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New Fangled Crazy Electronics

Being retired, I have to do my own work.

My newest vehicle, a 1988 Lincoln Mark VII, LSC, is extremely well maintained with 128,000 miles. Recently, the RED alternator light came on. I then noticed that the speedometer would wind up, like the tach when in PARK position; the odometer will spin at the same time as if I was actually driving down the highway; putting it into gear and reving the engine with the brakes on, give the same results. The voltage at the battery, at an idle, is 13.75 WITH the headlights on. An old fashioned hand held ammeter indicates +25 amps from the alternator, with the lights and accessories operating. The red light will DIM when the headlight swith is turned on at 2000 RPM. This alternator is rated at 105 amps and was replaced as a PREMIUM “Re-man” 1/1/2 years ago, as was the external regulator.

Help me folks, this will wipe out my next month’s SSI if I bring this to a garage.

I’m guessing a low resistance short to ground at the vehicle speed sensor. That could cause a signal to the gages you mention as if the car were moving and also allow current drain that could trigger the alternator light, which is actually just responding to a drain on the battery beyond the alternator’s abilities to keep it charged.

If unplugging the VSS causes the symptoms to disappear, then my guess is correct.

Thanks for your suggestion. This sounds pretty sensible.
Thanks, again,

Also, a bad or weak diode in the alternator will put an A/C ripple into your electrical system and cause these problems. Remove the alternator and have it tested. There are 3 diodes in the alternator. It will still function (sort of) when one fails. Try your radio on the AM band. If it has a loud buzz in it that changes with engine speed, you have found your problem…

You’ve heard the (new) old saying, “Knowledge will save you money”? It’s true. Get the repair manual and study the section on “electronic engine controls” (or, similar title).
Here is a picture of the vehicle speed sensor (VSS) on your transmission.,1987765/vehicleId,1076201/initialAction,partProductDetail/store,1140/partType,00430/shopping/partProductDetail.htm The VSS detects the car’s speed (from how fast the transmission is rotating) and sends its signal to the engine computer (EE-IV). The engine computer uses the signal to send a mph signal to the speedometer, and controls the cruise control. The rpm gauge is fed a signal from the ignition module. What may have happened is that some of the connecting wires in these circuits may have gotten a little too intimate with each other.
Examine the wiring, under the dash and in the engine compartment for any signs of the wires, or wire bundles, rubbing against corners or other protuberances. If in doubt, unwrap…the wire bundle, that is, and examine the wires.

Yes, I’d thought of spurious voltages.

There may be AC ripple coming from the alternator causing the problem as Caddyman suggested. One easy thing you can do to see if that is so is to disconnect the small plug going to alternator while the engine is off. Then start the engine and see if the symtoms go away. You will have a battery warning of course since the alternator is in-op.

Courgar, and Caddyman,
Thanks for your suggestions on this. The radio noise was prevelant, and the symptems went away when the alternator was unplugged, as suggested. My first thoughts were with the spurious voltages, as well, having had to deal with redundant foreign, and large class 8 wiring systems.
I’ll let you know more when I get the aternator tested.
Thanks for everyone’s help on this. It’s good to be on an intellegent forum.

How did you come up with your header? 1988 is not in the “new fangled” era. Still dealing with a external regulator?

Are the voltage and amp figures you give being taken when the RED alternator light is on.

Be careful when combining symptons.

To Oldschool and everyone else who responded to this problem:
For me, 1988 is as new as I want to go and still be able to work on my own stuff. Having said that, after taking all your fine suggestions, and working with “old school” tools and meters, the easiest thing that diagnosed the problem was the disconnection of the alternator which totally eliminated the spurious voltages to the gauges. After replacing the alternator everything was great; undoubtedly a bad diode/set. What threw me off was the actual 13.75 voltage coming out of the alternator, and the +25 amps with the headlights and accesories working while at an idle. Having a charged battery didn’t allow the alternator to go beyond the existing indicated output amperage to indicate diode problems, except for the red light and the speedometer going crazy.
Thanks to all for your suggestions; these were all good answers. Now, I’ll have time to rebuild the voltage regulator on my AC D-12----Spring is coming :slight_smile:

Glad you got it fixed Frank and thanks for the update. The newer systems really aren’t too hard to figure out. It may take a little study to understand them but you can do it.

You see now how when reading a post you must filter and catagorize and put a value on (and suspect) information supplied by the OP. Here in this case you gave alternator output readings that were inconclusive coupled with the premium level component recent replacement.

You as the OP must make sure you don’t “pollute the pool” in regards to a helpful answer comming back to you.

Again, Thanks for the update.

You have a multimeter (old stuff). With it, what ac voltage did you read?

FWIW 6 diodes in an alternator. 3 phase full wave bridge.

One last reply:

  1. The multimeter read 13.75 volts (DC) at the out put lead on the alternator @ +25 amps.
  2. The actual alternator bench test read 17.75 (DC) volts; no amp reading; and a shorted diode in the stator ring. I didn’t want to try the AC test on this old meter.
  3. Replacement made by the jobber after the actual bench test on this “Life Time Warrenty” premium re-manufactured unit.
    Again, thanks for everyone’s help; I did share all your comments with the local service technicians.

Once again your welcome SnowshoeFB.

Doing the AC voltage test would really have been no problem on the meter. One thing to be aware of with older analog meters when measuring AC volts is most will show the DC component along with the AC component you want to measure. All the digital meters (mainly Fluke) I have used seperate the DC out when measuring AC voltage. If you want to eliminate the DC component with the older meters you need to place reasonable size capacitor in series with the test lead.