Frustrating electrical problem...kinda long

My 87 XJ is giving me problems. When at idle it seems like it isn't getting enough power. Lights dim (interior and exterior, wipers barely move, and radio will sometimes cut out. It idles at about 500rpm. At about 1000rpm the volt gauge jumps right up to 14v so I don't think the alternator is bad. Battery has been recently replaced. Any thoughts onb the cause? It is really starting to get bad because now with the heater going as well in really dims down and has stalled. Please help.

BrianC

Adjust tps or replace ox. sensor so you can get 750/775 rpm and there should be plenty of voltage.

Sounds like you may have a problem with the idle speed control. My '87 (4.0l) always seems to idle in the 700 rpm range regardless of whether the A/C is on, or the tranny being in gear or park. Sounds like the idle speed on yours is not being increased as the load on the engine is increased by having all the accessories on, and it finallly stalls out. I've had issues with the idle speed in the past caused by dirty connections. I ended up cleaning the connectors for the idle air control, the throttle position sensor, and C101 connector by the brake booster. Cleaned and filled them with dielectric grease, no idle problem since (knock on wood).

Lights dimming etc. at idle may be normal (mine does it too) if you have everything on. Alternator can't keep up at idle speed, and if your idle speed is lower than specs, that will even make the alternator output that much lower.

So my .02 cents is to make sure the belt is in good condition and properly tightened, battery cables clean and tight including the engine block and body grounds, and get that idle speed checked out !!

:repair:

Thanks for the reply. I will check that out. I wonder if I put a smaller diam. pulley on the alternator, making it turn faster at lower rpm, if it would have problems at higher rom and a shorter alternator life. Any thoughts?

Your problem is electrical not an idle problem. You aren't getting enough juice at an idle. My first guess would have been battery. Otherwise it is probably a bad alternator or a bad power/ground cable somewhere. Check the ground wire from above and behind the distributor. Clean the connections on the alternator field connector. Also check the braided cable from the firewall to the drivers side rear head bolts. You could even have an old/loose alternator belt.

Otherwise pull the alternator and take it into Autozone and have them check it.

Tom Houston

BrianC
Have you added a body ground and done the basic charging upgrades on your '87?

The battery ground path for your of your '87 runs only through the engine connections (battery to engine; engine to body), not only will this rob some of your available current, it doesn't provide the "best" reference for the voltage regulator in the alternator -

Check-out Craig H's work He addresses some of the grounding thought about half-way down that page, the rest of the mods are useful as well - given the age of your XJ and all...