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1995 Chevy 6.5 Diesel tach problem - need help and advice

dbrannon79

dbrannon79

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Hello all, It's been a while since I've done anything in the forums and my first post! I am finding myself in need some help with my factory instrument panel tach. Not sure if this is the right are to post this but here goes...

I have a 1995 chevy 6.5l diesel c3500 truck that is all stock and my tach gauge is reading about 500 to 600 rpm too high. I verified this with a photo tach on the crank and with a snapon scanner (OBD1) the engine at idle showing on the dash tach is about 1200 when the photo tach and the scanner both are showing 580 to 600. cruising at 70mph the dash tach shows 3100 and the scanner shows 2450 or so. The dash tach seems to be consistent with no wild reading as if it's just off reading high.

since the tach gets the signal from the alternator I decided to replace the alternator thinking that a previous owner may have replaced it with the wrong one. this did not change anything. Is there a calibration that can be done on the gauge cluster or anything else I can check to help solve this?

Thanks in advance
 
Here is a photo of my pulley on the old alternator, the new one is the same. it measures 2 3/8" edge to edge. guessing about 2 1/8" where the belt rides.

Alternator.jpg
 
Just searching online I found a post in another forum showing the OEM pulley should be 29mm (1.142") wide x 65mm (2.559") OD pulley which is larger than what I have. but also calculating the difference in the shaft rotation speed between what I have and this larger one, the RPM would only change from 1200 at idle to about 985. this would still make the tach high but closer to what it should be. is there any other things to check? I do plan to call the Chevy house tomorrow (Monday) and see what they show and if available on the pulley.

edit: one thing I did notice that should be a dead giveaway for me is when I installed the new alternator, I noticed the springed belt tensioner was maxed out to the stopping point when the belt is tight. it was like this as well before I pulled the belt on the old alternator. I have a spare belt I plan to install but can the belt routing possibly cause an issue with the tach reading from the alternator? does anyone have a pic of the belt routing sticker under the hood to verify it wasn't installed correctly by the previous owner?
 
Last edited:
no nothing weird that I know of. the truck has had this tach rpm issue since I bought it in Feburary. the previous owner thought there was a transmission issue with excessive rpm on the highway (sold it to me real cheap) I've been toying with it ever since. just recently finding out theres nothing wrong with the transmission, its the tach that is reading high! the funny thing is that when I bought it, nothing worked, glow plugs, dome lights, A/C, radio, ect... replaced several missing fuses and now everything works perfict including the A/C blowing ice sickles! the previous owner never messed with any if it.
 
Not doing his maintenance means you get the pay off for fixing it!
your belt routing is probably right. With the smaller pulley what happens is people go to a smaller belt size usually because the old belt is stretched and when the do the new alternator with wrong pulley, they figure the belt is worn out also. Then a new smaller belt to fit. When you go back to the right size pulley you have to go back to the 101.5.
 
Well, called the Chevy house today and they couldn't tell me the dimensions of the pulley but did give me the part number for the alternator. I contacted a local starter and alternator rebuild shop and gave them the info along with the pulley from the old alternator. they contacted me later and told me that the OEM pulley calls for a 2 5/8" pulley and mine was too small! so needless to say a new pulley is on order. I will report back once I get it in and installed.

Thanks again with all the help :)
 
I just went through this same situation.

I'm am not suggesting to buy it from here and it looks like you have on the way, but all the dimensions are listed. The measurements are the diameter where the belt rides. These alternators typically come with a 57mm pulley. The pulley required is 62mm.

Amazon.com: New Pulley, Compatible with 6-Grooves, 0.67" / 17mm ID, 2.5" / 63.5mm OD, Delco / 10477425/24-1279, 24-1279-2, 24-1279-5/206-12012: Automotive

Buy New Pulley, Compatible with 6-Grooves, 0.67" / 17mm ID, 2.5" / 63.5mm OD, Delco / 10477425/24-1279, 24-1279-2, 24-1279-5/206-12012: Pulleys - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
 
As an Amazon Associate we earn from qualifying purchases.
Since I started researching this problem. I found that all Alternators can output the tach signal from connecting a wire to one of the field terminals before the internal regulator. I’m guessing it all depends on the speed the alternator is turning for us to get the right signal to the tach.
 
Just remember, when you change the pulley to a larger diameter, the amp generated will be lower due to lower rpm.

With the stock alternator, the truck will not get enough amp.
So, as the alternator becomes weaker, the accessories in the truck like AC will not be as cold.
I hope you live in the cooler part of Texas.

Just a warning. There is a price to pay for getting the correct rpm.
 
Here's the update. I installed the new pulley which is 62mm diameter compared to the 56mm pulley ( I measured them with a micrometer this time) and now were moving in the right direction. the tach changed from 1200 at idle to about 950 -1000.
I also noticed something which I didn't pay attention before is when I turn the key on without starting the engine, the tach sits at about 200. I'm guessing there might be some way to zero it out by pulling the cluster. haven't looked yet but I remember back in the day some gauges had a tiny screw terminal in the back to zero adjust it.

the only other thing I can do is go slightly larger on the pulley size. I found a listing for a 75.5 mm pulley that's listed to fit the cs-130 style alternator this truck has but that's almost 3 inches. might be too big and do exactly what JMJNet mentioned. lowering the amperage and creating another problem.

Going from a almost 2 1/8" pulley to a almost 2 1/4" changed the tach by 200 rpm. I think a 2 1/2" will get me to 800 rpm. Then maybe if there is a way to zero the gauge all will be perfect, but that's just a theory :D
 
Will L. said:
Ya might get a hand held tach and get actual reading so you know when it’s right.
Click to expand...
Already done that! handheld tach and SnapOn scanner both showing 600 rpm at idle. watching the scanner while driving shows the dash tach is about 600 rpm higher consistently, thinking there is ether a calibration issue or the previous owner replaced parts that weren't like for like. I doubt the cluster has ever been changed but I'm actually concidering scowering the local junk yard for one just to see if it reads different.
 
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