Speedometer problem on a 1993 Aerostar.

[micapeak]

Have a '93 Aerostar van, 4.0 automatic, 2WD, 156,000miles. Got it used, and have found the speedometer reads 10mph off at 65. Used a GPS to check this. Also seems the cruise control drops out sometimes. Tried to find something in the posts, but nothing that showed what I have. Seems the sensor is mounted in the rear axel pumpkin, top center. I pulled it out, but there are no gears, just a smooth, round, non-turning sensor. Will try to post a pic of item. Have it dangling at back of pumpkin to get pic. Have spent a lot of time searching for an answer. Guess I don't know what to ask!

 

[BrooklynBay]

This sensor doesn't have a gear like a transmission mounted VSS. It's similar to an ABS sensor which works on the Hall Effect. Check the internal gear in the differential for damage as well as the sensor in the above photo.

 

[micapeak]

This sensor, other than having been exposed to the elements when it was mounted, appears to be just fine. No damage visible. Would I have to pull the back plate on the pumpkin to look at the internal gear? Can't get my head in a position to look into the hole where this was mounted, since it was on top, and forward. Might be able to put a mirror and a light to look down the hole. Checked the wire for damage, and it also looks good.

 

[BrooklynBay]

Check the sensor with a multimeter. You could use a mirror or an electronic scope to inspect the ring gear.

 

[micapeak]

Using the multimeter, what would I be looking for? Continuity, resistance, ??? Any ideas what the specs would be? Guess, if I can see in the hole, I'd have to prop up the axel so I could turn the wheels to make the gear turn. Would be next step be pulling the cover off the pumpkin, if I can't see down that hole?

 

[BrooklynBay]

Check the resistance first, then set it to a volt range. Place a magnet near it, and see if you get a small AC voltage as you move the magnet. These sensors have their own built in magnet. An amplifier circuit in the computer detects an expanding & collapsing magnet field when it's in the proximity of teeth on an internal gear in the differential.

 

[micapeak]

Pulled the sensor back out, and did what you said. Got about 190 ohms. When moving the magnet past the center post, rapidly, got it up to almost 2v ac. I imagine, as the ring spins past this post, the faster it goes, the higher the voltage? But not able to see into the hole where sensor is mounted. If the vehicle was raised, and the axel was allowed to hang down, there might be enough room to look into the hole. right now, I don't feel comfortable wedging my head into that small space. Must be my safety meter going off!!

 

[micapeak]

Found a small mirror on a flex handle, and was able to look inside the mounting hole for the sensor. Saw the teeth, looks sort of like a sprocket. Need to get some more support stands to hold up the van, so the rear wheels are free of the ground, and I can put it in neutral and turn the wheels to look at all the teeth, to make sure none are broken off. Don't know how this would make the speedometer read a consistent 10 mph off at 60, when speedometer reads 70. This is verified by GPS.

 

[BrooklynBay]

There might be an easier way to test this sensor, and inner gear. Connect both test leads onto the sensor, raise both rear wheels, and turn the rear wheels. This is a little tricky since both wheels will have to turn. A lot of times one wheel will turn clockwise while the other is turning counter clockwise. I don't think that the inner gear will turn when that happens, but I could be wrong. Another option is to run test wires into the vehicle from the sensor, and test the sensor while you're driving. If you have access to a lift, you could sit in the van, and watch the meter while the rear wheels turn while it's on a lift.

 

[micapeak]

Will try to do the wheel rotate tomorrow. Have come down with a nasty cold, and feel like a hot shower and bed right now!!

 

[micapeak]

Well, my sensor is ok. Need to find out what the Programmable Speedometer Odometer Module looks like and where exactly under the dash it is mounted. Would really appreciate a picture if possible.

 

[BrooklynBay]

I looked at a schematic, and a component location view on my Ford software, and it looks like it's near the inertia switch on the passenger's side lower kick panel. There is a programming connector in the same area.

 

[micapeak]

Thanks for the reply. Will take a look-see tomorrow. CIAO

 

[BrooklynBay]

You could contact James@HensonPerformance.Com if you want to find out what's involved with reprogramming the module. He's a vendor on this site as well as an expert on vehicle programming. It's possible that he might just tell you to replace it unless you want to change the size of the tires. Either way, please post what you learn here to share the knowledge.

 

[micapeak]

BrooklynBay,
I tried to use the email address, but was rejected. Tried several variations, since sometimes these are case sensitive, but it was a no-go. Here is the message I was trying to send to him.

Jim,
I received your email address from BrooklynBay on the Ford Aerostar web site. He says you are also a member, and that you might be able to help me out.
My '93 Aerostar van has a speedometer problem. It reads about 10 mph slow at 60 mph, as checked by GPS and TomTom. I did some checking, with help from the forum, by looking at the sensor in the rear pumpkin. It seems to be in good shape, not damaged in any way. It shows about 1900 ohms resistance, and the sensor responds by producing voltage, when a magnet is passed over it rapidly. Now, I've been directed to look at the PSOM. Not being a trained expert in this device, I really need some help. My question, if I can find it under my dash, is it repairable to make my electronic speedometer function correctly? Can I do this, or do I need to go to a shop? Or, would I be able to swap it from one taken off a junker at the salvage yard? Any help, especially a picture of the above PSOM so I know what to look for, will be greatly appreciated.
Thanks

 

[BrooklynBay]

Here's his Q & A thread: http://explorerforum.com/forums/showthread.php?t=213225. Here's a link to his profile: http://explorerforum.com/forums/member.php?u=38028. You could send him a PM or an email. Send him a link to this thread along with your original message.