Screeching noise
Using my Pico NVH kit, I tracked down a screeching noise on a 1990 Opel Kadett E to the THM125C automatic transmission. It is coming from the transaxle end, rather than the sidepan, valve body end. From the ATSG General Motors manual on the 125C, I eliminated the possibility that the speedo-driven gear needs lubrication – it’s well lubricated and does it with the car static – but it does sound as if it’s coming from the governor/speedo-gear body. I thought it might be an alternator lack-of-bearing-lubrication noise or a loose alternator fan rattling and contacting the alternator body, but the NVH kit showed that not to be the case. I’ve attached the original Pico.
The noise only occurs at one engine speed, with the car moving or static. I can get it in neutral and in gear, although it’s easier to achieve and stay at the exact revs when in gear, which is about 1200rpm. I wasn’t able to measure the revs precisely because all four channels on the Picoscope were in use.
I’ve dosed the transmission with Wynn’s ATF Flush and extracted all the oil from the sump and refilled it three times while collecting the waste fluid from the return oil-cooler pipe. I used AC Delco Dexron III ATF. I have some Wynn’s Automatic Transmission Treatment, but I’m reluctant to add this to a proprietary lubricant that meets the gearbox manufacturer’s standards, especially if it’s going to start swelling seals that are fine at the moment.
With it being a resonant noise, I’m not too worried, but I wonder if you can shed any light on it? Looking on Youtube at a teardown of a THM125C, I have a mental image of a set of plates loosely vibrating at resonance.
Martin Rubenstein
With noise vibration harshness (NVH) issues, it can be very difficult to pinpoint the source. One of the problems in tracing such problems is that noise and vibrations can travel and isolating them may not always be straightforward, even with the sophisticated kit you have. Because of this, and because you mentioned that at one point you suspected the alternator bearing, I would remove the alternator belt just to be sure that one of the driven pulleys is not the cause of the vibration. You can then be confident this area is not to blame.
The Turbo-hydramatic 125C gearbox is quite a complex unit, but the earlier units contained a torque converter clutch that locked up at around 50mph using vacuum and temperature sensing; later units use a solenoid activated by the ECU to lock up the impeller. As the noise occurs at only 1200rpm, it may be due to the impeller vibrating within the torque convertor, although I appreciate that this is not near the area you suspect. One other possibility is that the chain driving the main shaft has stretched slightly and vibrating through the output shaft, which would travel the length of the gearbox.