that.
Let's think about it a little more.

I'll preface this with saying that it'd be nice to know what his car does while stationary at ~ 2K RPM.

Generally speaking, idling is when alt-output is lowest. While the vehicle is moving (and the RPM's are 1.5-6K), the output should increase. The OP's issue is the inverse of this.

Another tell is the increasing fluctuation in another gauge. That's generally a sign of a bad ground. I think Jrichker hit the nail on the head.

A time when your theory could hold is if the belt was slipping anytime after idle. A solid electrical system should handle that without any large drops in voltage (it'd be running off of battery for a bit, but that's still 12.0-12.6 (ish) Volts). The OP's issues are way too large for that to be likely.

You're right HISS, if an alternator is failing your temp gauge wouldn't spike. When it comes to trouble shooting I'm all about checking for potential problems that cost me nothing first before I start throwing $$ at it.

#1 Check grounding per Jrich

#2 Expanding on Hissin comment, Check your serp belt to see if it's stretched, your tension arm may be maxed out and no longer pulling the slack. If it's not spinning the alternator and water pump normally then your alternator would be draining your battery (from not charging it), your water pump wouldn't circulate coolant fast enough causing an increase in temp, and your battery wouldn't be able to pick up the slack from your alternators lack of output so your system will fail under heavy load.

I think you have a solid start on problem solving with $0 out of pocket.

Luis911 said:

Problem solved, the cable was disconnected from the engine to the fire wall. A few weeks ago fixed the engine and did not connect all the cables and accessories. Thanks to all for your help.

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Another successful hit...

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.

Thanks Luis for coming back and letting us know what the culprit was. Most people that post their problems on here and then get good advice end up never coming back to confirm what the actual problem and that leaves other people still wondering what the fix was.