Gearbox seizes in reverse

[JelleP]

Hi all,
 
I have a Fiat 500 related question, which I hope is OK - my car is from 1969 and is fitted with a synchronized gearbox, presumably from a Fiat 126. This forum seemed to be the most active of all classic Fiat forums I've found through Google.
I've bought the car together with a friend of mine a few months ago and after giving it some TLC it became road legal last week!

Today, the following happened: we had to reverse down a road for a bit, hitting about 30 km/h. As soon as I pressed the clutch without touching the brakes, the rear wheels blocked completely causing the car to stop so violently the engine hood flew open and the tyres were screeching on the asphalt. It scared the hell out of both of us and it happened again after we tried to replicate it (a bit slower this time..).

Is there anyone out here who can explain what happened? Did we do something really stupid we aren't aware of (like driving in reverse that fast) or is it some known issue in the 126 gearbox? Thanks for your replies!

Jelle & Simon (The Netherlands)

[Pete126]

Hi, you have been a little bit silly 😛 the 126/500’s have a straight cut gear for 1st/reverse and it’s quite easy to chip teeth off the cogs if you’re not gentle when you’re going up and down the box, what’s probably happened is that the gearbox has peace’s of metal floating around that has got jammed in the gears which locked up the drive, the only way to find out is to take the box out and inspect, it is a common fault that’s why they do a repair kit.
Once you’ve sorted it out remember never select 1st/reverse when the car is moving,  don’t try doing burnout’s 😂 and no J turns in 1st and reverse  😂

[JelleP]

Thanks for your reply! Yes, we know we shouldn't select the 1 and R gears while the car is in motion, even in modern cars they are often not synchronized. But in our case, the sudden stop happened after just pressing the clutch and not touching the shifter or the brakes. Also, a floating piece of metal couldn't possibly cause a seize twice in exact same situations could it? 

[Gadge]

To lock a car up like you decribe i'ld say you've suffered a major mechical failure down such as a selector fork breaking or two gears engaging at the same time such as 3rd and reverse or maybe a diff pin dropping out and jamming up the differential. One other area it could be is the cluth. As you say that it locked soon as you pressed the pedal it could be that the spring plate has failed and the clutch fork has punched its way through it locking  it up. It's certainly not a common thing so the only way to be sure is to whip the box off and investigate it further.

[JelleP]

Thanks for your suggestions. There's just one thing I have to add, as I don't think I've described the gravity of the problem well: the car drives and shifts totally fine, even after the incident I described. There's absolutely nothing that indicates a major mechanical failure in the gearbox. Gently reversing is no problem at all.