hard shift, engine light and code p1870

hard shift, engine light and code p1870

Postby danvan [OP] » May 28th 2009, 3:09pm

I am re-asking a question over here at this site before I dig in and try anything to my trans. I posted this at another site and was given some good info about a front pump spring and TCC sticking, but then was told I may be able to fix it with just changing fluid and filter and adding a cooler. Changing the spring involves dropping the valve body which I have never done.

So i wanted to get a 2nd opinion here before attempting any repairs. Since my original post, the same exact thing happened the following weekend also while towing the same light load.

Here goes the question,

This weekend while towing a utility trailer with 2 small quads on it through the mountains of Vermont, my 'service engine soon' light came on and the trans started shifting very hard.

At 1st it was just the 1-2 shift, then later on it was the 2-3 also. Both shifts were also very delayed no matter how hard or soft I was on the gas pedal.

I checked the fluid and it was good.

On the return trip it acted up again after it was fully warm, like 75-100 miles of driving. This time we stayed out of the mountains and stuck to the 'big' road.

On the return trip we stopped 2 times, (food and gas) each time the problem stopped for a while and then eventually acted up again after getting hot again.

The light has not been off since it originally went on, but the trans is shifting normal the last 2 days going to work and back, about 10 miles each way.

I do remember reading something here in the past about a shift solinoid causing a harsh shift, could this be my problem? Can I replace this myself w/o getting inside the tranny?

How do I pull a code and shut the light? I remember something years ago with my last astro, using a paper clip and testlight maybe??

I dont have a tranny cooler installed, but do have one that I will now install before the weather gets real hot.

This is a 2000 astro with the 4 speed overdrive trans, with the tow/haul button.

I did leave it in the tow haul mode for 80% of the trip. What does that do for the trans, it seems to shift a little later with in engaged, not sure if the OD engages or not when it is in tow/haul, if it is doing it its going in very late and soft. I wish I had a tachometer.

There is a little over 100k on this van, not sure if the trans fluid has ever been changed and not sure if I should change it now or not. Ive always heard that if its never been done by 100k, leave it alone.

Thanks
Dan
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Re: hard shift, engine light and code p1870

Postby Matrixx » May 29th 2009, 5:08am

It sounds to me like it's getting to hot or not enough fluid circulation after a period of time, which would lead me to believe it maybe a restricted filter, wrong filter, incorrectly installed filter, wrong fluid, or the pump itself.

I would suggest to you the same as what has been mentioned already and see what happens at that point.

Definitely a transmission cooler.

Hope this helps.:)
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Re: hard shift, engine light and code p1870

Postby dannyg40 » May 29th 2009, 9:44am

Not sure if this would help but I had a very hard shift and no check engine light. Took it to a trans shop and he tried the servo, I think accumulator spring, shift solenoid and problem would not go away. The problem turned out to be ........ The Computer itself! The computer kept one of the solenoids open and something about the trans constantly had 185lbs. of pressure and that made him think it was a solenoid but turns out it was the computer
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Re: hard shift, engine light and code p1870

Postby gillbates » May 29th 2009, 10:30pm

This sounds like the solenoid issue you mentioned. It only happens when the car is at highway temp and should just be the 1-2 shift and it feels like someone hit you in the rear when it happens. There are several TSBs on this. When it happened to me I had a new valve body put in and it fixed the problem. Since then I've heard they have a $60 dollar kit to fix it which is probably the replacement solenoids. Any good trans shop should be willing to put it in for you for less than $200.
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Re: hard shift, engine light and code p1870

Postby msgale » June 7th 2009, 7:10pm

I have the same exact problem: i could even predict which traffic lights it would happen at, since i often travel the same route: about 45 min of driving, in warm weather

It's a well known gmc tran defect

There is a solenoid inthe trans, easily accessibld if you remove the pan, for about $30, that control 1-2 shiftng. easy replacement once pan is off,.

also another for i th9ink 3-4 shifting, but for some reason the 1-2 solenoid fails first.
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Re: hard shift, engine light and code p1870

Postby danvan [OP] » June 9th 2009, 1:23pm

Thank you all for the replies. Ive been trying to find the time to get this repaired and its just not easy. I stopped at a local tranny shop this morning, told him the symptoms and he is aware of the very common problem in these transmissions, but told me it would cost me $400.00 at his shop, plus $60.00 - $100.00 to install the cooler. Also he will not sell me the parts. The 400.00 would also include a tranny service, which I assume is not a flush, but just the pan dropped and new fluid and filter.

I am in the process of getting the name and number of another tranny shop my friend recently used to see what price he gives me.

He told me the parts are only available at a dealer and I would have to but the whole valve body for around $700.00.

Does any one here know where I can get the parts for this repair if I decide to do this myself? He refered to it as a valve body update. Can they be ordered online? Part numbers any one? Is there a TSB number that I can use at a dealer to get just the parts I need and not an entire valve body?

Thanks again, you've been very helpful.
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Re: hard shift, engine light and code p1870

Postby twitchit72 » July 9th 2009, 3:38am

I joined this forum just to answer your question. your 1870 is a code that pops up because a piston wears in its bore in the valve body. this is a comon problem. Sonax sells a kit that cosist of a new sleve and a piston. it retales for between 40 and 60 bucks. the install requires a piloted reamer. that tool is another80 to 120 bucks. add a filter and gasket set and you are looking at well over 200 bucks. this has to be done off the tranny. You have to pull the valve body to keep from getting metal shavings in it. There is also an issue with clutch material in the TC gm used normal clutch material up to about 99 and then went to a "woven carbon" material after that. The two are not compatable and the sonax kit has to be installed a certain way per which TC you have. install it the wrong way and the van will violently lurch when the TC locks up or if it is the other way the TC will burn the clutches. This is 99% sure your problem is NOT the solenoid! Nor is it a dirty filter. You won't be abel to fix it by changing the fluid. The reason it goes away when you stop is because when you turn off the ignition the computer resets. The reason for the hard shift in laymans terms is the computer sees slipping in the form of heat, aand boost line pressure. Thanks GM!On the bright side this is a permenant fix. 300 bucks seems reasonable to me
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Re: hard shift, engine light and code p1870

Postby twitchit72 » July 11th 2009, 3:08am

here is the link to sonnax kit. Yhere are differant kits for year models so you have to know which one to get. Also worth noting, it doesn't come with the reamer. That alone is high dollar. This fix is not snake oil! it is the proper way to fix this problem. Most tranny rebuilders incorperate this fix. If you are thinking of doing it yourself, I wouldn't. Id doesn't cost much more for it to be done by a pro. One last word, I do not rebuild trannys. I know all this because I have a good friend who makes his living on building trannys. He did mine in my 98 safari and not only did it fix my van it lowered my operating temp by 30 deg.http://www.sonnax.com/instructions/77754-04K-IN.pdf
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Re: hard shift, engine light and code p1870

Postby Matrixx » July 11th 2009, 6:44am

And that's what it's all about (helping each other out). Great information their for sure twitchit72, Thanks for posting it, and welcome to the Forum.:)
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Re: hard shift, engine light and code p1870

Postby guyonearth » July 11th 2009, 5:01pm

This issue is fairy common with these transmissions, but to say emphatically that it is one or the other cause is premature without acess to the vehicle. Beware shops that want to install parts without doing a proper diagnosis including a running pressure check and reading the computer outputs with the vehicle running. That being said, it probably is a valve body issue. Wear or too much initial tolerance in one of the bores will cause a pressure drop in the circuit which actually results in the pressure in the rest of the system being too high, because the system tries to compensate for it. This is what causes the harsh shifts. The valve bodies are widely available on ebay, among other places, and are a relatively simple bolt on part. I have done two successfully, and the problems were solved. You should also install a cooler and upgrade to Dexron VI or it's equivalant.
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Re: hard shift, engine light and code p1870

Postby danvan [OP] » July 13th 2009, 12:00pm

Thanks for the helpful reply.
Since posting Ive added a tranny cooler and changed 6 qts of the old fluid with new. I noticed a smoother shift right from the beginning with the newer fluid, but have not towed a trailer with it yet for a long enough period.

I guess I will have to have a pro fix it at some point.

Thanks again.
Dan
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