Voltage bounce, high oil pressure

[jnbqm9]

Hey guys,

I picked up my "new" '99 Tahoe tonight and as I was driving home I noticed something kinda weird. While on the highway, the oil pressure was really high (~80 psi, max on gauge). Then when I pulled off and came to a stop, it dropped to about 50 or 60 psi and the voltage starting jumping between 14 and 15 volts. While accelerating (no matter how fast or slow) the voltage steadies out at 14 volts and the oil goes back up to 80 psi. I pulled it into my garage and put it in neutral and revved it up, but the lights didn't get brighter or anything, so I'm not sure if it's the alternator.

The guy I bought it from is a senior account manager at a local shop who rebuilds engines and transmissions (www.dahmerpower.com). He is also a drag racer on the weekends (with one of my friends), so I trust that he's taken good care of it (the battery died two days before I bought it and he went out and got a brand new Interstate battery!). He did buy it with a blown engine about a year ago and got a rebuilt engine from his company. Is it possible he put a high pressure oil pump on? Would this affect the voltage at idle? The oil was also changed today, although I'm not sure if that matters. Let me know what you guys think or if you've seen this before. Thanks!

 

[95TwinTT]

The first thing I would do is hook up a volt meter to make sure the dash gauge is accurate. An easy way to do that would be to get a cigarette lighter plug and use it to hook up the volt meter. You can't do this kind of test under the hood very easily.

There is a possiblity that a connection problem exsits in the dash, or between the gauges and the engine. You will know instantly if the volt meter reads different than the dash. If that's the case, your oil pressure gauge is probably sharing the same ground and or problem.

Almost all of the gauges in the dash are just volt meters, with different dampers on them.

 

[jnbqm9]

Yeah, I was planning on doing that this weekend. Unfortunately, I don't own a voltmeter, but I've been meaning to pick one up anyways. I'm guessing it will end up being more of an alternator problem than gauge problem. Thanks for the help!

 

[jnbqm9]

I just took the tahoe out for lunch and noticed that the voltage only oscillates when the turn signal is on, not at any other time as I previously stated. I also spoke with the previous owner and he said that he noticed this as well. He said he had the alternator checked and nothing is wrong with it and he and his friends suspect it is a problem with the gauge in the dash. I'm going to check it with a voltmeter to make sure but I may end up getting a new guage. He also said that his remanufacturing shop does use high volume oil pumps, so it is okay that it reads high. Has anyone replaced an instrument gauge? It looks like you were right 95TwinTT!!!

 

[95TwinTT]

It has been my experience that those problems are simply connections that need to be cleaned up.

I would guess that the back of your dash is the vinyl sheet that looks like a clear flexible circuit board. Often the clip in connectors just need to be taken out and re-installed. I use a new pencil eraser to clean up the contacts.

Don't buy any parts until you have a chance to get into it and look around. I ran into one of those problems once that was cured by adding another ground wire. Check the connector that hooks to the vinyl sheet for hot spots.

DC electricity shares current equally on both the hot wire and the ground, so the grounds are very important.

Good luck.........

 

[jnbqm9]

I plan on taking a look at it this weekend. Have you ever replaced the lights behind the dash? Actually, I don't think I need to change the lights. Here's my story: I've noticed my daytime running lights indicator doesn't show up, nor does my parking brake (not sure about anything else). This is probably specific to these options right, not a simple light behind the gauges?

 

[GMCYukalade]

the lights are very easy to change, and the cluster is equally as easy to get to, there aren't any bolts untill you get to the cluster, and then its like 4 7mm.. very easy.. dash snaps on and off and, then you have the headlight switch harness, and the push buttons on the right side of steering wheel, lower gear shifter all the way (put e-brake on) and lower steering wheel all the way, for ease of taking it out

 

[jnbqm9]

Thanks, GMC. I've also noticed when behind another car that my drl's are the same as my regular head lights (aka when I turn the head lights on, the parking lights are the only new thing showing). This is normal right? Or is my drl indicator not showing up because someone wired the reg head lights to show up all the time? Regardless the parking brake indicator is not lighting up. Also, two of the 4wd buttons aren't illuminated. Have you ever seen whether these are easy to change or are they built into the buttons? I just picked up my tahoe and wished I would have test drove it at night!!! The passenger under-dash light is out, 3 out of the 4 door lamps are out, the driver's door power window switches don't light up and one of the license plate lights is out. Oh, the joys of buying a 10 year old vehicle...

 

[GMCYukalade]

the 4wd lights that switch is prone to going out... it just has to be replaced (the lights i do belive are LEDs so they aren't just going to go out)-and my door lights are out as well, never really payed attention to the window roll up and down lights.. - the DRLs are the low-beam lights so thats not a problem either

 

[jnbqm9]

Thanks for all the help. I spent all day today replacing various lights in the Hoe. The door lights are easy. FYI: you don't need to pop the whole panel out to get to them. The plastic white and red light portion will pop out and the bulb just pulls out of it. I've been looking around on the net and after looking at the window/lock controls, I believe you have to replace the buttons and all, not just the lights. Just like the 4wd buttons, they're a black box that just has a harness going into it. Now for the dash: the DRL and emergency brake indicator lights are very easy to replace. What's interesting is the DRL is a one piece bulb and plastic locking piece (that locks in into the back of the instrument panel), but for the emergency light, the bulb actually pulls out of the plastic piece and you just replace the bulb. Kinda weird that they're different.

I'm still not sure what's going on with my voltmeter. I've tested the alternator and battery. I think the panel is just starting to go bad. I'm going to go with TwinTT and start checking the ground. Any advice on the best way to do this? Where is the instrument panel grounded?

 

[95TwinTT]

I'm not certain about the lay out of your particular model, but I suspect that there is a flexible circuit board laid across the plastic shell that makes up the back of the intrument cluster. It would be in that area I would expect to find some gremlins.

Your comment about the turn signal having an influence over the voltage gauge is very significant. They used to have a cinch connector that slipped over the edge of the circuit board. I would look there first. I don't have a schematic in front of me, but you can usually identify the ground because of wire size. They try to gang them together. Often the perimeter of the circuit board will be ground, just for ease of connecting the patterns.

I will bet if you follow the copper trace on the circuit boarcd you will find the turn signal lamp and the volt meter share the same ground.

It is probably just a connection and will not require new parts. Worst case senario would be having to add a external ground. Go for cleaning the connections first. The circuit boards can be soldered to, but it's tricky. You must use a soldering iron that is just the right temp. Some of the little butane soldering irons get too hot and will do more damage than good. Good luck..........:cool2:

 

[GMCYukalade]

i pulled my guage cluster out again tonight, and found the ground - if you look at the connector in the dash, it is clip in out type, pull it out, and look on the right bottom, 2nd wire over from right bottom, it is black with a white line - that is the ground.. i attached a new ground to mine, and ran it down grounded it to a bolt, and to the ground the radio runs from.. that didn't fix any of my probs tho.. o well, i know that it is grounded good

 

[95TwinTT]

Was there a vinyl circuit sheet with the copper traces? And if so, could you follow the lines to see if there was a common connection to the turn signal. I would have been looking for cracks or separations of copper from the base. Or any indication of corrosion or heat.

It alomst has to be in that area to have an affect on the volt meter. I have schematics in another computer around here somewhere. I will see if I can see something in the schematics.

 

[jnbqm9]

Yeah, it was a vinyl sheet with a bunch of lights plugged in the back and a bunch of flat copper lines running to the connection point with the car. I didn't look to see if it shared anything with the turn signal, I'll have to do that. If you can find those schematics that would be great.

Maybe I'll just buy a new instrument panel??? I really like the way that escalade one looks on Yukalade's ride...

 

[95TwinTT]

There is a common circuit in your dash that ties the turn signal lamps through the "check gauges lamp driver" to the oil pressure, fuel and volt gauges.

I need to see if I can get a schematic that goes a little deeper. It could be something getting tired in either the oil pressure guage or the check gauges lamp driver. There does not appear to be anything lethal going on here, just some minor things that can be irritating.

I'm looking at a borrowed schematic, and have not had a chance to get my own computer together yet.

I'm busy at work right now, so it may take me a few days. Sorry for the delay.

 

[jnbqm9]

Dude, you shouldn't be apologizing for free help...:thumbsup:
No rush. I appreciate all the help you've given me so far. Without your input, I would have no idea where to begin. Also, there's only so much you can do without actually taking the dash apart and looking. So, if you want to go ahead and schedule a flight, that would be great. Ha!

 

[95TwinTT]

I have been studying the schematic and I keep coming back to the ground. Did you look close at the cinch connector that slips over the edge of the vinyl sheet? I would be looking for any discolored pins or any indication that the traces in the vinyl itself, might have been hot.

Of the three guages sharing the ground with the turn signal lamps, the volt meter is the only one that would react directly to resistance in the ground. It is also possible for the wire connecton inside of the cinch connector can be soft.

I ended up soldering wire directly to the copper trace on one of those vinyl circuit boards. It is tricky because there is no heat sink. I scraped the copper clean and then tinned the trace on the vinyl, then tinned the wire then put them together, so the time spent on the actual solder joint was quick.

That was because the ground pin on the cinch connector was burned. No way to overcome it's resistance without going around it.

Anyway, it's just a thought. You probably have better things to do than tear your dash apart.

 

[jnbqm9]

Ha! I wish I did. I'm putting in a new stereo head unit this weekend anyways so while I got it torn apart, I may as well take a look at it. I'm going to be wiring an iPod connector and extra cig lighter into my center console as well, so I really will have the entire thing taken apart. Thanks again for the help and I'll let you know what I find.