My problem is that my transmission RPM sensor on my '98 Astro is telling ECM that engine is turning too many RPMs for highway speed being traveled. This results in trans going into 'limp home' mode which is the only true problem. Vehicle rated at 21 mpg is getting 24 mpg when ECM makes this conclusion that there is too much transmission slippage due to the RPMs being more than 133 over nominal programmed amount.

I am perfectly happy getting 24 mpg. I don't know if there is an abnormal amount of slippage, but if there is, I don't want to fix that. I want to know if I could have a bad RPM sensor that reads too high. I would also like to know where this sensor is so I could look at it. I would also like to know if this sensor could be altered to get it to read about 5-10% low, as that would definitely fix problem? I would also like to know if I were to get a replacement sensor, could I chose one that was made at the extreme of the tolerance range so that it reads the RPM's lower? If the reading is a voltage, could I add a resistor to lower the voltage? The alternative repair is $750 or more to replace the control module in the transmission. I would like to avoid this option. To make matters worse, I replaced the original tires with larger radius tires, which increases the speed and the wind resistance at a given engine RPM.

 

Make sure your torque converter is locking up. That would be about the margin you are seeing. And you are properly sceptical about the need for major tranny work. I would do very serious research on the web. It's unlikely you're the only one who has had this problem and it might well be something simple, which they'll fix and charge you $750 anyway if you don't know any better. And make sure you're running the right fluid if it's been touched lately.

Regards

cww

 

I am sure a well spent conversation with a knowledgeable GM repair service tech will reduce the overall cost of the repair. Given that the transmission is slipping, if you do't fix the problem soon, you could be looking at a repair bill well into the thousands of dollars.

If you attempt to alter the signal from the sensor, you could creat other problems and again be back to spending money you either don't want to or have to spend. You can also go down to a local auto parts store and see about getting the required part and replace it yourself... You can go to places like large auto salvage yards and pull a part off of another vehicle yourself.

Sometimes is is cheaper, simpler and faster to have the vehicle repaired by a dealer, plus you get a warrenty on their work, if you screw it up, you will be paying them anyways to fix it.

mjh

 

I am very familiar with Ford Transmissions. In fact, they are the reason that the foreign car makers have had success in penetrating the US market. The transmissions are junk. A friend of mine used to own a AAMCO shop, and he said that Ford and Mercury kept him alive.

More seriously, go to a used part supply house (previously known as a junk yard) and get the parts you need. If the transmission is actually slipping, then I would definitely not plan any long (or short) trips.

I had a 1990 Mercury Sable that I kept in excellent repair. The transmission started acting funny at 58,000 miles, and I told Mercury about it. They made light of the situation and actually laughed at me. So, the transmission went out at 60,000 miles when the warranty ended, and Mercury refused to act even though I had made them aware of the problem. That was the last time I bought a Ford and will never buy another regardless. In fact, I think that I may have cost for a substantial amount of money because of my large circle of friends, associates, and relatives whom I made aware of my Ford problem, and they in turn largely avoid buying Ford junk.

So, good luck (junk the Astro)

 

Is the torque converter clutch being "forced" to lockup earlier due to "thinks slippling"? If you do not have the factory manual(what's $120 when you spend $1000's on a vehicle?) get one( yours is $120 at http://www.helminc.com Part #: GMT98MLF), borrow one, or do some library time. This will locate the sensor(s).

Also, buy/borrow/rent a realtime scantool to interface to the ECM to "see" when & where this happens(I've saved $100's in "diagnostic" costs which paid for the tool by having a realtime scantool from ACTRON).
http://www.actron.com
http://www.autotap.com/

The manual, scanner, wiring diagrams, and your intelligence(you are at control.com) will help you decide if you can fix this, drive it until it grenades, or get it replaced.

 

Try an internet search on the device like "transmission RPM sensor". Sometimes the automotive boys use the term "sending unit". I've seen signal protocols described for various automotive sensors. Or, try looking at the voltage, current, and waveform of the sensor output, as the engine runs at various speeds (while the sensor is still connected to the ECM). This should tell you what the signal quantity is (voltage, square wave frequency, etc.), and help you figure out what your options are.

 

> I am perfectly happy getting 24 mpg. <

No wonder you guys didn't want to sign up to Kyoto, I'd have my car at the garage if it dropped below 10miles/l (45.5 mpg)

marc

 

This is an automation board, not an auto repair board.

 

Most of us do drive cars as well. And it is an electronic automatic transmission. :^) Servoes, etc. I forgot to mention the ATC manuals available from J.C. Whitney. Very useful and condense the experience of many transmission pros. The hazards of an ill spent youth :^)

Regards

cww