4L60 questions

[RedBed]

I just got my truck back from having the TH350 swapped out for a ('91) 4L60. We did not change the rear end gears (yet, anyway).

I don't know squat about transmissions. I do not know what rear end gears I have either. Were 3.08 stock?

Anyway, as I accelerate and get up to about 60-65, the truck feels like it bogs for a second (almost like slight braking) and then I give it a little gas and it seems to get up out of it. But I can't really tell if it's a lockup problem, or if it is just not getting into OD. If I have 3.08s, maybe there's not enough rpm to get into OD? Or maybe the lockup is going in/out?

I will be taking it back to the shop, but I am just wondering if anyone has had this problem, or any ideas?

(note: I do not currently have a Tach, but will install one in a couple of days)

I'm trying to learn a little...

[wilkin250r]

With a 3.08 rear end, the overdrive kicking in at 60mph should drop you from roughly 2000rpm, down to a little under 1500.

1500 rpm is low, but I wouldn't say it's THAT low. It's not like you're bogging the engine completely out of it's powerband, that wouldn't occur until you get down to about 700rpm or so.

[RedBed]

Ok, thanks.

A lockup is not a necessary thing right? Could I disable the lockup and run it that way to see if it still happens?

And how would I do that, by disconnecting the vacuum and plugging it? Or remove the power wire from the vacuum switch?

[Alex V.]

Is the current transmission a 4L60 or '60E? If it's an 'E lockup is governed electronically; not sure about its predecessor. Lockup benefits fuel mileage and fluid temperatures, so if you're not going to use it you've partially lost some of the benefit of ditching the Turbo 350.
Asking an engine to tote the truck down the road at anywhere near 700 RPM is asking for a quick death of said engine. I've spent some time in a '93 truck with a 700r4 and what I believe were 3.73's, with 31" tires, and the 60MPH engine speed of about 1,700 RPM was just sad. Forget about towing anything in overdrive. On the other hand my dad's Suburban with the Vortec 350 and '60E, 3.42's and 29" tires runs pretty well, turning exactly 2,000 RPM at 70 - but it has all the benefits of the Vortec injection and induction systems and concert of computers working together.

[RedBed]

4L60 - (not E)

I only want to disable the lockup temporarily so I can see if that is where the problem is.

BTW - my tires are 29.5"

According to the online calculator I used, if my gears are 3.08, then I should be at 1473 rpm at 60.

[kwmech]

The 350 regardless of cam isn't going to be happy at 60 miles an hour running just barely off idle. With the 29'' tire you should look at putting in at least a 3.73 gear. Personally I would go with a 4.10. You should also run the lock-up, without it, it adds additional heat to the trans. A little math for you.

3.73 x (4th gear overdrive of 30%) .70 =2.611 final
4.10 x .70 = 2.87 final

[RedBed]

HA HA, joke's on me.

Had a bad FUEL PUMP. That was what was making it hesitate at speed. I guess I had blinders on.

Seems to be doing fine now. Now I will be able to test it for a while and see if the setup works. Once I get the Tach installed I should be able to calculate what gears I really have in the differential.

At least the Fuel Pump was still under warranty!

[wilkin250r]

Gears are really easy, and you don't need a tach.

Jack both rear tires off the ground, transmission in neutral (block the front tires to prevent rolling) and spin one side by hand.

-If the opposite tire spins in the opposite direction, you have an open differential.

-If the opposite tire spins in the same direction, you have a limited-slip differential.

So the basic method is to count the rear tire revolutions, and the driveshaft revolutions, and do the math to get the ratio. The only thing you have to remember is that rear is designed to spin BOTH tires the same. If you're spinning by hand, and both tires are not spinning the same, you need to adjust your math to correct for it. The more revolutions you spin, the greater your accuracy will be.

If you have an open differential, I'd suggest dropping one side on the ground, and spinning the other side double to compensate. So let's say you spin your free tire 8 times, which results in 4 revolutions of the ring gear (that's the adjustment in your math I was talking about). You also count right about 13 and 3/4 driveshaft revolutions.

13.75 / 4 = 3.44

The closest to that would be 3.42, that would be your answer!!!

If you have further questions, just let us know, we can help.

[RedBed]

Very handy info, thank you.