i've got an 88 s-15 jimmy 2.8L auto trans...

it isn't shifting at the right times... instead of shifting at 2000/2500 rpm it shifts at 4000/4500 rpm. basically drives like i've got the pedal thru the floor all the time. i've checked the vacuum lines and they are all in tact...

i've been told to replace the modulator (is that the same thing as the solenoid?) and even to replace the ecu (i was told there was a separate one just for the tranny is this true?).

any suggestions?

First make sure your transmission fluid is full and not burnt. If that checks out o.k, check the T.V. cable and make sure it is connected and not kinked or broken at any point. If that checks out, then you will have to adjust your cable. If you don't know how to do that, try searching this website, if you find nothing(unlikely), then just come back here and ask. Hope that helps you out.

First make sure your transmission fluid is full and not burnt. If that checks out o.k, check the T.V. cable and make sure it is connected and not kinked or broken at any point. If that checks out, then you will have to adjust your cable. If you don't know how to do that, try searching this website, if you find nothing(unlikely), then just come back here and ask. Hope that helps you out.

the fluid was the first thing i checked... fluid level is fine and doesn't smell burnt. every cable that i could see connected to the tranny is in tact. i should add that the tranny was replaced about 50k mi. ago.

is the modulator a part that is known to go out? and is it located inside the pan?

Check the vacuum lines going to the tranny.

Your symptoms are a text book case of a vacuum leak where the engine has to race to compensate for the vacuum loss so the transmission with shift.

Check the vacuum lines going to the tranny.

Your symptoms are a text book case of a vacuum leak where the engine has to race to compensate for the vacuum loss so the transmission with shift.

The tranny in this truck is a 700R4(4L60), it does not use vaccuum for shifting, it has no vaccuum modules or vaccuum lines, it uses no vaccuum at all for any transmission function. The line pressure is controlled by the throttle valve cable which controls shift timing and downshift patterens, based on the angle the gas pedal is at. If the TV cable is disconnected it will give you exactly what you are saying, a high rpm shift, because there is no response from the TV cable for adjusting line pressure as needed for shift points. This too which I may add driving it this way will destroy your transmission because your putting all kinds of stress on the transmission internals (band, clutches etc). You said though that the TV cable is alright, so try adjusting it and see if that changes your shift points - it should. If it does not change anything you either a) have a bad TV cable, or b) you have something wrong with your transmission.

yes it does, it is a vacuum controlled transmission.

700R4 is not vaccuum controlled....check your information.

Then what is it then.

The transmission line pressure is controlled by the TV cable, which gives the transmission proper line pressure for shift points, and downshifts etc.

The transmission line pressure is controlled by the TV cable, which gives the transmission proper line pressure for shift points, and downshifts etc.

sorry to disappoint you but blazerLT is right... this tranny is controlled via vacuum lines there is no tv cable on my tranny. i have talked to numerous "chevy guys" and all signs point to modulator and/or vacuum leak.... so my question is:

How do I check for vacuum leaks?

BlazerLT I am sorry then, my mistake. Is this a three speed or a four speed transmission. I am curious as I have never seen anything other than a 700R4 in the shop, all the way back to '83. All four speeds on these trucks are 700R4, which have always been TV operated. I personally have never seen a three speed in these trucks, but they may have had them which would indicate something other than a 700. Heck, I've personally had a '87 2.8TBI auto which was the 700R4. I drove it without overdrive and I think it was 1st gear for like a year before I rebuilt the tranny. The most sluggish truck I have ever owned, absolutly no pick-up, but also the most dependable truck I had too.

sorry to disappoint you but blazerLT is right... this tranny is controlled via vacuum lines there is no tv cable on my tranny. i have talked to numerous "chevy guys" and all signs point to modulator and/or vacuum leak.... so my question is:

How do I check for vacuum leaks?

You can check for vacuum leaks by tracing the vacuum lines and looking for cracks or where they have become unattached from their respective connections.

For the effort, it is better to just replace them seeing it will require the same effort.

BlazerLT I am sorry then, my mistake. Is this a three speed or a four speed transmission. I am curious as I have never seen anything other than a 700R4 in the shop, all the way back to '83. All four speeds on these trucks are 700R4, which have always been TV operated. I personally have never seen a three speed in these trucks, but they may have had them which would indicate something other than a 700. Heck, I've personally had a '87 2.8TBI auto which was the 700R4. I drove it without overdrive and I think it was 1st gear for like a year before I rebuilt the tranny. The most sluggish truck I have ever owned, absolutly no pick-up, but also the most dependable truck I had too.

No worries bro, we all make mistakes.

It is called learning.:D

I make a shit load of mistakes and that is how I learn so much. ;)

You can check for vacuum leaks by tracing the vacuum lines and looking for cracks or where they have become unattached from their respective connections.

For the effort, it is better to just replace them seeing it will require the same effort.

well i've been up and down every vacuum line that i could find, going to and coming from the tranny, without finding any cracks or detached lines... i heard that a few shots of wd40 to where the line connects can let you know if there's a leak at that specific connection or not. all the lines are quite supple.

which would be easier... replacing the vacuum lines or the modulator?

Yea, try carb cleaner instead of wd40 to find the leak.

The engine idle will start to raise or burp a bit once you have found the source.

Yea, try carb cleaner instead of wd40 to find the leak.

The engine idle will start to raise or burp a bit once you have found the source.

thanks... i'm still wondering about the modulator though...