Intake Air Heater Question

[JHylton]

In the last several weeks, my '91 runs for miles with the charge indicator in the negative... I know it is normal for the ammeter to cycle between discharge and full charge until a sensor somewhere declares that plenty of heat is in the intake manifold and don't need the preheat coils any longer. Lately it never gets to that point... it sporadically stays on the way low side. Then, all of a sudden, it will jump up to full charge, the lights get bright, the turn signals go to normal operating speed, the heater blower picks up speed, etc... It acts like the hot air coils in the intake are stuck in the "on" position and don't kick off. How and what do you test to figure out what is broken? The problem is sporadic. Some mornings it works as new... . other days, it stays stuck on the discharge side. The battery is getting recharged, because it starts easily. Oh, and I plug the block heater in at night as it is still chilly up here in NW Oregon.

 

[Bob Beauchaine]

Mine usually returns to normal within a couple of blocks so something is not right. The first thing I would check is the relay that controls the pre-heaters. It is located on the drivers side inner fender - can't remember which one it is but I can get the info for you if you need it. When I installed the cowl induction scoop on my truck mine would cycle in the winter with temps in the mid 20s so I just block the outlet off during winter months.



Bob

 

[Carl Cook]

What I did with mine is I disconnected it on the firewall and run it through a heavy duty switch. When it's real cold out I raise the hood turn the switch so the heater will come on to warm the intake when the key is turned on, then when it starts I turn the switch off under the hood on the firewall. That way it's only on for a moment while I'm starting it.

 

[KOG]

Don't know about the '91, but the 91. 5 has two large relays mounted on the left fender well. I unplug them in warm weather (just take off one of the small leads) because my engine controller is flaky and turns them on seemingly at random when the truck is running.

 

[JHylton]

Mine is a late 91... a 91. 5 ... . some of the parts we have found on it are 92 year model parts..... I found the relays mounted way up under the left fender well. I am going to try unplugging the lead that runs to the OBEC and see if that is the culprit. Somehow I think it is, just because of the random on/off cycle that is does.

 

[cerberusiam]

Check the wiring to and the air gap on the CPS. A false signal there could be corrupting what the ECU thinks is going on. Check all your body grounds also and the hot battery cable connections for corrosion.

 

[NIsaacs]

It might not be the pre-heater at all. It might be the regulator doing it's intermittent charge, no charge thing. Very common on the early 1st gens.



Nick