95 Tahoe Engine Electrical Issues?

Orion 762 · September 14, 2006

I've got a 95 Tahoe, 5.7 liter, TBI, OBD-I, etc., with 270,000+ miles on the original engine. I've had this truck since 1997 and it's been very reliable with normal parts replacement when needed.

Recently, she started giving symptoms of IAC problems and shifting at weird RPM's. Would crank fine but idle at 1200 RPM, then drop to 800 occasionally, then back up to 1100 RPM, erratic. Also, the "Service Engine" light came on, BUT no OBD codes were "set" each time I brought it in for checking. The "problem" was intermittent . . . sometimes she'd run fine for an hour or two, then start surging again.

What I did was replace the IAC and the MAP sensor. After driving above 40 mph, this new IAC is supposed to "reset", but it never did. After a couple weeks of more erratic idle and poor shifting (hanging on in 1st, then hanging in 2nd, not wanting to go into 4th after it made it to 3rd, etc.), I got another IAC solenoid.

After installing this new one (it also never reset correctly), I noticed that my Gas Gage was erratic. When accelerating, the fuel gage would go to Empty, then back to the correct level after I was maintaining speed. This happened even going from 60 mph up to 70 mph with a slow acceleration. ANY acceleration caused the gage to move fast to "Empty". Once I levelled out on the gas to maintain speed, it would go back to working OK.

Then last week the Gas Gage went all the way to FULL ++, and has stayed there ever since. (Have only about 1/2 tank of gas for sure, so I'm certain the Fuel Gage is "broken".)

Now here's where things get interesting! The erratic Idle and Shifting problems come-and-go . . . occasionally she will smooth out and run fine for a few minutes at at time. I've noticed that when the engine is erratic (as above), the Battery Gage shows charging at near 16-17 volts. (It's normally around 14 volts.) When everything is smooth and running ok, the Battery goes back to the normal 14 volts.

Anyway, this seems like a multi-symptom and multi-problem issue, and I'm not sure where to stab first. Possibly related is that the fuel pump has never been replaced on the truck, though the fuel filter has had regular replacement.

(I'm a little shy to take this to our Chevy Dealer service center, as last time they told me I needed a new $2K transmission - after doing all their computer checking, even driving around with a "diagnostic computer" on the seat. That time, 3 years ago, turned out it was the catalytic convertor and NOT the transmission at all!! The Dealership folks never figured this out, even with all their high tech gear.)

Hints or guidance will be appreciated.

Regards,

Orion 762

1999 Tahoe 4x4 · September 22, 2006

I'm not sure it this helps, but check your ignition switch. The reason that I'm suggesting is because this one controlls the transmission DC and so the shift as well. I had to just replace mine. My symptoms were basically a dash that light up like a christmas tree. AC worked intermittently and 4wd, ABS lights came on. THERE ARE SEVERAL CONTROL CORCUITS in the swithc that the switch operates. So other symptoms may arise in other circuits. The switch is about $200 as the GM dealer. You will need a deep socket E4 and E5 torx. Upon changing the switch I opened it up and checked the contacts. Everything burned, I was suprised everything else worked. Hope this helps.

Orion 762 · September 23, 2006

1999 Tahoe 4x4,

Thanks for the advice, and you may be on to something. My wife mentioned that occasionally the "key" comes out before the switch is fully turned to "off" position, leaving various accessories still ON. Years of cranking have it worn a bit. Now to find the "deep socket torx" setup!

Orion 762

Mr. Smith · September 23, 2006

1999 Tahoe 4x4,

Thanks for the advice, and you may be on to something. My wife mentioned that occasionally the "key" comes out before the switch is fully turned to "off" position, leaving various accessories still ON. Years of cranking have it worn a bit. Now to find the "deep socket torx" setup!

Orion 762

<{POST_SNAPBACK}>

Remember the ignition switch and the lock cylinder are two seperate things.

1999 Tahoe 4x4 · September 23, 2006

That is right! So IF the switch is bad, you will need the electrical component part of the ignition switch. So not the lock and cylinder. If your car has a passlock teft deterrent system don't worry about it. You won't have to reset it or anything for this job. The Torx tools that you need are DEEP E4 and E5 sockets. (NOT T4 and NOT T5 that most people have). The E-Series Torx refers to a female torx sockets. Snap-On or has them of Course and (I could not find them at e.g. Sears, Craftsman) You will not be able to use anything else than the right quality tool for this job because there is no place and you can't get to the bolt with anything else (this is more true for the E4 than the E5 bolt) IF the switch is bad or not, it is a good idea to change the switch no matter what, even as a precaution. Like said before it is too important and it controls too many circuits. It has about five contacts inside and all five were worn out and burned on mine.

Another idea for electrical problems as you described would be to check how the alarm was installed. In my car some smart guy switched the alarm system in row with the ignition switch. He cut the harness and inserted the alarm. So in other words all the circuits form the ignition switch have to go through the alarm first and only then the current will be able to go to the circuits. So if this cheep aftermarket the alarm has some problems it is also questionable how that affects function of all other circuits.

Finally you can check for a voltage drop for before and after the ignition switch. I have a picture of the diagramm, I just have to figure it out how to post it.

Orion 762 · September 25, 2006

1999 Tahoe 4x4 . . . I just sent a PM with my email address.

Mr. Smith . . . thanks for the reminder on the two parts. Looks like I might need both. I've got the key-lock cylinder and ignition switch apart - still cranks OK using a screwdriver!

Next challenge is to look for shorts/opens along the cabling. After 270,000 miles, I guess these could be anywhere.

Other updates: I installed a new fuel filter (just in case), and figured out why the fuel gage went to max. Short story from a long one . . . we have a Daschund dog that really "hates" mice. Well, a "mouse" apparently got under my Tahoe - and up into the under-carriage. My dog went after him, and most anything else he could bite/tear away trying to get the mouse. He got one brake line, most of the cabling for the SIR systems running under the engine AND he mostly bit-thru the cable bundle running to my gas tank. Chewed 2 cables in two, and almost got the 8 gage "hot-wire". Fixed these and the gas gage works ok now.

Not worried about the SIR/airbag setup, as I've had this disable quite a while. After getting the "bomb" off the steering wheel this weekned, I think we'll have a "fireworks show" in the backyard to render it "permanently safe". Thinking of backing off a hundred feet or so, and direct wiring it to a battery. Anyone with better hints on how to do this? I'd rather not have that disabled "bomb" in front of my face on the steering wheel, but would like to save the "parts" so my horn works right.

More later . . . Orion 762

1999 Tahoe 4x4 · September 25, 2006

I have received your e-mail address and sent you the diagramm. What you have to do is to probe the switch and the wiring diagram is the key for that. You will HAVE TO HAVE 12V before the switch AND after the switch. Any voltage drop can be caused by burned contacts. Normally in this case heat would develop and the switch would melt. HOWEVER in this case the switch is only switching control circuits and relais. Any voltage drop and relais don't operate and no current flows, so the switch does not get hot and does not burn. In my case after changing the switch I opened it up and inspected it. Almost all circuit-contacts in my switch were burned.

Opening the ignition switch is a one way operation by the way. This ignition switch unit itself seems to be not serviceable. Once opened it is nearly impossible to reassamble. So it can can be changed only.

Be careful how you take out the passlock switch. It is a liitle white plastic switch in the cylinder. IT HAS TO BE TURNED to take out and to put back in. I know it sounds too simple to understand. You'll see as you work you if perhaps won't be able to figure it out. Then you'll just will have to remember the turn trick and it will be easy.

BTW: GENERALLY I don't believe in shorts in wireing harnesses without the car being in any accident or under water or sound system installed etc. If you have never had a burned smell in your car than I BELIEVE chances for a short are very slim. I more belive in some component failiure.

Orion 762 · October 13, 2006

1999 Tahoe 4x4,

Thanks for you help on this, and the schematics came in handy. It turned out NOT to be the electrical switch, and I'm glad I did not "open it up" just to see . . . as that part costs about $250 !! I did replace the key cylinder though.

On to the Problem! After all the "shouting" was done, it turned out to be the TPS. Got a new one from the Chevrolet dealer (AC/Delco), and all the problems disappeared. The old TPS apparently had an intermittent "open" in the near idle position. When the OBD detected a "weird" TPS condition, I guess it just set the idle "somewhere close", which was about 1000 rpm.

The diagnosis problem was compounded by the new IAC I installed. The 1st new IAC came from autozone (made in Mexico) and the instructions said to simply "run vehicle above 40 mph for 3 minutes to reset", etc. When I got a different IAC (this one from AC/Delco made in Canada), the reset instructions included:

a. turn key on (but don't crank) for 30 seconds, then

b. turn key off for 30 seconds, then

c. crank and let engine idle for 5 minutes

During "part c", the OBD apparently adjusts the IAC pintle position to get the 500-600 RPM curb idle.

So with the autozone "instructions", the IAC was never going to get reset. Then with the bad TPS, the OBD never even tried to reset the IAC in the 1st place.

With the new TPS, and following the AC/Delco instructions for the IAC, everything is now "reset" and idling just fine.

Strange how all this "parts" can interact in weird ways!

Lessons learned . . . (1) defang the dog, and (2) my "airbag disposal exercise" has "convinced" by kids to always wear their seatbelts!

Thanks again for you help with this.

Orion 762