Hello c body gurus: 78 Dodge Magnum w 400 engine, automatic. Chasing a vibration that occurs at idle and when in gear, not caused by alternator or power steering pulleys. Could see a slight wobble in the harmonic balancer when opening the throttle, so replaced the HB. Bad vibes still there, again present at idle and up through 1800 rpm. Looking at the old harmonic balancer it had the round type (not the oval type) no additional weights on it - should it? I have been told the 400 ci eng were externally balanced at the HB and at the torque converter. I see weights on the torque converter, but not the HB, thus why I am asking. The transmission was replaced 5 years ago by the original owner (and I assume the torque converter was too, no paperwork to prove it). Should i remove the weights on the torque converter (in case this is an internally balanced engine), or, since it still vibrates when not in gear is that not a factor. Should i install a weighted HB? Any ideas on how to chase this down?
Chasing vibrations
- Thread starter Gtguy
- Start date
The '78 engine if it is original should be externally balanced. There was some internally balanced 400's with steel cranks, but they were long gone by '78. You should research the old balancer number and find out if it was the correct one for a '78. What did you replace it with? The universal units that they sell at Auto Bone only work with internally balanced engines which is why they fit a wide variety of Mopar engines. Check your balancer #3614371 steel crank, 3614374 cast crank covers most of them
Dave
Check the casting numbers on the block to see if it is the correct year for the car. If this is a '78 engine, it should be externally balanced. The problem on a 50 year old car is that sometimes parts get mixed and matched. The transmission is for an externally balanced engine, so one could probably assume that type of engine was in the car to start with and whoever rebuilt the transmission left it the way it was if they were competent (sometimes a dangerous assumption). The VIN number should also be stamped on the block, there is a machined pad on the passenger side just above the pan line at the front on most models. If the VIN matches, you know this is the original engine. The police and high performance engines usually had steel cranks until mid '74, the rest were iron cranks. After mid '74, they should all have been iron cranks, but with Mopars, never say never, as there always seem to be some outliers. The only sure way on an engine that may have been rebuilt with an uncertain pedigree of internals would be to pull the pan and check the crank number. Did you replace the original balancer with the same type? If this is an externally balanced engine, the replacement balancer would need to be for that type of engine or you would still have the same problem. Do not attempt to remove the torque convertor weights until the type of crank is known for sure as that will just generate the need to replace the convertor if you guess wrong. Removing the weights is always a last resort as it is very easy to damage the convertor housing and ruin a good convertor.
Dave
The 2BBL 400 engine used crankshaft # 3462923 from 72-78.
There was also a "premium" cast crank for some of the police and high performance 4BBL engines #3751877 and this crank was also used in most truck 400's.
Dave
There was also a "premium" cast crank for some of the police and high performance 4BBL engines #3751877 and this crank was also used in most truck 400's.
Dave
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Hi Dave, attached is a photo of the HB taken off the car, cleaned it up and there is a number 1604-2 and A2. Not sure if it's original. And yes, the HB installed was the same as the one pulled off (not weighted). Had a chance to drive the car a bit today, I'd say it's vibrates less, wondering if what's left is due to "a more aggressive cam" that the original owner says he installed (no paperwork). How can I tell if whats left is a cam related vibration? Unfortunately i cant reach the original owner. Its not terrible, not noticeable above 2k rpm or at highway speeds. Car starts, rides beautiful other than this issue. Thank you for your advice, I really appreciate it. As far as inspecting the crank, I want be able to do that until fall, given my workload.
That looks like an internally balanced unit. Are there any cut outs on the back side.?
Dave
Dave
An after market cam should not cause any abnormal vibration. If you rev up the engine to 2k rpms, with the car in neutral, do you still fell the vibration? If so it is probable engine related. If it only does it in gear down the highway, it might be drive line or rear end related, usually going to be u-joints. As your '78 has an early locking convertor, that could also be what the issue is. '78 convertors were noted for having chunks of the friction material for the locker come off and leave the unit unbalanced. If that is the case, you will need a new convertor and probably a transmission rebuild as when these units fail there will usually be friction material through out the transmission.
Dave
Dave
No Dave, no cutouts on the back, see photo. Regarding your comment that the HB is for an internally balanced engine, that's what my mechanic guy said, and hence the confusion. There are weights on the torque converter.
Could we put a weighted HB on as test? Would a weighted HB from a 440 work (sorry, that's prolly a stupid question), we have one of those handy. I could probably pull the access cover for the TC tomorrow and get you a picture.
Could we put a weighted HB on as test? Would a weighted HB from a 440 work (sorry, that's prolly a stupid question), we have one of those handy. I could probably pull the access cover for the TC tomorrow and get you a picture.
The part number is different for a 440. There is a 400 cast unit currently on e-bay but for a test unit $50 is a bit steep.
Dave
Dave
You clearly have mismatched components (internal balancer/external torque convertor), but the only way to know for sure if it is the wrong convertor or the wrong balancer will be to verify the crank. My guess, unless the engine was rebuilt with a steel crank or premium cast crank installed, it is probably the harmonic balancer. Up to you to decide if you want to bite the bullet on another balancer.
Dave
Hi Dave ,
Wanted to follow up with you on this issue, I was finally able to find a old school mechanic willing to look into this. I pointed him in the direction you indicated- mismatched components (harmonic balancer and torque converter). If you recall, you indicated the wrong balancer was on the car, it required a weighted balancer (a non weighted one was on it) I found a NOS weighted balancer for the 400 engine on line. My mechanic installed it and the car no longer has the vibration. Anyways, I wanted to thank you for your advice and expertise in this matter - you kept me going in the right direction, and although it took longer than i wanted for me to resolve it, the wait was worth it. The car runs great. Thanks again, it's so nice to pick the brains of experts...
Wanted to follow up with you on this issue, I was finally able to find a old school mechanic willing to look into this. I pointed him in the direction you indicated- mismatched components (harmonic balancer and torque converter). If you recall, you indicated the wrong balancer was on the car, it required a weighted balancer (a non weighted one was on it) I found a NOS weighted balancer for the 400 engine on line. My mechanic installed it and the car no longer has the vibration. Anyways, I wanted to thank you for your advice and expertise in this matter - you kept me going in the right direction, and although it took longer than i wanted for me to resolve it, the wait was worth it. The car runs great. Thanks again, it's so nice to pick the brains of experts...
Your welcome.
Dave
Dave
oldkimmer
Active Member
Is it even a 400? Crank may have been changed. What is stamped on the pad under the distributor? Maybe it’s a 383.
It's a 400. Stamped on the block. Original build sheet also confirms 400.
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