Well when I first start the truck 98 explorer 4x4 4.0L OHV volt gauge reads fine. Then when I put it in gear only sometimes the gauge drops lights dim and the heater fan dies right down. I put a volt meter on the battery only when it is not doing this and it reads good. It only does this when it is in drive usually when it is cold out after I have driven it for a min and then come to a stop. As soon as I hit the gas to go again the guage jumps up does this sound like an alternater or another issue.

 

The first thing to do is to make sure the battery cables are in good shape, connections are clean and they are tight. If all of that is fine, and the self tensioner pulley is providing enough tension that the belt doesn't slip, you probably have a weak battery.

-Rod

 

I guess I left that part out. My battery died a month or two ago. So It is a brand new battery in that thing. I will check the tensioner is there a particular way to test this. it just seems odd that it only happens when it gets put in gear and is at a idle if in park does not exhibit the same results.

 

You need to check the wiring for green rot, all connections shiny/greased and all grounds secure.

You should have NO MORE than .05vdc difference in any cable run...... Philip

 

Thanks I am going to clean everything as much as I can. Retighten it all see what happens.

 

Just not sure why it would only do this in drive. And not at an idle

 

Well I could list several items, one being

• a ground strap that is tensioned when there is torque on the engine (in drive), or
• a shift in the RPM pattern, or
• consider if there is not enough to create a field in the windings, the Alternator cannot produce any charge current, OR!!!....

But guessing wont fix the issue, diagnostics rocks!... DVOM and DC Amp clamp.... Philip

 

One simple thing to understand is that the output of the alternator is proportional to the speed that it is driven at, up to the point that the maximum output is reached. Since the engine slows down a bit when placed into gear, the theoretical output of the alternator drops. This reduction in voltage will be seen as the symptoms you described, especially when lights, heater, defroster, etc. are being used sumultaneously. The fact that the voltage gauge in the cluster doesn't change means very little, as it is not sensitive enough to see the volt or so change that occurs. As aquanaut20 stated, putting a DVOM on any B+ circuit will help you understand what is happening. Any damaged wiring will make these effects more pronounced, so careful inspection of the cables and connections is certainly a good first step.

Additionally, most alternators have three separate windings in the stator. The loss of one of these windings will allow the alternator to continue to function, at a reduced output. It will take an oscilloscope to see if this is the case. In some instances, the alternator can be heard to have a harsh growl instead of a steady whine when under a high load. You may be able to notice this if you are paying close attention.

Finally, I will say that the symptoms you described are normal, to a degree. When the 4.0 first came out in the Explorer and Ranger we had many people bringing their vehicles to the dealership complaining that there was something wrong with them, that the lights would dim and the blower motor would slow down when they were stopped at a light. The fact is it was normal and due to the fact that the pulleys were arranged to turn the accessories at a slower speed for more power and better fuel efficiency. This was common in the 4.0 OHV and was still noticeable in the 4.0 SOHC and the 3.0 in the Ranger.

Jay

 

Thanks I am still going to go through everything I suppose it wont hurt. Thanks for the info.

 

The one thing I forgot to add is that the guage does drop down so much that it is almost at the complete bottom of the guage.