My D250 has been acting funny for several months and at least 20k kms. After start-up, the alternator seems to cut in and out at about 10 second intervals. Voltage drops quite low, lights dim and then brighten. After a couple of minutes everything is normal. I had a poor battery in it and thought it might be due to its inability to accept much of a charge. This theory was incorrect as it now happens with a good, high current battery. The battery does not discharge.

I have known this fluctuation to happen with worn brushes but it quickly turned to total failure. Any thoughts about possible regulator or relay problems.

I have tested voltages and everything is fine except after cold start. This fluctuation happened only once when I restarted after about an hour.

Should I pull the alternator and give it a once over or should I be looking elsewhere? Battery connections, cables etc seem to be in great shape. I have gone over them very well.

 

Sound like heater grids cycling to me. How cold is it getting up your way these days?

 

The fluctuation you speak of sounds like a perfectly normal case of the grid heaters cycling when cold. This occurs on all year models that I am aware of and is nothing to be concerned about.

 

If it was doing this (cycling grid heater) during warm weather it could be the temp. sensor in the intake is bad? ???
Wouldn't that be a possibility?

ramguy

 

Yes

 

Definite possiblity. But it's cold enough to set the grids off in Alberta in September. If they're cycling in July, you definitely hve a temp sensor problem.

 

It could be the heat grids. I had not realized that they keep on working after the engine starts. The sensor could be the problem. A lot of the minor electrical stuff is iffy. I guess I would know for sure if I undid the main connection. Does anyone know where the sensor is? I removed everything when I drilled out the intake for a boost guage connection. I did not notice anything there.

 

Me again. This fluctuation happens even on days when the wait to start light does not come on.

 

Definately sounds like the sensor is bad. The intake temp sensor is mounted just to the back of the intake horn on top of the intake plenum. It is right by the KSB sensor.

 

My truck is an early 91 without an aftercooler. I am not sure if it would be the same as your 92.

 

Sorry, thought yours was an intercooled model. The non intercooled models will still have an intake sensor located somewhere on the intake plenum. 2 wire harness coming into a sensor. Maybe someone can post on here that knows the correct location on a non intercooled but I seem to recall the sensor being under the intake horn or mounted on the side of the intake plenum ???

 

I know on my 90 the two wire harness plugs in on the drivers side. I belive just under the air horn. Too cold to go outside and look right now to make sure. Anyway it should be easy to find, looking from the drivers side.

 

Thanks for the information. I will look for it in the morning.

At least my alternator is not about to commit Hari Kari.

 

FYI, My 93 does this when cool or cold, It just started recently on start ups, as the morning temps hit the below 50 mark, My wait to start light always comes on for some length of time hot or cold.

Apache.

 

Here is a jpg file that has the temperatures and times for the grid heaters out of the service manual. I got this off of Bushy a while ago.. Good info..

FWIW I noticed that the 89/90 models seemed to have lower temperature setpoint for the grid heaters. Soon after I got this truck I found it would cycle the heaters at warmer conditions than the 89 I had.

J-eh