'98 k1500 transmission issue

Bkpigs · Feb 8, 2017

My '98 K1500 Chevy will change rpms while going down the road at highways speeds. It seems like the torque converter is locking and unlocking. It changes just a few hundred rpms. I first noticed it when I would let off the throttle partly. Now it seems to do it more often. It shifts through the gears fine and when I get 70+ there is no problem. I am thinking it is the transmission ( supposedly there is a spring and check ball that is responsible for the torque converter locking up) but I am reading about other causes like a dirty maf sensor or a failing TPS sensor. Anyone had this problem before? She is my hay hauler and feed getter so she can't go down. Appreciate the help.

d9gdon · Feb 9, 2017

I'd look at the speed sensor(s) in the transmission.

jimg.allentown · Feb 10, 2017

There is a possibility that it could be a sensor. There is also a possibility of a bad brake light switch. The computer locks the converter when it sees the correct conditions. It will immediately unlock the converter when you either touch the brake (enough to light the lights) or lift your foot off the gas pedal. This is to prevent the "chucking" that you would get if it stayed locked like a stick shift.
Also, there is a common problem that comes up on the 4L60 and 4L65 transmissions. They use steel check balls in the valve body. As mileage accumulates, they beat their way into and finally through the separator plate. Every one that I have rebuilt has failed at least in part because of check balls either stuck in the separator plate or have beat their way through the plate.
There is also the possibility of a weak lockup solenoid.

oldsarge · Feb 10, 2017

1870 code???????????????? listen to jimg

jimg.allentown · Feb 10, 2017

P. S. I have been rebuilding automatic transmissions for about 45 years.

Bob · Feb 10, 2017

Don't forget to tell him that a clogging fuel filter, or even air filter, or other issue that causes low power/a weak engine will cause the MAP sensor to show low manifold vacuum/high engine load and the 'puter can drop out the TC lock when it sees those sort of conditions.

jimg.allentown · Feb 10, 2017

I'm not sure what the spec is on GM, but on Ford, they do not use the MAF for TCC lockup modulation. Ford also did not continue use of a MAP sensor. They are looking for TP voltage to drop below 1.2 volts or go above 3 volts or a high signal from the brake circuit. GM should be somewhat similar. This is a fairly common complaint on GM vehicles. Many times it turns out to be a problem with the TCC solenoid.
To be sure, there is only one sure way to solve it, and that is diagnosis BEFORE throwing parts at it. A good live reading scan tool can be a good friend for this problem.

Sprint 6 · Feb 10, 2017

TCC bore in the valve body is worn egg shaped. Sonnex makes a kit to bore it oversize and put in a new valve. Or buy a remand valvebody
and spacer plate. P1870 was a money maker when I built transmissions at a Chevy dealer.

Bkpigs · Feb 10, 2017

Thanks guys. I will give the fuel filter a change just to rule that out. Other than that it looks like I need to find someone above my pay grade to diagnose it. I don't have a live scan tool, too much cost for that toy.

oldsarge · Feb 11, 2017

Auto parts stores will read your SES codes,but will not reset,After recent problems I don't really believe in sonnex
valves. I went through two "guaranteed" valve bodies.One no drive,one no reverse. Local guy put a spring in my
original V.B to over ride the TCC.Shifts sweeter than it ever did.Original problem was weak lockup in OD

'98 k1500 transmission issue

Bkpigs

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d9gdon

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jimg.allentown

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oldsarge

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jimg.allentown

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Bob

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jimg.allentown

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Sprint 6

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Bkpigs

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oldsarge

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