Weird Tach?

[6.5 Detroit Diesel]

Recently I put a different dash cluster in the truck as the tach wasn't reading right. A short time later the alternator went as well. Since I put the new alternator in, the tach has been reading higher. Where it would read 1,750RPMS at 90/kmh before, it is now reading almost 2,000RPM at 80/kmh. Which one is the correct reading? 2,000rpm at 80 sounds really high to me, even with the 4.10's.

[DmaxMaverick]

Your alternator failing at the time you replaced the IP is coincidental. Sometimes it just happens. Since you replaced your alternator, it has the wrong pulley on it. The tach is driven by the alternator, and if the tach is reading too high, the pulley is too small. You'll need to replace the pulley with the Diesel pulley, or recover your old pulley and install it.

[6.5 Detroit Diesel]

Thanks, I forgot all about that as I was following another thread that dealt with something similar. Only problem is, the old one has been returned for a core already. Can I get just the pulley? And what size was it again?

[DmaxMaverick]

I don't remember the size, and I'd hate to guess wrong. The size is around here somewhere. You can get the pulley from the dealer, Napa, etc. Try an application search at the parts counter. gmpartsdirect.com has one listed for $16.32. No part # or size, though.

[6.5 Detroit Diesel]

I was fortunate enough to get the old pulley back. One of the benefits of dealing with a smaller autoparts shop. There is a 1/4" difference between the new and the old. Thanks again

[DmaxMaverick]

Yep, that'll do it. Lucky you.

[JoeX87000031]

ALCON,

I have read a couple of posts in here about alternator replacements and the tach reading high due to the wrong sized pulley being installed with the new alternator. This has occurred to my truck, and I am also reading appx 300+ RPM high. I went to the dealer and they said they don't make the pulley anymore although they gave me a list of dealerships across North America that have a few in stock still. Anyways I went to the Alternater shop here an Anchorage, turns out these pulleys all have numbers stamped into the front of them. The number on mine (an incorrect one) is 964 with an M on the bottom. Could someone who knows they have the right pulley pop thier hood and post the correct number off thiers so I can get the right one from the ACDelco shop here. Also my pulley measureS 2.21", in a previous post, (now closed) they said it should be appx 2.40" Thanks to all and take care fellow diesel owners.

[JoeX87000031]

I went to the alternater shop and this is what they found for me. It is 1.410" on top of the groves. This should be much closer to what size it should have. The bad pulley had the belt tensioner right on the loose edge of its limit. This one is right in the middle. Should work great. I will have my truck back together from some upgrades this weekend, will compare GMSCANTECH RPM reading to dash tach and post again to confirm. As for now, this pulley is a 6 groove serpentine NUMBER 104, also with an M on the opposite side as the number. Still if someone out there knows they have the right pulley and would post its number on here it would be great to know the OEM #. We will find out soon whether this is a suitable substitute. Take care all.

[More Power]

The numbers might mean something, but I don't know. The pulley diameter is the important thing...

A 62mm diameter pulley is the stock 6.5L diameter. In tests I performed a few years ago, I learned that there is a 33-rpm difference for each one millimeter change in dia. On my 6.5, I would have needed a pulley somewhere between 64-66mm to get the engine rpm exactly right-on. :)

Jim

[JoeX87000031]

More Power,

What I got from the technician at the ACDelco shop was that the pulley numbers just identify them, like 6 groove ?? diameter hole, 62 MM OD. I asked him what the M means, He said he didn't know, could be a w for all we know. Searched and searched the internet for an explanation, could find none. Anyways, the 104 M is approximately 2.41 inches, 61.2 MM. Pretty close, might be to small. I will check and see how the DASH RPM to SCANTOOL measurements sync up. I just wanted to help out anyone who might run into this after me, by giving them a number they can look for knowing it will work, rather than digging through ten boxes of pulleys measuring each with a caliper. Take care and thanks for those numbers More Power. Also, why does your truck take a bigger pulley than stock?

Also, on my last post I meant to say 2.410 inches, I think a 1.410 inch pulley might read just a little to high.

[More Power]

More Power. Also, why does your truck take a bigger pulley than stock?

I owe it to the "close enough" accuracy of the GM alternator driven tachometer. For those who are picky, you should be able to get the accuracy to less than a 33-rpm difference between indicated and actual, which would take a highly calibrated eye to discern when seated in the driver's seat anyway. ;)

Jim

[Kenneth]

I know my pulley is 62mm. It is pretty accurate. With 3.73 rear gears, and at 65mph my tach reads ~2050rpm. I do have the taller tires (265/75/R16), without a modified vssb, so my speedometer is off, but by very little at that speed. I calculated it once and the difference between the 245 and the 265 is about a 3% differenence. Anyway, my tach is close enough for me, and by the book it is on.

[More Power]

Changing tire size has no effect on the relationship between the speedo and the tach - unless the VSSB has been modified. :)

Jim

[Kenneth]

Right. The relationship does not change depending on tire size. But, if I wasn't sure I had the right pulley and I knew my speedometer was off it would be hard to find the right pulley without having the vssb modified correctly or knowing the % error of the speedometer, right? The only reason I bring it up is for the accuracy of the speedometer. If the speedometer is not accurate then your tach could be off in relationship with the error of the speedometer. Man, sometimes I confuse myself.

[More Power]

Assuming the VSSB is factory original, and has not been modified, the following chart will apply - no matter the tire size.

6.5TD engine rpm (4L80E automatic transmission) at exactly 65 mph indicated:

3.42 gearing - 1880 rpm
3.73 gearing - 2050 rpm
4.10 gearing - 2250 rpmOf course, if someone has changed the VSSB or attempted to re-program it, you'll need to re-calculate the speed/rpm relationship. :)

Jim