Hey guys,

I have a 1990 Ranger, manual tranny, 2.9 L V6.

I've been having issues with my RPMs lately. After the truck has been running for 30 minutes the idle RPMs gauge drops to roughly 600, although the truck does not always feel or sound as if the RPMs are this low. Along with that, the charger gauge drops to about 12 volts, normally at 14-16 volts. The headlights dim as well, which is expected if the charging system is not producing enough juice to power them. Just recently, the oil pressure gauge is now fluxing from low to normal during the idle period too. Everything is fine when driving, problems only occur at stops, or when the clutch is pushed in. Everything started during an extreme cold spell we had for about a week. I just wrote the problems off from the cold weather, but now it's warm and they still persist.

I think it could be my alternator is going bad, although the low RPMs have me thinking that is could be something else. Any ideas or suggestions?

 

Yes, alternator has lost 1 or 2 fields.

All the gauges are voltage based, including oil pressure, so all electric system will be effected by low voltage.
Speedometer is the only mechanical gauge on the '90

An alternator has 3 fields that produce AC voltage, each field has a set of diodes that changes the AC to DC.

Over time a diode or field wire will go bad, so an alternator that was 90amps is now only 60amps, with stock electrical system this might produce slightly dimming headlights at idle depending what other electrics were on, i.e. heater fan.

When 2 fields fail you have to replace alternator.

If you have a voltmeter check battery voltage with engine off.
Battery should be 12.2-12.8volts

Start engine, battery voltage should now show above 14.5volts to recharge battery.
Then after a few minutes it should drop down to 13.6volts
Turn on head lights, voltage should drop and then come back up to 13.6v

If voltage is below 13.5volts with engine at idle then battery will slowly discharge, raising RPM increases the amperage the alternator can produce so voltage may be near 13.6volts at 2,500 RPMs, but over time you will ruin the battery and still have to get a new alternator

 

Ok, so I replaced the alternator, but all the problems are still there. I'm thinking it could possibly be the IAC unit, only because I've had issues with it before where it affected the RPMs. If it's not the IAC unit, then I'm at a loss for the problem is.

 

Just replaced the IAC Unit. Seems to have fixed the problem, although I will know when I go to work later tonight. I'll update after I've driven it without problems for a couple days.

 

i was going to say if its idling lower then normal then it would effect every thing around the lower idling since everything is turning at a slower speed it would result in lower oil pressure,lower voltage reading and ect so did it fix it all the way i also have a 2.9

 

Warm up engine and then let it idle, should be about 700-750rpm.
Unplug IAC Valve control wires, idle should drop to 500-600rpm, engine may even stall.
If that happens then IAC Valve is probably OK, could be cleaned, but no need to replace.

If RPM does not change with IAC Valve unplugged then replace it.

IAC Valve uses a Step Motor, a Step Motor can be turned to a set point by "pulsing" the Voltage, so it is, in essence, a digitally controlled motor, Computer sends a "Morse code" type pulse to the IAC Valve and it turns to point "A", a different pulse and it turns to point "B", ect........some stepper motors have 100+ points

Because this needs a "clear" signal from the computer to work, make sure the wires are in good shape.
If you have replaced IAC Valve before then I would check voltage at IAC Valve and give the wiring a good long look, IAC Valves usually last the life of the vehicle, IF everything else is in good shape.

 

How did this work out ?

Since you had a problem with the IAC before now a second time, my question is why ?

If you continue to have problems start checking connections, pins in the connectors, grounds, wiring, make sure things are clean, tight and no broken or chafed wires.

Some conductive paste helps too !