Tachometer fluctuating at highway speed

[goser]

Hi all, 2001 Blazer vin W with 4L60E. Bought it off a friend when his radiator failed and coolant mixed with tranny fluid. Put in a new rad and flushed the fluids. Low speed (<50km/h) test drive performed normally so I had the vehicle safetied and plated. Still drives okay at low speeds but when it gets up to highway speed the tachometer starts to fluctuate by a few hundred RPMs...most noticable between 1500 and 2000 RPM. Also, my wife noticed a vibration when she drove it up to my storage garage; she said it was a rythmic vibration similar to driving over a rumble strip when she was doing 95-100 km/h. It could just be an unbalanced wheel or something, but I thought I should ask if that could be coming from a transmission component.

I remember reading a post on here about a similar situation and testing the TCC (torque convertor control?) solenoid by tapping the brake at speed and watching the RPM's... my RPM's did increase when I touched the brake, but I can't seem to find that original post now to see what that indicates.

I don't have a scanner capable of reading tranny codes, so I'm hoping someone can recognize these symptoms and tell me where to start looking for the problem. I know I will eventually have to rebuild/replace the transmission from the fluid contamination, but if I can solve this by just replacing the torque convertor or a solenoid that would buy me some much needed time before shelling out for a full rebuild.

Thanks in advance!

[MT-2500]

Originally Posted by goser

Hi all, 2001 Blazer vin W with 4L60E. Bought it off a friend when his radiator failed and coolant mixed with tranny fluid. Put in a new rad and flushed the fluids. Low speed (<50km/h) test drive performed normally so I had the vehicle safetied and plated. Still drives okay at low speeds but when it gets up to highway speed the tachometer starts to fluctuate by a few hundred RPMs...most noticable between 1500 and 2000 RPM. Also, my wife noticed a vibration when she drove it up to my storage garage; she said it was a rythmic vibration similar to driving over a rumble strip when she was doing 95-100 km/h. It could just be an unbalanced wheel or something, but I thought I should ask if that could be coming from a transmission component.

I remember reading a post on here about a similar situation and testing the TCC (torque convertor control?) solenoid by tapping the brake at speed and watching the RPM's... my RPM's did increase when I touched the brake, but I can't seem to find that original post now to see what that indicates.

I don't have a scanner capable of reading tranny codes, so I'm hoping someone can recognize these symptoms and tell me where to start looking for the problem. I know I will eventually have to rebuild/replace the transmission from the fluid contamination, but if I can solve this by just replacing the torque convertor or a solenoid that would buy me some much needed time before shelling out for a full rebuild.

Thanks in advance!

Is it a 2W or 4W?
If the transmisson has a problem you need to find someplace with a transmission/engine and if 4W transfer case capable scanner to check it out.

Vibrations can be caused by many things?
Do some more road testing?
Try to pin point.
At what road speed in MPH and on what type of roads nd under what driving conditions do you feel the viberation?
Does tach act up same time you feel viberation?

Another note is the Blaziers have a lot of instrument cluster problems with tach and or speedometer.
Does speedometer work good at all times?

[goser]

As always, thanks for the reply MT.

It is a 4WD. I know that I'm just stabbing in the dark until I get the codes read, but it will be a few weeks before I have the time or money to have it done.

Unfortunately I haven't been able to recreate the vibration that my wife felt, so perhaps that was a passing problem. It was on the freeway at 60MPH, but she doesn't remember if the tach was fluctuating at the same time.

You make a good point about the instument cluster. I thought I could feel the engine changing speed when I took it out on a test run after noticing the problem the first time, but perhaps I was imagining it. My first course of action was going to be connecting my scanner to see if it showed the RPM's fluctuating, but my laptop screen died so that will have to wait. (when it rains it pours lol.)

Now that you mention the instument cluster, the fuel gauge has been acting up since I bought it... I suppose that could be an indication of a loose connection somewhere. My other thought was that if the sending unit is going, perhaps the fuel pump is also growing weak. It has been hard to start the last few days, which I attributed to corrosion on the battery terminals, but I haven't had a chance to check it since charging it up again. I had immediately assumed the transmission was the culprit because of the contamination, but would a weak fuel pump cause RPM fluctuations? I guess I should take it out for a spin with a fuel gauge just in case.

Cheers.

[MT-2500]

Originally Posted by goser

As always, thanks for the reply MT.

It is a 4WD. I know that I'm just stabbing in the dark until I get the codes read, but it will be a few weeks before I have the time or money to have it done.

Unfortunately I haven't been able to recreate the vibration that my wife felt, so perhaps that was a passing problem. It was on the freeway at 60MPH, but she doesn't remember if the tach was fluctuating at the same time.

You make a good point about the instument cluster. I thought I could feel the engine changing speed when I took it out on a test run after noticing the problem the first time, but perhaps I was imagining it. My first course of action was going to be connecting my scanner to see if it showed the RPM's fluctuating, but my laptop screen died so that will have to wait. (when it rains it pours lol.)

Now that you mention the instument cluster, the fuel gauge has been acting up since I bought it... I suppose that could be an indication of a loose connection somewhere. My other thought was that if the sending unit is going, perhaps the fuel pump is also growing weak. It has been hard to start the last few days, which I attributed to corrosion on the battery terminals, but I haven't had a chance to check it since charging it up again. I had immediately assumed the transmission was the culprit because of the contamination, but would a weak fuel pump cause RPM fluctuations? I guess I should take it out for a spin with a fuel gauge just in case.

Cheers.

Fuel gauge and weak pump and tac are are seperate problems

[MT-2500]

You are welcome.
And yes test drive with fuel gauge on it.
Also watch cold start fuel pressure.
For a good cold start you need 64-65 lbs fuel pressure.
Engine cranking.
Full fuel pump pressure should be 95-105 lbs.
On the road after 20-30 minutes you pressure should range 62-65.
Fuel gauge and pump are seperate but may share ground wiire ground.
If fuel pump does check bad always replace with Delphi fuel pump.
Say no to airtex or aftermarket pump.
Good luck