95... Sensor(s), Relay(s), or computer?

[StangsandMavs]

Over the past couple of months, the speedometer in our Explorer has been a little erratic...

Well, a couple of nights ago, it got wildly erratic and when it does steady, it is a good 10 - 20 miles below the actual speed... coinciding with this change, has been a change in the driveability:

1. Transmission shifts erratically, especially at cruising speeds with slight changes in the accelerator...
2. 4WD lights started flashing 6x
3. When in 4WD, the front tires are doing a bunch more clunking, especially when turning.

I replaced the VSS last night... and that didn't change anything.

With all of this happening simultaneously, I'm thinking that something is getting a bad or intermittent signal rather than it being a mechanical failure. I thought I'd ask for other opinions.

Thanks for taking the time...

 

[Glacier991]

As I was reading this I was thinking VSS.... then I saw that you replaced the VSS. It certainly appears to be somethign related to that input, so if the sensor itself has been replaced with a known good one, then next I'd check wiring and connectors.. I wish I was more help. You are on the right track though.

 

[StangsandMavs]

Thanks for the reply... man I hate electrical problems.

According to the schematics that I have, the VSS goes directly to the PCM... so I would think that unless it's the wire/connector itself, I'm looking at replacing the PCM.

With everything going whacky at once, it does make sense...

 

[Cstoliker]

A bad Hall Effect speed sensor could account for the 4wd symptoms but I do not know if the speedometer takes any input from them. I think it just uses VSS.

 

[Cobraguy]

Just for grins and giggles, where was the VSS sensor located that you replaced?

 

[wyochimneysweep]

it maybe be the gear that drives the vss on the tailshaft is slipping. ussually the lower the speed the more its right then the faster the more its off

 

[StangsandMavs]

Just for grins and giggles, where was the VSS sensor located that you replaced?

Transfer case beside the shift position sensor...

 

[StangsandMavs]

it maybe be the gear that drives the vss on the tailshaft is slipping. ussually the lower the speed the more its right then the faster the more its off

At higher speeds, it tends to be steadier... but well off. Lower speeds it can go from 10 to 30 a couple times per second...

I didn't have a chance to get to look at it this weekend. I had my brother's F150 taking up the open spot in the garage... but it looks like we're going to get slammed with some weather this week, so I need to take a look at it tomorrow because the Mustangs stay parked in crap weather.

 

[StangsandMavs]

Alright... had a chance to take a look at it. The book says that the VSS connector should have 1.5 volts with the Key On, Ignition Off. I only 0.75. I checked output at the PCM, and it was 0.75. So, the harnesses are fine.

Anyone with a reason not to believe that it would be the PCM?

 

[Glacier991]

My experience is all. PCM's can/do go bad, but I bet 75% of PCM replacements are unnecessary.

That said, you have narrowed it.... The only remaining Q is whether the worm gear on the shaft is being held properly, or whether it might be slipping. A look at the 4405 Rebuild Diary will explain what i am talking about.

If you are confident the worm gear is firmly held on the shaft, then your PCM potential diagnosis has merit.

 

[StangsandMavs]

Thanks again... I wouldn't think it's the gear because the input voltage going to the VSS is low, at least according to my manual... I'll check it out to be sure, though.

 

[StangsandMavs]

My experience is all. PCM's can/do go bad, but I bet 75% of PCM replacements are unnecessary.

That said, you have narrowed it.... The only remaining Q is whether the worm gear on the shaft is being held properly, or whether it might be slipping. A look at the 4405 Rebuild Diary will explain what i am talking about.

If you are confident the worm gear is firmly held on the shaft, then your PCM potential diagnosis has merit.

Well... I've been chasing this "electrical" problem for a couple days when I get a chance...

I had checked around for a PCM... but the only place that had one was the dealership, and they wanted $400. The chain parts stores could get them for cheaper, but it was going to take a week.

I had remembered that there was a Ford Truck salvage yard in my area... called them up and they had the exact PCM (mine was a weird number) and wanted $50 for it. I figured the worst that could happen is that I'm out $50 and I would have a spare PCM for the next time, at least I could rule the PCM out if it did the same thing. Well... the worst that could happen was the worst that did happen, and I ruled out the PCM.

So, I went back to the manual and checked again the routing... obviously, it had to go the the speedo... so, tore the dash apart today to check what the connections to the speedometer look like. They looked like crap. Looked like someone spilled chocolate milk over the circuit board... I have no idea what it was. So, I thought I thought I'd clean it up and try it out.

Well, I used denatured alcohol and went through a handful of Qtips... I replaced the cluster bulbs while I was at it, 2 of them adjacent to the speedo were toasted.... and gave things a fresh coat of dielectric grease.

Put it back together and voila, the dash was brighter... but the problems persisted.

So... I took the VSS back out and tried to reach in there and see if I could feel the gear move... I couldn't feel anything, so I thought I'd try to stick the VSS on a drill and just see if at least the information is getting sent.

Well, you probably know how this story ends. The speedometer read a constant 40 MPH with the drill. No fluctuations... no abnormality. So, it's got to be with the gear that you mentioned.

Good call.

Now... does this mean a total rebuild of the transfer case? Or is there a fix I can do while it's on the vehicle?

 

[Douce_8]

I was having the exact same problem with mine and i changed the little plastic gear on the end of the speed sensor and the problem went away for awhile then i looked at the gear again and the teeth were all tore up again and it turned out the gear inside the transmission was bad and tearing apart the new gear each time,so i woudl suggest looking to see if the gear on the speed sensor is chewed up because this was causing my speedometer to bounce and upshift and downshift erratically

 

[StangsandMavs]

I was having the exact same problem with mine and i changed the little plastic gear on the end of the speed sensor and the problem went away for awhile then i looked at the gear again and the teeth were all tore up again and it turned out the gear inside the transmission was bad and tearing apart the new gear each time,so i woudl suggest looking to see if the gear on the speed sensor is chewed up because this was causing my speedometer to bounce and upshift and downshift erratically

At first look, it looked alright... but it was getting late last night and a bit chilly. I'll take another look at it...

I had swapped rims and tires (from the 15" to 16") when I got it back in 03... the speedometer was pretty close as far as I could tell, but I could probably use a different gear no matter what... certainly spending a couple bucks to try it out isn't going to hurt.

Thanks for the suggestion.

 

[StangsandMavs]

I checked it out... and there's some sides worn (had to look closer than I thought I'd have to) on 1/3 of the gear. I used the pinned thread and see that I should move down a notch on the tooth count, from tan(19) to gray(18), so I called up the local dealership and they have the part in stock for $2. Heck it will cost me more in gas to get the part than the part, certainly worth the effort. I'll pick it up later and try it out tonight.

Thanks again.

 

[StangsandMavs]

Good news... bad news...

I swapped the gears, and it works a lot better. It's taken care of the 4WD issues and the shifting and the speed is right. The speedo still has a twitch though, and I'm sure that's just a sign that the "fix" will be a temporary one, but hey, at least it was cheap!

So, it's down to replacing the gear in the transfer case, that takes me back to my question about whether I can swap the gear in the transfer case without a complete rebuild.