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Discussion Starter #1
I hooked up a VDO tach to my R terminal on my alternator, programmed it and now I have a problem. At idle it jumps all over the place sometimes up to 2000 rpm. It gets even worse if I turn on the head lights, radio ect. I have it grounded to the battery. If I give it just a little throttle It works fine. Do you have any ideas. The alternator is the older delco si 10 I think? And it is a "one wire". Is this my problem?
 

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The alternator tachs operate by picking up electrical pulses from the alternator. When you turn on accessories it changes the pulses some and can cause some inaccuracies. Common problem depending on type of tach installed. I've been told that it helps to move the sensor on the alternator if it is that type of tach. Otherwise check your power source and ground connections and all wiring connections one more time.

Part of your problem is the alternator itself. It doesn't put out steady juice until you spin the engine up some. So at lower rpms that alternator's electrical pulses are ragged, uneven and intermittent. There's a fix for those alternators, consisting of putting in a different diode pack for low rpm power production. I might be able to find the part number and instrucions if needed....
 

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Discussion Starter #3
That sounds like It might be the problem since it works so good off idle. Like 50 to 100 RPMS off idle it works fine.
 

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I am using a VDO electronic 0-4000 RPM Tach on my Delco 10si R terminal, single wire alternator. Mine works great and is stable. You will find that most, if not all, single wire alternators will not put out a good charge when first started at idle. Blip the throttle and you will see the alternator cut in and start a normal charge. Then when it returns to idle it will still charge. Just needs the RPM's up on first stat to get it to kick in. Both of my 10si/r terminal on my trucks operate this way. Its something about the alternator needing to be excited but I forget the exact description.

Paul
 

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Discussion Starter #5
paul you are right on about having to throttle up to get it working but when mine goes back down to idle the needle starts bouncing until i give it 50 to 100 rpm increase then it works great. Should I turn my idle up, idle is at about 750 to 800 RPM now if It idles at 850 it works great is 850 to high of an idle?
 

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paul you are right on about having to throttle up to get it working but when mine goes back down to idle the needle starts bouncing until i give it 50 to 100 rpm increase then it works great. Should I turn my idle up, idle is at about 750 to 800 RPM now if It idles at 850 it works great is 850 to high of an idle?
Flazyj;
You should not have to increase RPM's to get a stable reading. VDO rates that 0 to whatever the top limit of the tach is. I am doing this from memory, you have 12Volts to one terminal on the tach. You have a ground going from a Tach terminal to a GOOD ground and you have the single line going to the alternator. Check all those connections real good, especially the ground. What model VOD are you using ? Does it have Dip Switch Jumpers recessed on the back of the Tach ? Maybe you have the wrong dip switch settings.
And maybe you have a bad Tach Head. Let us know the model and what dip switches are in what position. About 6:30pm I will go down to my shop and get my VDO install sheet. I have my settings marked on it. Mt Tach also has a Adjustment screw on the side of the tach. The adjustment screw is covered when the plastic gauge lock ring is screwed in place. I used a General Radio Stroboscopic tach to calibrate the VDO. I am deadly on with the General Radio
Stroboscopic and the VDO stays accurate.

Paul
 

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Discussion Starter #7
Paul I double checked the dip switch settings and they are right, So I increased the RPMS about 50 to 75 RMPS to about 850 and it works perfect even when I turn on electrical components. My alternator must need to be turning just alittle faster to give a good pulse for the tach? My tach is an older vdo 3/3/8 0 to 3000 RPM with 8 dip switches. I think it is out of a tractor or some kind of equipment.
 

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Paul I double checked the dip switch settings and they are right, So I increased the RPMS about 50 to 75 RMPS to about 850 and it works perfect even when I turn on electrical components. My alternator must need to be turning just alittle faster to give a good pulse for the tach? My tach is an older vdo 3/3/8 0 to 3000 RPM with 8 dip switches. I think it is out of a tractor or some kind of equipment.
Glad to hear your all set. I did neglect to answer your question on idle speed. The data plate on both my CPL 857 engines is stamped Low Idle 800. However, I run my engines at 850 R's. Nothing like resolution to a problem.

Paul
 

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I'll toss in my two cents worth of information. Ten years ago I was the local supplier for building GM 10SI one wire alternators that were being installed on farm tractors. I ran into a problem one time when a fellow installed one on a diesel. I found out about the required speed for the voltage regulator to kick in. From memory it was somewhere around 3200 rpm for the alternator's shaft speed to trigger the regulator. Divide the crankshaft diameter pulley by the diameter of the diameter of the alternator pulley then multiply the result by the rpm and you should end up in the 3000 rpm range. I would venture to say that if you had a test light rigged up as an idiot light circuit the light would flicker every time the tach jumped upward as one side of the regulator was going to ground and possibly shorting the tach to ground causing the deflection on the tach. Also note that initially when a triodiode (the triple connection diode that connects the three stator coil terminals to the one regulator connection) looses one of its legs the same condition can result. Bumping up the rpm's is the fix in your case. I have a tractor that I can start on low idle and the idiot light always stays on until I initially bump up the rpm's to where the regulator kicks on then it is fine for the rest of the day.
 

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Discussion Starter #11
BobS, that was a good explanation of what must have been going on. My idle was just alittle to slow for the alternator. I drove it most of the day today and did not have a problem. Thanks for all you guy's help it is good to have this sight!
 

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