Engine RPMs

[Ricker]

On my '82 automatic here what is now happening. Sitting at a red light in Drive only, the engine RPMs go from 875 to 500 for no reason and almost stalls the car. I also noticed, maybe unrelated, the instrument panel voltage goes up and down from 13 to like 10 volts. I measured the reality new battery engine off; it is 12.4 voltage. The alternator is at 13.8 volts engine on and same with load. The ISM is relatively new and humming. The car has never really wanted to hold an RPM. Can it be the Control Module? Ideas?

[mhanch]

Mine was doing this when the cooling fans were kicking on and drawing too much power. It would almost stall out.

I ended up putting new fans on from Delorean Parts NW, and also putting in a new alternator since mine was original and not working great.


The combo of those solved it for me. YMMV

[Ricker]

Mine was doing this when the cooling fans were kicking on and drawing too much power. It would almost stall out.

I ended up putting new fans on from Delorean Parts NW, and also putting in a new alternator since mine was original and not working great.


The combo of those solved it for me. YMMV

The fans were put in new from DMC Texas a few years ago. This was happening when the A/C was on, so the fans were running constantly. Can a voltage regulator being going bad, or does it just work or not? Would low voltage cause the Controller to act funny for the RPMs?

[mhanch]

For me, i saw my engine stumble if voltages dropped too low. Updated Alternator helped there.

[Timeless]

Any vacuum leaks? Also ensure grounds are clean and tight including ballast resistor connections.

[Ricker]

Some data. Battery after sitting overnight 12.56 volts, engine not running. Engine running and dash showing 10 volts reading on a voltmeter 13.9 volts. Same but with a load (lights, A/C, blinkers) it dropped to 13.2 volts. Do these indicate an alternator going bad? It is only 3 years old from Deloreango. Belt is good and tight. I'm cheap and don't want to get a new alternator and that no be the issue. Also, note the RPMs drop from 875 to 500 when the dash reads 10 volts.

[painterdave72]

check the main power cable going to the alternator from behind the ballast cover..

[Rich]

Some data. Battery after sitting overnight 12.56 volts, engine not running. Engine running and dash showing 10 volts reading on a voltmeter 13.9 volts. Same but with a load (lights, A/C, blinkers) it dropped to 13.2 volts. Do these indicate an alternator going bad? It is only 3 years old from Deloreango. Belt is good and tight. I'm cheap and don't want to get a new alternator and that no be the issue. Also, note the RPMs drop from 875 to 500 when the dash reads 10 volts.

~12.6V on a battery at reast means it's 90% charged. No worries there, especially after a year or three on the battery.

Good to see the data from your Volt-Ohmmeter. Let's refer mainly to the system voltage per your VOM, preferably measured between the jump start terminal and any good engine metal.

The dash gauge is way off, obviously. Your (accurate) VOM shows 13.2-13.9V means your charging system is doing fine under that condition. Anything over 12.6V means the battery is still charging but >13V will charge it faster. Most references point to 13.0-14.5V as the range.

Whatever's happening to the idle speed may be isolated to only that system, not necessarily a problem that will run down the battery and strand you. The fact that your battery sits at ~12.6V means it's been charged well and is ready to fire up the engine.

About the dash voltmeter - a good upgrade is to check the chassis ground to your binnacle harness AND clean the plug terminals (including the ground terminal) on whichever harness plug terminals are tied to either end of the dash voltmeter. DMC dash voltmeters aren't the most accurate but you want them to give the chance for the best/highest reading if only for your peace of mind during a drive. Worst case you can figure out the % difference between the (low) dash gauge voltage and the (real) system voltage, then use that info to monitor the performance on the road. If you're good at math.

[Ricker]

Well, I took the alternator out and took it to a rebuild shop. It worked fine EXCEPT the output post was a little loose. It seems the plastic piece that holds it in place was breaking and falling out. Got it rebuilt and it appears that problem is fixed. However, the RPM issue is still there. It seems it drops from 875 to 500 and back pretty quickly when at a stop and in Drive. I tried, at a stop, and put it in Neutral and the 500 RPM didn't happen. What would cause the ECU to do this based on engine load (and not temperature)? More testing today to try to isolate when this happens. And, as in the past, in neutral it sometimes goes to 1500 RPMs. Not worried about that as when in drive it doesn't. Would a new redesigned ECU work around these issues? Thanks guys!

[Rich]

Nice to know the alternator is now solid, at least in a bench test.

To close that cause out can you confirm the voltmeter is no longer showing intermittent drops to 10V since that repair?


Comment about the ECU: per DMC Classic the Idle ECU has a very low failure rate. See their note at bottom of the page for that part. So try looking elsewhere for now.

Try checking/cleaning any idle control system elec. connections. I?d start with whichever of the 5 bulkhead connectors have input or output wires to/from the Idle ECU. The wiring diagram will show these along with their colors or just check/clean all of them.

It?s also worth disconnecting-reconnecting the Idle ECU?s multi plug if you haven?t already.