Depending on how you cleaned the battery contacts, it is very possible that you caused damage to them. Most battery contacts are made from a cheaper metal that has been plated with layer of metal with better electrical conductivity. This is cheaper than making the whole contact out of a more expensive metal. Battery corrosion is something that vintage toy collectors/restorers deal with quite frequently. A quick google or youtube search for vintage toy battery contact restoration should yield some good results. It sounds like you have already addressed this issue.
My personal experience with using AA sized rechargeable batteries is that they tend to be a little shorter/not as long as AA alkaline batteries. Again, you may need to adjust the battery contacts to make the best contact with the batteries you are using.
I did some online research into the Sears Lobo (Nikko BlackFox) buggy as I wasn't familiar with this car. Based on some pictures of the box, it looks like your car uses 8 AA sized batteries. It could use either 8 x 1.5v akaline or 8 x 1.2v nicad batteries. It also looks like there was a version or later model used a removable 9.6v battery pack. So this got me to thinking. Are the electronics on your car limited to only allowing 9.6 volts to get to the electronics? That would seem to make sense based on your readings.
- 8 x 1.5v alkaline batteries = 12.0 volts fully charged (I just tested a brand new Duracell AA and it was around 1.5-1.6 volts).
- 8 x 1.2v nicad/nihm batteries = 9.6 volts fully charged (I just tested a freshly charged Energizer nihm and it was around 1.3-1.4 volts).
So if the battery circuit on your car is only allowing around 9.6 volts to get to the rest of the electronics with the alkaline batteries, why are the nihm/rechargeables only around 8v? Without seeing a wire schematic, I can only guess what is happening. Here are a few options...
- Your nihm batteries aren't fully charged. Since you appear to have a volt meter, I would suggest you check each battery individually out of the car.
- Your batteries aren't making good contact. It seems like you have already suspected it. Since the batteries should be installed in series, just find the negative end of the 1st battery and use that as your starting point. Before you test make sure the vehicle is turned off. Then use your volt meter to test the positive end of the 1st battery in the series and it should show around 1.2 volts. Then test at the 2nd battery positive and it should be around 2.4. Continue down the series and check at the positive end of each battery. Depending on your battery compartment this may not be easy and you may need to use a wire to reach the battery contacts.
- Your car may have a physical or electronic switch to limit the voltage from the battery to the electronics. While I highly doubt this is an option, it wouldn't surprise me if there was some kind of switch to set which type of batteries were being used in the car. Since the car was designed for rechargeable batteries, the switch may have been needed to allow charging of the nicad batteries while installed in the car.
- Since this car is over 30 years old, there may be a bad electrical component on the circuit board. Capacitors can wear out over time, resistors can fail, etc. This doesn't necessarily mean the vehicle won't work at all, but it may be working in a less than optimal state. The easiest solution with r/c cars is to upgrade the electronics when possible. On cars such as yours that isn't always a practical option. Unless you have electronics knowledge and schematic, board level repair may not be an option.
Good luck getting your vehicle back into running condition. I hope you can make the rechargeable batteries work as that is a more cost effective option. Based on videos I found online, it looks like that car would be very fun to drive for a 4-yr old.