Does anyone have an idea? Please Help!

[PotentialTahoe]

RPM and MPH gauges are constantly turning off and on, both going to 0mph and 0rpm regardless of speed, This also effects the power of the truck. The truck seems to be very sluggish when accelerating when the gauges go out, but when they are on, the truck drives fine. All the other gauges work fine e.g. fuel, oil, amps. they stay on regardless of the rpm and mph needles going out. Just wondering if anyone would know of what is happening????? Please Help!

 

[mygreen98]

check the ignition switch. could be going bad

 

[PotentialTahoe]

Just curious, how does the ignition have to do with the tach and speedo going out or quitting and the power of the motor? It is intermittent as well. The truck starts and idles great, its just the initial power to the wheels that it is lacking and seems to be losing when the tachs go haywire.

 

[mygreen98]

u said car runs bad as well, posible bad feed 12v + key on, affecting the computer and dash assembly. for both the tach and speedo to blank out, it must be ither a bad ground or loss of 12v. jiggle key when it happens and see if the dash starts to work.

 

[PotentialTahoe]

u said car runs bad as well, posible bad feed 12v + key on, affecting the computer and dash assembly. for both the tach and speedo to blank out, it must be ither a bad ground or loss of 12v. jiggle key when it happens and see if the dash starts to work.

Ok, I tried the jiggling method, nothing, Today I will go out and try and wiggle and tighten the battery connections. Thanks for your help!

 

[95TwinTT]

After spending some time looking at a schematic. I would try checking the connector at the VCM. Look for any kind of evidence of wires being worn, or corroded.

There also appears to be a 23 pin connection somewhere in the wiring harness. Possibly between the engine compartment and the interior. I can't tell for sure from the 4 pages of schematics I'm looking at. Anyway, I would open that connection and inspect the wires and pins if possible. It will be the only 23 or 24 pin connector in the harness.

The wires in question at the 23 pin connector are pin 10 (white wire - tach) and pin 14 (dk green wire - speedo)

The problem would seem to be upstream of the ECM or VCM as it sounds like it is pulling timing when it fails.

Odd things can happen to these vehicles through the years. It could be damage from an accident that was not initially found. Who knows.

In the process of checking the connectors, I would also scan the wring harness that you can for any outside influence, like rubbing on body parts etc.

Good luck..........:skep:

 

[PotentialTahoe]

After spending some time looking at a schematic. I would try checking the connector at the VCM. Look for any kind of evidence of wires being worn, or corroded.

There also appears to be a 23 pin connection somewhere in the wiring harness. Possibly between the engine compartment and the interior. I can't tell for sure from the 4 pages of schematics I'm looking at. Anyway, I would open that connection and inspect the wires and pins if possible. It will be the only 23 or 24 pin connector in the harness.

The wires in question at the 23 pin connector are pin 10 (white wire - tach) and pin 14 (dk green wire - speedo)

The problem would seem to be upstream of the ECM or VCM as it sounds like it is pulling timing when it fails.

Odd things can happen to these vehicles through the years. It could be damage from an accident that was not initially found. Who knows.

In the process of checking the connectors, I would also scan the wring harness that you can for any outside influence, like rubbing on body parts etc.

Good luck..........:skep:

Thanks for the info, I attempted to wiggle the wires pos/neg to the battery. to no avail. Now the speedo is stuck on 100mph and the rpm is stuck at 0rpms at all time. Again all the other gauges are working properly. I agree with you on the wires affecting the timing, because I loose a lot of power when the gauges went out, especially from start to about 20mph. I no longer can just ease along with the brakes, it takes some GASSSS to get it just to 10mph. I am confused on where that 23 pin connector would be, what it looks like etc. etc. Do you have any pics on this deal because I would be looking for a needle in a haystack. Do I need to pull out the dash as well? Thank-you for all the input gentlemen.

 

[95TwinTT]

It will easier to find now that it is no longer intermittent.

It sounds like something has crossed those two wires. The white and dark green wire. It could be happening in the computer, hard to tell. The fact that is affecting performance would point to a problem closer to the computer, if not in it.

Do you know if this truck has been in an accident?

I would also exercise the plugs on the ECM as the very next thing. It is probably behind the glove compartment. It is important to look for any kind of physical damage around the wire harness and plug. If you can remove the computer try shaking it to see if anything it loose or moving around inside.

I am just looking at a block diagram in a Haynes manual. You can go to any library and find specific diagrams that will explain where connectors are located and show more clearly the paths they take through the chassis.

I can't tell for sure from my manual where the drivers are that operate these gauges. I would have to believe that they are in the computer itself.

If I were there, I would scope the white wire and dark green wire at the connector on the computer. The speedo is at the top and the tach is at the bottom would indicate to me that it is seeing the normal voltage spread of it's circuit. If it were going way past 100 mph or the tach were going way below zero, it would point to outside interference. It may be a circuit board problem in the computer.

If you check out the computer and do not find anything wrong, I would get to a shop with a scanner that can check all of your sensors circuits. It might point to where the problem is coming from. It would be able to see what is pulling timing. That could get you to a cure more quickly. :thumbsup:

 

[PotentialTahoe]

ok it is now intermittent again, the speedo and rpm has started working but I still have a significant loss of power at the pedal. If I take the truck and have it scanned, is this going to scan like the OBD type scanner through the computer? Don't they have those scanners at advanced auto zone? Just wondering. Thanks.

 

[95TwinTT]

It is my understanding that there are several levels of “Scannersâ€.

You need to get to a sophisticated scanner that can look at engine details while it is running, like timing, sensor inputs from TPS, MAF, MAP, IAT, etc.

It needs to be in the hands of someone who knows how to use it and what to look for.

I would guess that Auto Zone may just have “today’s genius†holding a scanner that is only capable of reading trouble codes.

The issue you describe does not sound like a “normal†failure. Maybe it is and there is a trouble code waiting to be read, but I doubt that.

Your computer is the OBD-2. It is very complex and needs experience to diagnose as a rule. Simply looking for error codes could be a waste of time.

 

[96ProCompTahoe]

the scanners they have at Autozone aren't going to give you anything but trouble codes. you may get lucky, but your looking for an actual shop with someone who doesn't have to search for 20 minutes to tell you what a trouble code means.

 

[PotentialTahoe]

I certaintly agree with you both, autozone scanners are very limited in what they read. I guess I will have to take it to a mechanic, probably the only mechanic I trust specializes in AC repair, but I'm sure he knows more than just that. Thanks again for the info gentlemen.