Help diagnose this electrical & idle problem
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Senior User
Joined: Apr 2003
Posts: 280
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From: Iowa
Help diagnose this electrical & idle problem
Lately (like since this fall) my truck has idled really rough. It runs fine (seemingly) at any other RPM though. Today I noticed that when I come to a stop light or park, that the voltage meter drops way down (it happens instantly, and then if you rev the engine, it jumps up to where it normally is immediately.)
I took a video clip of the voltage gauge, and will upload it for you all to view here in just a sec (it's on my other computer)
I ran the DTC tonight and got this pattern:
l-ll-l____l-ll-l__________l-lllll-lllllll____l-lllll-lllllll
I guess I translated this as 121 and 157
When I looked up these codes in a Chiltons manual for a ford Taurus (I was at a parts store), they came out as
TPS voltage was above or below expected
and
MAF voltage was above or below expected
First of all, are these the correct codes, and did I read them right?
Also, do you guys think my alternator is crapping out on me, and also caused these two conditions?
I need to hook up a volt meter and see what kind of readings I'm getting too.
The headlights do dim, and the blower fan slows down at stop lights.
So what do you guys think?
I took a video clip of the voltage gauge, and will upload it for you all to view here in just a sec (it's on my other computer)
I ran the DTC tonight and got this pattern:
l-ll-l____l-ll-l__________l-lllll-lllllll____l-lllll-lllllll
I guess I translated this as 121 and 157
When I looked up these codes in a Chiltons manual for a ford Taurus (I was at a parts store), they came out as
TPS voltage was above or below expected
and
MAF voltage was above or below expected
First of all, are these the correct codes, and did I read them right?
Also, do you guys think my alternator is crapping out on me, and also caused these two conditions?
I need to hook up a volt meter and see what kind of readings I'm getting too.
The headlights do dim, and the blower fan slows down at stop lights.
So what do you guys think?
Fleet Mechanic
Joined: Jul 2004
Posts: 1,552
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From: va
codes
These must be engine running codes. See the list of engine trouble codes at www.fordfuelinjection.com under getting codes. Or http://www.dalidesign.com/hbook/eectest.html under three digits codes. Test the two sensors and wiring with the smallest code first. Do you have a Haynes manual? It has the voltage number you will need. If you need to replace a sensor don't forget to unhook the ground on the battery for a minute, to reset the computer. I think you know most of this already. Sorry I can't help more.
Thread Starter
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Senior User
Joined: Apr 2003
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From: Iowa
Here's the link to the video clip of the voltage gauge when the truck is running:
Fuel Gauge, the movie
(230 kb, small)
when it spikes quickly, I have the pickup running about 1000 rpm
versus about 675 at idle, where the voltage falls off suddenly.
The high point you see there is where the voltage typically has always read when I drove the truck.
Is this normal for it to drop that much at idle? I worry mostly because it's single digit degrees right now and Xmas time, and I'm driving cross state later this week, and if I need an alternator, I need to buy it tomorrow
Fuel Gauge, the movie
(230 kb, small)
when it spikes quickly, I have the pickup running about 1000 rpm
versus about 675 at idle, where the voltage falls off suddenly.
The high point you see there is where the voltage typically has always read when I drove the truck.
Is this normal for it to drop that much at idle? I worry mostly because it's single digit degrees right now and Xmas time, and I'm driving cross state later this week, and if I need an alternator, I need to buy it tomorrow
Thread Starter
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Senior User
Joined: Apr 2003
Posts: 280
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From: Iowa
The fiancee is taking off for xmas in the daily driver, so if I pull the alternator, I'm stuck at home
Can they do any sort of tests with everything still in the pickup?
As far as having a manual, I have a Haynes I believe, and the Ford dealer service manual on CD Rom somewhere around here (lost in the big move last winter)
I'm hoping that it's the alternator, which is sad, because that's 100-200 bucks depending on what one I'd want to get, but I'm sure it's cheaper than a MAF or TPS
As you can see in the video, the dash lights dim quite a bit at idle. It's not as bad as it looks in the video, but they do dim noticibly.
Can they do any sort of tests with everything still in the pickup?
As far as having a manual, I have a Haynes I believe, and the Ford dealer service manual on CD Rom somewhere around here (lost in the big move last winter)
I'm hoping that it's the alternator, which is sad, because that's 100-200 bucks depending on what one I'd want to get, but I'm sure it's cheaper than a MAF or TPS
As you can see in the video, the dash lights dim quite a bit at idle. It's not as bad as it looks in the video, but they do dim noticibly.
Fleet Mechanic
Joined: Jul 2004
Posts: 1,552
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From: va
testing electrical system
I did this test because my truck sometimes was hard to start. Every year this test is done ( up hear) because the battery won't hold a charge when it gets real cold. It's a free electrical test it checks most of the electrical on your truck, all at once. Just drive up to a shop and ask for the free test, maybe call first and ask what they call it. They hook up to your battery and check your battery under a load. You start the truck, then turn the lights on takes about 10 minutes while you sit in the truck. It will show if all the grounds are good, if the alternator and silinoid is good. You can have the alt. tested off the truck but it cost $. If you replace the alternater check for codes again. I see alot of posts problems with tps. Do a Advance Search above under tps.
"When my tps started to fail, i got a minor hesitation on acceleration when the problem first occurred, then a major hesitation much later on acceleration. (very irritating)" found this under search.
"When my tps started to fail, i got a minor hesitation on acceleration when the problem first occurred, then a major hesitation much later on acceleration. (very irritating)" found this under search.
Postmaster
Joined: Jun 2004
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From: B/CS, Texas!!
Nice video capture 
.
If you're having rough idle, this could cause it. Your RPM's drop with that bad idle, thus less centrifugal force on the alternator=drops voltage. When you Rev it up, more centrifugal force on alt.=voltage goes up. I would assume alternator is good.
The TPS is a cheap($20-30) piece and could be causing this, but test it. I don;t know about a MAF, since my truck isn't equipped with that.
.If you're having rough idle, this could cause it. Your RPM's drop with that bad idle, thus less centrifugal force on the alternator=drops voltage. When you Rev it up, more centrifugal force on alt.=voltage goes up. I would assume alternator is good.
The TPS is a cheap($20-30) piece and could be causing this, but test it. I don;t know about a MAF, since my truck isn't equipped with that.
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Senior User
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Advance auto & Auto Zone do these test for free, they can test the alt & bat
on the truck. i had a simular prob with my 90 f150 300 I6 scarey anoth it turned out to be my starter lol.
id try the sensor tests should be in the haynes manuel if there ok id run to advance or autozone and get your bat, alt and starter tested, good luck
on the truck. i had a simular prob with my 90 f150 300 I6 scarey anoth it turned out to be my starter lol.
id try the sensor tests should be in the haynes manuel if there ok id run to advance or autozone and get your bat, alt and starter tested, good luck
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Check your battery cables and all the grounds. Engine block, frame, ring connectors on post on radiator support in front of battery, PCM ground on firewall near left hood hinge. Get the metal-to-metal contacts clean and shiny. Doesn't cost anything but your time.
The truck should be capable of running off the battery, even if the alternator is not charging at all. The codes you are getting, and the stalling, are consistent with low supply voltage to the computer, which could be coming from poor battery and ground connections. The truck might be OK at higher engine speeds, as the alternator adds enough voltage to compensate for the excessive voltage drop through the connections.
See my other recent posts about the 2nd-gen alternator. The voltage drop-off at idle can be normal with enough electrical load (both the headlights and blower fan). The amount of drop in your movie is fairly typical, and not, in and of itself, enough to worry about.
The truck should be capable of running off the battery, even if the alternator is not charging at all. The codes you are getting, and the stalling, are consistent with low supply voltage to the computer, which could be coming from poor battery and ground connections. The truck might be OK at higher engine speeds, as the alternator adds enough voltage to compensate for the excessive voltage drop through the connections.
See my other recent posts about the 2nd-gen alternator. The voltage drop-off at idle can be normal with enough electrical load (both the headlights and blower fan). The amount of drop in your movie is fairly typical, and not, in and of itself, enough to worry about.
Thread Starter
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Senior User
Joined: Apr 2003
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From: Iowa
Ok, I had the battery and alternator checked today and they both checked out good. They told me most likely it's the TPS on the fritz. So I purchased a TPS, and have it in my h and, and I've been looking around my throttlebody for where to put it and can't find it.
Where is it located at?
This is a '94 with a 302 engine
Where is it located at?
This is a '94 with a 302 engine
Thread Starter
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Senior User
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From: Iowa
Yep, I saw that after I went out with a flashlight. It's currently -4 with a -15 windchill. There's a water passage in the TB, so yeah, this is going to wait until March.. I returned the part to the parts store, got my cash and went and bought myself some pizza
Does this definately sound like a bad TPS though? The problem is that the truck idles at about 500 RPM according to my sun tach (my truck doesn't have a stock tach) at about 700 RPM the battery voltage jumps up to its highest point on the gauge, so I'm thinking that the truck is simply idling too slow. This is all happening with the headlamps and blower fan on, which seem to significantly suck the current.
When you replace the TPS, does it have to be in a specific position? One test I was reading (mustang board) mentioned turning the tps until you get a certain voltage reading, but there is no way to adjust this sucker on the F150 with everything installed. So do you just basically pull the old one (two screws) and put the new one on?
Does this definately sound like a bad TPS though? The problem is that the truck idles at about 500 RPM according to my sun tach (my truck doesn't have a stock tach) at about 700 RPM the battery voltage jumps up to its highest point on the gauge, so I'm thinking that the truck is simply idling too slow. This is all happening with the headlamps and blower fan on, which seem to significantly suck the current.
When you replace the TPS, does it have to be in a specific position? One test I was reading (mustang board) mentioned turning the tps until you get a certain voltage reading, but there is no way to adjust this sucker on the F150 with everything installed. So do you just basically pull the old one (two screws) and put the new one on?
New User
Joined: Dec 2004
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This sounds like a similar problem I had on my 94 F150 302. Like 93GreenF150 said above the rough idle is casue the voltage fluctuation. I replaced the air bypass valve/idle sensor. The valve was carboned up. It was about 40 bucks at AutoZone.
Posting Guru
Joined: Dec 2003
Posts: 2,155
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From: Centerville, Iowa
My truck's voltage guage will drop from the A in NORMAL down to the O or R when at its lowest idle with the headlights on and blower going (I'm in Iowa too bro). It's normal. Though your situation seems to have an added issue. Give my truck a blip of the throttle and it instantly returns back to the A again. My dash lights also like to dim a bit, as you mentioned, but all is normal. I'd only worry if your truck takes more than 3 tries to start. THEN you've got some trouble...or a straight 30 weight in the crank case!