96 Rodeo, Won't Charge Properly

[mutley630]

Was working on my 96 Rodeo today for a charging issue. Battery good, Alt only putting out 12.6 v, while running. Originally, suspected Alt just bad, but after replacing, still same problem. Checked connections, voltage at plug, etc and everything looked good, except voltage on PCM control pin was zero. Checked to ensure no shorts or wire harness problems and couldn't find any issues. At this point, considering PCM itself. Just to be sure, snipped the PCM control wire to Alt (red/white wire on PCM connector side) for "troubleshooting" and sure enough voltage jumped up to the a normal 14.6 v, while at idle.

Question: I know when working properly that this control signal from PCM is meant to decrease output of Alt under high RPM's. Anybody know the risk or downside in keeping this wire disconnected? Short-term (so I can drive this weekend) or long-term to avoid laying out the cash for a PCM from the local salvage yard that may or may not have other issues? I'd sure hate to spring for a new one at $300 or so.

Ideas? Thx.

[Ramblin Fever]

Who's alternator did you buy?

These trucks are very picky about alternator quality; if you spent less then $150 on one from a box store, that's probably the issue.

A good high quality one from Napa with a lifetime warranty should do you good.

Also, it's very common that when the alt goes, battery usually follows.

Keep us posted....but it's most likely due to a cheap alternator replacement. I went through 4 checker alternators in one week before throwing out around $200 for a new/rebuilt Bosch one.

[mutley630]

Thanks Ramblin.

I think the battery's fine as I can remove the PCM control input to the Alt and charging voltage jumps right up to where it should be (about 14.5 v) and battery stays charged.

Could be the Alternator as I did get a rebuilt one that certainly wasn't OEM. The only point to mention here is the "old" alternator (one thats been in the Rodeo for a few years) was doing exactly the same thing at that point. It was charging at a very low rate about 0.3v - 0.4v over what the battery voltage was when checked static, not running. Additionally, when I checked the voltage on the control pin from the PCM, it read close to 0v. I read someplace that this is exactly what the PCM "should" tell the Alt to do when you step on the gas (said it saves fuel or something.) I do remember, when it was charging correctly that the voltage gauge on the instrument panel would fluctuate from about 14v - 15v when idling or cruising and would jump down consistently to about 12.5v when I stepped on the gas, then back up again when I let off.

I really think that this one is related to this PCM control signal. Just looking to see if anyone knows of any potential long-term consequences (other than a slight decrease in gas milage), like "terrible" milage, overworking the Alternator or battery etc., in running with this signal wire disconneted.

[Ramblin Fever]

In 200k miles and 12yrs on one Rodeo, and 180k miles and almost 13yrs on another Rodeo, I've never experienced any issues with the PCM.

Any voltage issues on mine were always due to either an intermittent issue with an alternator, weak battery or a bad, worn out ground wire. Or, as what happened a few months back, my battery terminals had weakened over the years and were very brittle.

Sorry...not quite sure where to help you at. Keep us posted.

[mutley630]

I know what you're saying. In over a dozen vehicles I've owned in the last 30 years, including the 96 Rodeo for 13 years, I've never had any PCM/ECM issues on any of them. So of course I was real skeptical in this case as well and kept trying to find reasons why it "wasn't" the cause here.

However, with the PCM control wire disconnected since Saturday, charging voltage consistently back up around 14.6 V the entire time. Just hooked it back up this morning to be sure and again voltage went down to about 12.6 V and stayed there. I even measured the voltage on the control line with the connector to the Alt disconnected and reading about 0 V (believe it should be something considerably greater at idle.) Removed and back up to 14.6 V again.

I think I'll leave it disconnected and continue driving for a week or so and if still looks like it's charging the battery like it should, I'll probably spring for a used PCM from a local salvage yard. Actually, found one for $75.

Will post back with final results.