I have a 1992 DOHC Nissan Maxima SE which has been to 3 different mechanics and been at the dealer for a couple of months and they have conceded to the challenge of diagnosing the problem. The car downshifts erratically at random times, when the engine is hot or cold, whether you are going fast or slow. I did notice that when the outside temp was very high and the car had been sitting in the hot sun for some time, it acted up way more. The list of parts replaced is as follows: Fuel pump, TPS, MAF, spark plugs, Injectors, Speedo head, TCM, Coil packs, transmission, Ignition control module and probably some others I have forgotten about. AS I said, the dealer is baffled by this as well as others. A friend recommended that I contact you for advise.
The first thing that I would ask is whether or not - in all of those parts replacements - each circuit associated with each part was checked and double checked. E.g. a bad TPS would do this to a transmission. But an intermittent wiring fault to the TPS could also do it.
Has the thing been on a scanner? Was any of this parts replacement done with actual diagnostic info or is everyone just guessing. It looks a lot like guessing.
Yes, it has been at the Nissan Dealer and they used all the diagnostics at their disposal. They did talk about only a speed sensor code but said that the sensor tested good. They then determined that it was the speedo head and replaced it but that didn’t work so they gave it up and didn’t charge for anything (this was after replacing the water temp sensor and telling me it was the TCM which I replaced) Could it be the ECM if there are no codes? The car does have a sport and comfort mode on the tranny. I noticed that when it is doing its thing, the sport mode light flashes. Anyway, yes they had the car hooked up to a diagnostic tool while driving to do the testing.
“The list of parts replaced is as follows: Fuel pump, TPS, MAF, spark plugs, Injectors, Speedo head, TCM, Coil packs, transmission, Ignition control module and probably some others I have forgotten about.”
I’m just having a hard time believing that all of this gets done as a response to actual diagnostic procedures. It looks like a lot of guessing is going on. Guessing about the speedo head (as you mention) is a case in point.
It seems to me that the sport mode light flashing would indicate something. You’ve been to 3 shops. Was any one of them a local transmission specialty shop?
Has the wiring been checked each time a part was replaced or not?
Sounds like they’re just throwing parts at it.
Since the code they find is the speed sensor, and the sensor tests good, sounds like some bad wiring or a bad ground somewhere. If the TCM is getting a weird signal from the sensor, I imagine it would confuse it. You say the sport mode light flashes. Does the car do the same thing if you put it in sport mode?