YACT: (updated) Alternator appears to be cause

[LordMorpheus]

Update:
Checked to voltage at the cigarette lighter, gave me a reading of 13.4 which is outside the 14-15 volt range given in the Hayne's manual so it would appear that my alternator could use some work.

The question, then, is how can I test the alternator to determine if I am facing a new alternator (150) or a new voltage regulator (50)?

Any advice would be appreciated.



1997 Lincoln Town Car Cartier

When I start the car, the speedo goes crazy, reading anywhere from 0 to 120 mph (the full range) and jumping around like all get out. Once I get moving, say, 3 mph it settles down and behaves normally, even at a stop, until I start the car again.

Idea on a lincoln forum seems to be that I'm getting bad power from the alternator, and the irregular voltage is causing this problem.

Now the Ford 4.6 alternator is very easily accessable (it sits on top of the engine in the front of the bay) and has an internal voltage regulator that isn't very internal and is readily accessable just by opening the hood.

It's got some text that says "Ground here to test" with an arrow pointing to a lead. Naturally, I want to test it, but "Ground here" wasn't all that specific as to what I should be looking for, and I was hoping one of the car gurus here could give me some advice.

The other option is to try to get at the speed sensor, which is attached to the back of the transmission. Hayne's says I should see 10 volts there, so if I could get in there and test the power I would have an idea. The problem is as I'm a college kid away from home, I don't have jackstands or anything and I'm sure as hell not getting under the car while it's on the tire change jack, and I'll have to do this while it's running. Haven't tried to get underneath it before, will I be able to get below with jacking it up at all?

Thanks for the time.

 

[Cattlegod]

have you hooked up a volt meter to the cig lighter output? see if it is jumping all around at low speeds.

also, hook a volt meter up to your battery.


do you have a pic of the ground test to here? ground means just putting it to the black terminal of the battery (or body sheet metal as it is connected to the black terminal of the battery).

also, how does your car behave when it is working normally? does it go from 0 to max then back to 0 and stays there when you turn your car on?

 

[LordMorpheus]

Originally posted by: Cattlegod
have you hooked up a volt meter to the cig lighter output? see if it is jumping all around at low speeds.

also, hook a volt meter up to your battery.


do you have a pic of the ground test to here? ground means just putting it to the black terminal of the battery (or body sheet metal as it is connected to the black terminal of the battery).

also, how does your car behave when it is working normally? does it go from 0 to max then back to 0 and stays there when you turn your car on?

Its a digital readout - Everything on the dash lights up with it first starts (so it says 188), and then it says 0 after that, normally.

I'll voltmeter the cig lighter, that'd be an easy way to check for dirty power.

I'll have a pick of the voltage regulator up in a second.

http://www.rockauto.com/ref/Motorcraft/Detail.html?GR821.jpg

I know how to ground it, but I have no idea what to look for to see if the part is good or not.

 

[LordMorpheus]

any other ideas?

 

[LordMorpheus]

and . . . back onto the first page.

 

[The Stigenator]

battery?

 

[LordMorpheus]

Originally posted by: TheGoodGuy
battery?

doubtful, the car starts fine and I'd expect it to not start before the dash started going berserk.

I'd like someone with experience to tell me how to test the voltage regulator on the alternator.

 

[LordMorpheus]

bump

 

[Evadman]

Grounding that bolt will full field the alternator. It will basicly put out the maximum amperage that the alternator can put out. This is for testing the output of the alternator, and can cause damage if grounded for a decent amount of time as it will overcharge the battery and raise the charging voltage.

13.6v is fine unless you are having charging related issues such as it dips to or under 13v under load. You may also be pulling more amperage than the alternator can supply at a higher voltage. Again, not an issue unless it drops below about 13. Anythign above 12.6 will charge the battery.

This sounds like a classic speed sensor issue either with the sensor itself or with the wiring to the sensor, not an alternator issue. Messy voltage should not effect the spedo unless there is an issue with the computer as well. The computer will filter almost any input voltage between 8 and 18 (or so) volts to what it needs, just like the brick transformers you put in the wall at home. If this really were a alternator issue, then you would have other symptoms than just a speedo acting eraticly.

 

[akubi]

you can get alternators tested for free at autozone..... battery too

 

[lightpants]

Put your meter on the battery terminals with the car off. It should read close to 12 volts +/- maybe .5 volts
Turn the car on, the voltage should go up at least to 13.5 volts with the car at about 1000-1500 rpm.
Now load the electrical system - turn all the lights on, turn the rear defroster on, blower motor on high. See what the voltage at the battery reads. It should stay above 13 volts, if it does the alternator is working correctly.

It would be a lot easier to take it to a garage and have them test in on their charging system analyzer, I have seen alternators that will output the correct voltage, but by looking at the diode output pattern for 5 seconds you can easily see that it is not charging correctly. Then you will know for sure.
My money would be on a faulty ground in the dash, a bad speed sensor, or the digital dash itself.
The car shouldn't have to be running for you to check the power at the speed sensor, the key will just have to be in the on position.

If you can't figure it out tonight, PM me tomorrow 8-5 EST and I can look over the wiring diagram for you, and see if there is any easier way to eliminate a faulty speed sensor.

Jim

 

[LordMorpheus]

Originally posted by: lightpants
Put your meter on the battery terminals with the car off. It should read close to 12 volts +/- maybe .5 volts
Turn the car on, the voltage should go up at least to 13.5 volts with the car at about 1000-1500 rpm.
Now load the electrical system - turn all the lights on, turn the rear defroster on, blower motor on high. See what the voltage at the battery reads. It should stay above 13 volts, if it does the alternator is working correctly.

It would be a lot easier to take it to a garage and have them test in on their charging system analyzer, I have seen alternators that will output the correct voltage, but by looking at the diode output pattern for 5 seconds you can easily see that it is not charging correctly. Then you will know for sure.
My money would be on a faulty ground in the dash, a bad speed sensor, or the digital dash itself.
The car shouldn't have to be running for you to check the power at the speed sensor, the key will just have to be in the on position.

If you can't figure it out tonight, PM me tomorrow 8-5 EST and I can look over the wiring diagram for you, and see if there is any easier way to eliminate a faulty speed sensor.

Jim

Thanks for the response. I'm having the brakes done tommorow at a shop, I'm going to go ahead and ask the mechanic what he thinks would be the cause of this, and I guess we'll test the charging system there.

I'll update with results/more questions.

 

[catnap1972]

Did you ever get the problem fixed? Sounds like the same type of issue I'm having with my Escort...whether the speed sensor itself is causing my problems (speedo that occasionally stops/jumps at low speeds, plus associated transmission wackiness), or the alternator that I replaced (old one was kaput) is inducing just enough crap (ripple) into the lines to screw up the speed sensor reading.

 

[Pepsi90919]

it's probably the goofy digital readout or a speed sensor.

 

[Ronstang]

Originally posted by: akubi
you can get alternators tested for free at autozone..... battery too

Yes, and it is a very thourough testing. It will eliminate or confirm your suspicions in minutes. O'Reilley's does it also. Take it and get it tested before you waste any more time.

 

[Pikachu]

The other option is to try to get at the speed sensor, which is attached to the back of the transmission.

Strange place for the speed sensor. I would have thought it would be part of the cruise control module up by the left inner fender. Well, maybe the sending unit is at the tranny...

Anyway, check the connectors at that speed control for poor connections or corrosion. To check the sensor on the tranny, drive it up onto a high curb, to get access to it. That voltage from the alternator isn't that bad.

 

[thomsbrain]

vehicle speed sensor is my bet here. it's an easy and relatively cheap fix, too.

 

[LordMorpheus]

Originally posted by: thomsbrain
vehicle speed sensor is my bet here. it's an easy and relatively cheap fix, too.

Yeah, it's not a huge problem so it's on sort of the bottom of my to-do list. I'll get ahold of some jack stands eventually and get at the sensor (curb idea can't really work for me around here, not enough room on the streets).