For the past several months I've noticed that when I reached interstate speeds the RPM needle will suddenly jump 1-2000 RPM for a split second then return to normal. Initially it took some watching of the gauge because it was infrequent, happens only for 1/2 a second and their was no physical signs that the engine was surging. I measured MAP, Fuel Injectors and MAF and the readings were not consistent with an increase in engine speed at all.

During the past 2 weeks the frequency has increased 20 fold and occasionaly I will notice a mild clunk in the transmission, although this happens only once ever 10-15 times the guage does this. Today after about 10 minutes on the interstate at 70 MPH, I gently accelerated and the gauge shot up from 2200 RPM to 3500 and back about 5 times consecutively...the transmission also lightly clunk and has a momentary hesitation.

Here'e the vehicle Facts

98 Chev Blazer LT

10000 miles ago I replaced
Reman GM Engine and all sensors (MAP,MAF,O2, CRANK, temp)
Exhaust, radiator, Transaxle actuators.

30000 Miles ago I had a rebuilt GM Transmission put in.

Increasing frequency (may be heat related ???, started in winter)

All other gauges normal, No engine or trans codes.

Other than slight trans behavior change during, no signs of engine fluctuation.

Doesn't seem to be TC lockup as I do the Brake test before or after and the RPMS consistently increase ~300 RPM shen I hit the apply the brake and then decrease after a second or two.

Trans Fluid, Diff Fluid, Trans Axle Fluid and Engine Oil are all changed on a regular basis.


Is the sensor that controls the gauge used to determine shift points or connected to TC lockup ?

Mike

Two things to check on it.
RPM signal reading to and in pcm.
A good scanner will give you that reading.
And rpm signal at pin A3 white wire on IPC.
If both match replace tach or IPC.
RPM signal to tach has nothing to do with transmission operation.
Speed sensors control the transmission operation.

check your transmission fluid level. willing to bet its very low.

Guys...sorry for the delay in getting back.


Tach is still suddenly spiking....a bit more infor though

Used scanner to record PCM RPM's to a file.....The PCM either isn't registering the RPM spike or the scanner doesn't have the resolution to capture a spike that fast....the needle jumps and returns split second. When I manually match the PID RPM to the tach it matches consistently.

This is accompanied by a quick "bump" from the transmission...it nearlt always happens when I've accelerated and backed off on the throttle.
Sometimes it may happens successively 3 times right in a row and even at lower speeds...ie getting onto intertstate.

I checked the fluid and it is right in range....color is very good.

Transmission shifts smooth...scanner detects no codes.

I'd say tach....which wouldn't bother me...but whatever is causing this must somehow be impacting the transmission.