A few months ago, my wife and I started to notice that our minivan transmission was acting a little funny. When slowing down to a stop, the transmission would drop from second to first (we have the 4-speed automatic) with a clunk, sometimes so forcefully that the car would shudder as a result. Additionally, I noticed that the cruise control had gone all wonky -- I'd set the cruise, but the engine would fluxuate around the speed I set, as though it couldn't find the correct speed once I set it. Lastly, my wife noticed that the speedometer needle started acting up, and would shoot out to 120mph and back down to 0mph with no actual change in engine speed. Very weird.

Having heard for years that Dodge transmissions were notorious for failure (especially on the minivans), I braced myself for the cost of repairs. Between the transmission, the cruise control, and whatever was causing the speedometer to race around, I expected to end up with a repair bill that would be greater than the value of the van. However, I wanted to see if anybody else had experienced similar problems with their transmissions, and wondered if the cruise and speedometer issues were connected in anyway.

Transmission:

The first thing I needed to do was to identify my transmission. The 2000 Dodge Grand Caravan Sport (3.3 L) has the A604 transmission, as did any Caravan from 1989 to 2006. As it turns out, this transmission is entirely computer controlled.

A604 / A606
The transmissions above are 4 speed front wheel drive automatics that are all computer controlled. If your transmission starts shifting erratic or stays in second gear only, then make sure you or a shop run a scanner on the vehicle. You want to see if you have a sensor that has gone bad causing your problems. Also make sure you check the clutch volume index (c.v.i.). This helps determine the condition of the transmission. Chevy's and Ford's do not have this neat little feature.

Vehicle's with A604 transmissions have chronic input and output speed sensor problems and also with the little wire connectors to those input and output speed sensors. The dealer offers a repair kit for these wires instead of buying a whole new wire harness. On most of these vehicles, If you have a code 41-44 you should check for weak battery voltage first. now the shop needs to check the wiring for poor grounds. I have seen that The transmission controller often goes bad causing these same codes, so be careful.

Reading that a common fail-point on these transmissions is the input and output speed sensors, I kept researching to see if the other issues with my van could be caused by the same sensors.

Cruise Control:

This article I found referenced a 1988 Dodge transmission (which is a generation prior to the one in my van), but the reference to the speed sensors caught my attention:

Q. Now I have a few problems with my 1988 Dodge Caravan, 3.0 liter engine, is equipped with cruise control, A/C and has 141,000 miles. I have a Maintenance Required light coming on with a Code 15. I had this for about two months or so. During that time the cruise control worked just fine!

A few days ago I turned on the cruise control to set it at 55 mph and when I did the car started to accelerate more and more!!! It would not stay at 55 mph. Could code 15 cause this problem?

A. Code 15 is the Vehicle Speed Sensor and a bad VSS would most certainly cause the problem you're having with the cruise control.

Speedometer:

The last issue was the speedometer, and sure enough, the issues with the speedometer can also be linked to the speed sensors on the van.


'91 Dodge Caravan, Speedometer Jumps

Q: I recently bought a '91 Dodge Caravan with a bad transmission. I had a Chrysler rebuilt transmission installed. When driving down the road at 30 MPH the speedometer will start jumping around sometimes will peg out at the maximum 100 MPH on the dashboard. The engine will start jerking at that point. Then the speedometer will settle down and the vehicle will run smoothly. The tachometer will remain steady even though the speedometer is jumping around. The transmission shop did not think that the rebuilt transmission would be the cause. The OBD did not show any errors. When the technician checked the dashboard, the speedometer would only go to the half way point(50MPH) instead of the full range(100MPH). What should I be looking for? Sensors, gauges, body control computer? Wiring? The problem is intermittent and seems to be affected by bumps.

A: If I'm not mistaken your vehicle doesn't have a speedometer cable. It uses a signal generator in the transmission that creates an electrical signal in relationship to vehicle speed called a Vehicle Speed Sensor (VSS). This information is processed by the vehicle's computer for engine and transmission management and provides the signal for the body computer that controls the speedometer. If the VSS is failing it will not always set a code in the computer. Why? It all depends on how it is failing (lying). Also you have to know that the computer in many vehicles have a programmed speed limiter. The computer's program uses vehicle speed as an input and if a specified speed is seen by the computer it will cut the fuel injectors to prevent the vehicle from going too fast. Knowing this, and the fact that your engine starts running rough when the speedometer flips out at 100 plus MPH, I'd say fuel cut out is kicking in. For this to happen it is likely that the Vehicle speed Sensor is on its way out. Before it quits it's going you make your life difficult. Have your technician plug into the vehicle computer and monitor the vehicle speed sensor signal. It's likely your problem is an intermittently lying Vehicle Speed Sensor.


Having put together my research, and understanding that the issue may have been caused by a bad set of speed sensors, I did my best to put my head around the problem, which I believe can be summarized this way: these transmissions receive signal about the engine speed through an input speed sensor. The transmission changes gears when the engine reaches a specific speed. The transmission, through the output speed sensor, tells the car how fast it is going, and that information is sent to the speedometer and the cruise control system.

Confident that I had figured out the problem, I checked around a few local auto stores to see if anybody had the sensors. The sensors ran about $20 each, which is significantly cheaper than having to a replace the transmission. When you get them, make sure to have the store tell you which is the input and which is the output, because I think they'll both fit in either hole. The boxes on my sensors didn't say, so I had the store just write "IN" and "OUT" on the boxes.

I found a handy image of my transmission, which shows where the sensors are located:



And then I made sure I had the parts and tools necessary to do the job.

Parts/Tool List:
10mm socket (to remove top of air intake)
1" socket (to remove/tighten sensors)
standard screwdriver (to loosen hose clamps)
Vehicle Speed Sensor - INPUT
Vehicle Speed Sensor - OUTPUT


Removal/Installation:


The first step is to remove the airbox cover. This can be removed with a 10mm socket. Under the airbox cover is the actual airbox; remove the filter by unsnapping the clamps holding the lid in place. Maybe this is a good time to see if your filter needs to be replaced.


Once you've got the airbox cover and filter off, you should be able to see where the sensors are located. On the left is the input sensor, and on the right is the output sensor.


The input sensor is located just under two little hoses; remove the clamps and get those houses of the way. Make sure that you keep track of which hose goes on the left and which one goes on the right. To remove the sensor connector, there's a tab that lifts up, probably under the sensor. It lifts very easily, and the connector slides right off. If it's not coming right off, you haven't properly lifted the tab.


Here you can see the hoses and the sensor connector removed. To remove the sensor, you need a 1" socket which should fit right over the terminal. The socket should also be large enough to clear the hose nipples.


Here's a shot of the output sensor. Note that with both of these sensors, the base is plastic. If you use anything other than a 1" socket, you'll rip the plastic to shreds and never be able to get the sensor off. USE the socket.


In my situation, the output sensor was covered in sludge, which may have been the source of the problem. Both the input and output sensors use some kind of magnetic receiving unit, and those magnets will pick up tiny metal shavings that are suspended in the transmission fluid. I suppose 10 years worth of buildup is enough to cause the sensor to throw bad signals. But maybe something on the inside caused the problem... so replacing it outright was the best decision.

Anyhow, after replacing the two sensors, the transmission problems have completely been resolved. No more weird cruise control issues, no more erratic speedometer, and no more 2-to-1 gear drops when coming to a stop. Problem solved.


NOTE: This is part 1 of the Dodge Caravan transmission issue. Part 2 can be found here: http://newprotest.org/details.pl?1457