Transmission Overdrive

[DayRadebaugh]

Have a '99 2500 Suburban diesel with 66K on the clock. I don't drive it often, but had it out today and noticed that engine was at about 3500 RPM for about 65 mph or less. This didn't seem right. I was expecting a shift into overdrive, and looking for <2500 RPM for 65 mph.

Any thoughts?

Thanks

[DmaxMaverick]

It could be any number of things (or combination of things), not to exclude the need for overhaul. What diff. gear ratio, and tire size? Is the torque converter locking? Is the SES lamp on? What emission state (Federal or specific state)? What engine code (8th digit of VIN)? What does the fluid look and smell like?

Even with 4.10 gears and go-cart tires, 65 MPH @ 2500 RPMs still seems high in OD. 3500 RPMs is against the governor. Maybe your torque converter hasn't been locking for a while? If that's the case, serious overheating isn't far off under highway load.

[Robyn]

Sniff the tranny oil.

Should be red, clean and no burnt smell
Check the rig for DTC's (Codes)
Any SES light on ?????
A pending code can stop the 3-4 shift....

There is an actuator valve in the valve body that wears out.....poof.NO 4th gear.
Replace the VB or has a kit put in the VB that comes with a new valve.

There should be some codes showing....RATIO ERROR CODES

This may simply be a failed B solenoid as well.

Gotta get any codes and then go from there.

Keep us in the loop.

Good luck


A pending code can stop the 4th gear shift

[JohnC]

Does it shift at all? A fault in the ignition switch can disable the trans solenoids and it defaults to second gear.

[DayRadebaugh]

Keep us in the loop.

Good luck

Thanks very much. Pretty clearly there is a problem here, and I will check it out and post the results.

[ronniejoe]

Sometimes the PCM gets confused. If that has happened, simply shutting the engine down and leaving power off for a couple of minutes will reset everything. Upon restart, the problem may be gone. If there is a genuine fault, the SES (or MIL) should be illuminated and a code should be stored. Limp mode will exclude fourth gear and prevent TCC lockup in most cases.

[DayRadebaugh]

Took the Suburban to the transmission shop. Their diagnosis was that the transmission was fine, and that the tach was registering too high. I noticed the tach bounced around occasionally. If true, any suggestions about how to fix tach? I understand it gets its signal from the alternator.

Thanks

[ronniejoe]

Have you recently replaced the alternator? Have you checked engine speed with a Tech 2?

[DayRadebaugh]

Have you recently replaced the alternator? Have you checked engine speed with a Tech 2?

Have not replaced alternator. Will check to see whether shop checked the tach.

[ronniejoe]

The Tech 2 is the GM scan tool. Third party tools are also available. The scan tool reads engine speed off of the reluctor wheel on the front of the crank shaft. The tachometer in the dash reads speed from the alternator. The reason I asked if the alternator had been changed is if it had, then the pulley size might be wrong. If you haven't changed it, there is not much that can cause the tach signal to suddenly change, unless you have a massive voltage problem, then I would expect it to read low. The scan tool (Tech 2) can verify engine speed in comparison to the dash tachometer.

[DayRadebaugh]

Verified with transmission shop that a) trans was shifting correctly and b) tach running high on scan tool. Problem appears to be in either tach or alternator or both. Any recommendations for further diagnosis?
Thanks

[DmaxMaverick]

If the tach is operating linear and tracks engine speed (despite the accuracy), the alternator pulley is likely the wrong diameter, and not a tach issue. Simply replace the pulley with the correct one. If the tach is indicating too fast, the pulley is too small. You need one larger, so measure and make sure it is before swapping. I don't know if they are still available new, but you can certainly find one at a recycler.

[DayRadebaugh]

I haven't changed the pulley, however, and have never noticed this problem before. I believe I read somewhere that the tach "senses pulses" from a post in the alternator. It would seem possible that this piece of electronics, essentially a counter, has gone haywire. Make any sense?

[DmaxMaverick]

If it's the same pulley, before and after, perhaps. Usually when the alternator gives up, the take either quits or goes wonky for a while. If the tach is linear and tracking engine speed, it's probably dropped a resistor somewhere, perhaps the instrument panel. It was fairly common on previous models (87 and earlier), but I've not seen it on the GMT-400 trucks.

[DayRadebaugh]

If it's the same pulley, before and after, perhaps. Usually when the alternator gives up, the take either quits or goes wonky for a while. If the tach is linear and tracking engine speed, it's probably dropped a resistor somewhere, perhaps the instrument panel. It was fairly common on previous models (87 and earlier), but I've not seen it on the GMT-400 trucks.

This makes sense. The tach bounces around occasionally, and is definitely not linear. The question is, how to diagnose whether it's the alternator or tach. I suppose replacing the tach would be really expensive, involving maybe the whole gauge cluster. Any thoughts?

[DmaxMaverick]

Yes, if the tach has issues, you repair/replace the entire IPC. Your description sounds like a failing alternator. If/when you replace it, measure the pulley before/after. Parts suppliers get it wrong often enough to be an issue. If you get the wrong size, just swap them, and keep the original. You'll need a 1/2" impact driver or a solid wrench and a BFH to R/R.

[DayRadebaugh]

Yes, if the tach has issues, you repair/replace the entire IPC. Your description sounds like a failing alternator. If/when you replace it, measure the pulley before/after. Parts suppliers get it wrong often enough to be an issue. If you get the wrong size, just swap them, and keep the original. You'll need a 1/2" impact driver or a solid wrench and a BFH to R/R.

Thanks. I guess I will go for the alternator. The downside here is replacing one that doesn't need replacing, but probably cheaper than an IPC.

[DmaxMaverick]

An IPC repair is usually a flat rate hundred bucks. Some offer exchange to save time, but then you run into odo and pairing issues. The same applies to recycler IPC's. There is no new, that I'm aware of, so buying one outright will be a reman, likely a lot more than a hundred, and may require dealer programming. 6 one, half dozen the other, as far as function goes. It works, or it doesn't. It's probably just a failing alternator, so hopefully a lot of wasted talk.

[DayRadebaugh]

I'm curious to know who would repair an IPC for $100. Is that the dealer rate?

[DmaxMaverick]

eBay, of all places, as well as other online outlets. The sellers I've dealt with, and know of, are shops and individuals who advertise there. There are also "kits" for known issue repairs, such as resistors for older gages. It cost me many times the few bucks they offer, to have my 85 fuel gage repaired, for the same remedy. I've not heard of any bad experiences, although I'm sure there are some. I don't know what the dealer gets lately, but they were very spendy at last check years ago for remans.

[a5150nut]

Maybe take your Alternator to a re-builder and have it checked? That's if there are still any around.

[arveetek]

Check with this guy for repairing your instrument cluster:

https://drspeedometer.com/

Casey

[DayRadebaugh]

Yes, if the tach has issues, you repair/replace the entire IPC. Your description sounds like a failing alternator. If/when you replace it, measure the pulley before/after. Parts suppliers get it wrong often enough to be an issue. If you get the wrong size, just swap them, and keep the original. You'll need a 1/2" impact driver or a solid wrench and a BFH to R/R.
I had the IPC overhauled but the problem remains. Since the tranny is OK, the tach has been overhauled, this seems to leave the alternator. Is it possible to replace the sensor in the alternator that generates the signal for the tach, or do I need to put a new alternator in there?

Thanks

[More Power]

Alternators aren't that expensive. I'd replace it... But, before you do, I'd remove the original alternator's pulley, and replace the one on the new alternator. These pulleys are specific to the 6.5L diesel - for tach accuracy. The 6.5L diesel alternator pulley should measure 67mm peak to peak when measuring across the pulley's peaks and valleys.

The last alternator I replaced had a 59mm pulley... That produced about a 500-rpm error. What seems to happen is that the original bad alternator on a 6.5 would be traded in for an auto parts store replacement, not knowing the pulley was unique. Since more of these alternators (and smaller pulleys) were used for a gas engine application, that's what you'll likely get as a replacement.

[DayRadebaugh]

Thank you very much. I agree with all that you've said. I also concluded, based on a few Internet searches, that that little pulse generator gizmo in the alternator cannot be replaced separately, so I'll just R & R the whole unit. We'll pay particular attention to the pulley.

Will let you know.

[a5150nut]

If you can find a local re-builder for your original alternator you'll have a much better end product and the correct pulley.

[DayRadebaugh]

Replaced alternator, I'm told problem is fixed. Have a new IPC and alternator now.

I very much appreciate the advice. This '99 Suburban w/6.5 TD has 66K on the clock, and I intend to keep it forever. You've helped me toward this goal.

:)