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Technical Thumping noise in first gear??? Merc Overdrive

Discussion in 'The Hokey Ass Message Board' started by Franco51merc, Mar 11, 2021.

  1. Franco51merc
    Joined: Oct 22, 2015
    Posts: 30


    I have a 51 Merc sedan I've been driving and running for about 8 years, never had any major issues. Original Merc Flathead, 3 speed overdrive, original rear end recently changed to 3.31 rear gears.
    All of the sudden the car is making a loud thumping noise from the rotating assembly somewhere. But only in first gear. It almost sounds like a bad rod knock, but it can't be cause it only does it when the car is in gear.
    It's not the rear end cause I ran the car in gear on stands, and the noise is coming from the front. The driveline didn't seem to have much play when I put my hands on it. It goes into gear ok, the clutch works. The only thing I could think of is that the transmission has taken a dump. Anyone know what it could be? I keep my trans full of oil, never ran it dry. Maybe I used the wrong fluid and messed up something? I did try different gear oils
  2. Did you when checking/filling the trans check at both fill places, both the main case and the OD unit? While they share a common drain, level must be checked at both fill plugs and the OD fills separately..
    Hnstray likes this.
  3. Mac VP
    Joined: May 13, 2014
    Posts: 402

    Mac VP

    You can use the OD filler plug to prefill the OD section if the unit is empty but generally the transmission is filled and topped off through the regular gear box filler plug on the passenger side of the case. There are several passages between the OD section and the 3speed section for oil to travel through and even out the level. You monitor the overall level from the 3 speed gearbox side plug.

    The knocking sound you described certainly seems odd for a transmission that is still shifting okay. If you loose a first gear tooth you would hear a clicking sound. If several teeth (all next to each other) were broken off, you could hear something more like a knock I guess. Removing the side cover might allow you to see if there are any broken gear teeth. You may have to put the car wheels off the ground so that you can rotate the transmission via the input shaft. Bump the starter a couple times (coil wire pulled) to rotate the gears enough to inspect the teeth. Or pull out the OD lockout cable and rotate the rear wheels (clutch pedal kept to the floor so that you’re not trying to turn the engine too). The main thing is to observe the cluster gear and the low/reverse sliding gear for broken teeth. Should you see missing gear teeth the transmission will need to be pulled and overhauled.
    vtx1800 and Franco51merc like this.
  4. KenC
    Joined: Sep 14, 2006
    Posts: 800


    Wouldn’t be the first of those with a missing tooth on the low/rev slider. I caused that a few times in my reckless youth. If it knocks/clicks in rev too that is likely the problem. Rev noise will be much less
    Franco51merc likes this.

  5. v8flat44
    Joined: Nov 13, 2017
    Posts: 1,047


    Sounds like a missing tooth 2 me....been there.....hope you get it resolved !
  6. Franco51merc
    Joined: Oct 22, 2015
    Posts: 30


    Thanks for the relpys, I will try to see if it makes any noise in reverse. I think im going to drain the gear oil and see if any metal comes out. If I do find that the trans is bad I have an extra one that I can swap in. Anyone in California that rebuilds these old Overdrive transmissons?
  7. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,304

    from Quincy, IL


    exactly what spec gear oil are you using in your trans?

    also curious to know what prompted the change to 3.31 gears, since you have an overdrive transmission? That would be reasonable choice for non-OD, but when OD is engaged the ‘effective’ ratio will be in the high 2.xx range. That’s pretty tall for a stock flathead. What size rear tires are you running?

    Last edited: Mar 12, 2021
  8. Did the 3.31 gear change involve changing the whole center section, or just the gear set? How much backlash did it have?
  9. Franco51merc
    Joined: Oct 22, 2015
    Posts: 30


    For the 3.31 gears. The whole carrier had to be changed, I found a rear out of an automatic car and had a shop switch the gears and carrier.

    I forget what fluid I used last, but one time I put a quart of ATF in with the gear oil. I was trying to get the Overdrive to work. It worked for a short time, but never worked consistently. Not sure if it was the electricals or not, but I put in effort to get it to work and it didn't. I said screw it and put those gears in.
  10. Franco51merc
    Joined: Oct 22, 2015
    Posts: 30


    Well I finally popped the cover off the transmission, and found out what was making all that noise. The cluster gear on first gear is missing a tooth and has about 7 teeth that are chipped.
    Some chunks came out when I drained the fluid, and quite a bit of small metal. Guess I'll be rebuilding the spare transmission and doing a swap. The spare transmission is missing a tooth on First gear too, but ain't as bad, guess a rebuild is in order.

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