Ammeter: Abnormal Behavior

I’m experiencing an intermittent problem with my '73 Toyota FJ40 Land Cruiser.

After I’ve been driving for a while (about half an hour usually), my headlights and dash lights will suddenly dim noticeably. I noticed that when it happens, my ammeter gage drops into the negative (swings to the left of center). I believe this indicates the alternator has cut out (stopped outputting current), and that all my systems (including the engine ignition system) are running off the battery.

Strangely, I found that stepping on the clutch—momentarily cures the problem! I.e., when I step on the clutch, the needle immediately swings back to the middle and all the lights return to full brightness. When I release the clutch, everything is usually ok for about a minute or two—and sometimes, it stays “fixed” for a lot longer (sometimes until I complete my journey).

Oh yeah, one more hint—this only happens when my headlights are on. It never happens when I’m just using the running lights, or when I’m running without any lights!?!

Any ideas would be appreciated.


First thing I’d check is to make sure the belt isn’t slipping. Then I’d look at all the wiring connections and if this alternator uses an external regulator, I’d probably suspect that.

You may have a bad connection that is heating up or a fault within the regulator or the alternator, the brushes perhaps. Clean the battery and ground connections though that most likely won’t solve the issue. I suggest you check the voltages on the alternator wires while it is functioning and then compare them with readings taken when it isn’t functioning.

I’d go with worn brushes.

The brushes of an alternator consist of a soft metal that rides on the slip rings of the alternator. When they wear past limits, they only intermittently make contact with the slip rings, which manifests as alternator turning on/off randomly.

I’ll second the alternator. Either your brushes are worn or the voltage regulator is intermittant. The alt is not keeping up with the needs though. Or of could be connections but I’d just overhaul the alternator.

Is the regulator inside the alternator . . . or on the firewall?

I wonder if stepping on the clutch completes the ground between the engine block and the body of the car. There should be some kind of electrical connection between the engine block and the body. Check to see if there isn’t corrosion on the ground connection between the engine block and the body. I would check out the ground between the block and the chassis before spending money on a replacement alternator and having the same problem.

If this problem is not related to the alternator or the external regulator you might consider the possibility of this being caused by an iffy fuse.

I’m pretty sure these vehicles still used the old style glass fuses. With age and heat the solder in the end caps of the fuses will melt. Any electrical connection can be a real hit and miss when this happens.
Remove the fuses and note if the end caps on any are loose and/or whether there appears to be solder blobbed up inside the end caps.

Maybe the fuse that the amp gauge runs through would be a good starting point as that’s a circuit that is high current (translating to heat) compared to others.