'95 Sierra C1500 Hard Shift, High Idle, Service Engine Light

bugman74 · April 4, 2010

Hi guys,

I've been having this problem with my truck for a while, and just cant seem to solve it. Every once in a while when I fire it up, I get a high idle, plus the "service engine soon" light, and an extremely hard shift. The shift points are even 1000+rpm higher. Just going from park to drive barks the tires (with my foot on the brake). If I shut if off, and re-start a few times it will go away (sometimes takes up to 4 re-starts). It is driving me nuts. Any ideas? I do all the service on my truck, and I can tell you it is otherwise in perfect shape, fresh tuneup, etc.

Truck is a 1995 sierra, 5.7L, 2WD, 4L60 trans. It only 56,000 original miles.

Thanks.

hoorders · April 5, 2010

Hi guys,
I've been having this problem with my truck for a while, and just cant seem to solve it. Every once in a while when I fire it up, I get a high idle, plus the "service engine soon" light, and an extremely hard shift. The shift points are even 1000+rpm higher. Just going from park to drive barks the tires (with my foot on the brake). If I shut if off, and re-start a few times it will go away (sometimes takes up to 4 re-starts). It is driving me nuts. Any ideas? I do all the service on my truck, and I can tell you it is otherwise in perfect shape, fresh tuneup, etc.

Truck is a 1995 sierra, 5.7L, 2WD, 4L60 trans. It only 56,000 original miles.

Thanks.

I used to have a cherokee that had similar symptons. Sometimes really high idle after starting. After searching high and low to what this could be, someone told me to change the throttlt body base gasket. They said that if its sucking air through the base of the throttle body, it would cause this high idle. So I figured for the couple of bucks, I would try changing the base gasket, and low and behold, it was fine after that.

Not sure if this could be your cure as well.

bugman74 · April 7, 2010

I am now wondering if this is the "infamous" P1860 issue relating to TCC valve wear...

Mike G · April 7, 2010

I am now wondering if this is the "infamous" P1860 issue relating to TCC valve wear...

Is that the code you get when the SES lights up?

Badass69 · April 8, 2010

His truck is OBD 1 so there is no codes like that. You can use a paperclip to jumper two pins in the ALDL plug and get the service engine light to flash in code. I would suggest doing that before going on a witch hunt

bugman74 · April 13, 2010

His truck is OBD 1 so there is no codes like that. You can use a paperclip to jumper two pins in the ALDL plug and get the service engine light to flash in code. I would suggest doing that before going on a witch hunt

Ok guys, I read the flashing code today and I get the following:

12

22

42

AFAIK 12 is normal when pin A and B are shorted on the OBD1 plug. Looks like code 22 is throttle position sensor related, and code 42 is electronic spark timing circuit related. I will reset and check again (pull batt cable for a few minutes). I'm thinking code 42 may be due to me disconneting the "secret wire" beneat the glovebox to set the timing during my last tuneup...

I'm not sure any of these is causing the neck-snapping P-D, and 1-2 shift. I know a bad/misadjusted TPS can cause the high idle I'm experiencing, but not sure about the other issues.

In reading TSB# 01-07-30-023A, it looks like 2 consecutive trips with the p1870 are required to set a history code, so maybe mine hasnt met that condition yet and stored the code?

The high-rev and neck snapping shift happened again this morning. After re-starting it was back to normal. This afternoon when shifting into drive it almost felt like the torque converter didnt engage. I have also noticed in the past that there is a slight delay (longer than normal) in lockup for reverse.

I have attached a picture of what the transmission pan looked like during that last ATF change (done recently). Looks like a fair bit of clutch material. I cant feel it slipping though... yet.

So obviously I have some work to do to fix code 22 and 42, but any thoughts on the TCC valve issue?

bugman74 · October 10, 2010

I know it’s been a while, but I thought I would post an update since I have fixed the problem - this might be useful to someone who is experiencing the same issue:

There were actually 2 issues. The first was the TPS. After testing, I found it to be faulty. I replaced it, which helped but didn’t solve the full line pressure P-D and 1-2 neck-snapping shift.

The 'infamous' TCC valve wear issue (P1860) was indeed the second part of the problem. I ended up installing the Transgo SK-4L65 kit, and the Transgo modified separator plate (the check-balls had worn into the OEM one). I kept the OEM accumulator pistons (they were all the aluminum ones - I would have replaced them if they were the plastic versions). While I was in there, I also threw in the 093 "corvette" servo, and a summit aluminum +2qt capacity pan (which I only bought because I wanted a darn drain plug!). Obviously I used new separator plate gaskets, filter, and pan gasket.

The total cost wasn’t terrible. I took my time, so the whole process took me a full day in the garage to get it all done.

The best part is the problem is now 100% fixed (knock on wood).

alpha capo · November 12, 2010

i got a 96 with a 305 with 200k on it and i just had a issue where the engine was racing....i knew it was a vacuum leak. and then about a month or so later it kept getting worse and then all the sudden the tranny was shifting very hard....so i took it in. Misfire on number four cylinder.

the Throttle position sensor(sits on the side of the trottle body) was replaced and that fixed the trannny shifting hard problem. wich is cool becasue i thought the tranny or possible the clutch itself was going bad. but it was wierd because it was fine one day and pretty much undrivable the next as far as the tranny....and i know that trannys go out gradually usually, so i knew something was wierd.

i was hoping the vacuum leak was due to that crappy heater core hose that plugs into the manifold ..but replaced that and still had the problem

the vacuum leak was the upper and lower intake manifold gasket back by the distributor....and that solved the high idle... Lean mixture condition.

now she runs smooth and shifts like buttah.....i did the cap rotor plugs and wires why i was at it...also a new water pump....now all i need is to get a new hose for the power steering because its leaking too.