Trying to help out a club member who has a 3800SC stick swap. A scan of his PCM showed that the PCM is not getting the correct input from the VSS. Speed input is off by a factor of about 7. We attribute this to the fact that he is using the Fiero 4000PPM VSS unit while the PCM is expecting a 28,000 PPM signal from the 4T65eHD trans. This could be solved by reprogramming the PCM but the guy doesn't have any tuning software. If we use ours, there is a $99 PCM licensing cost ( HP Tuners) just to make this one simple change.
I suggested that he locate a signal converter and splice it in to the Yellow (VSS) wire at pin 64 at the blue PCM connector. However, I'm not sure that Dakota digital or anyone else make a 4000 to 28,000 PPM signal converter box. Any of you guys with a stick 3800SC swap want to suggest a solution how we can achieve the right VSS signal input??

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

I @$$ume he's using a 282.
It's an enormous PITA, but you could tear the transmission apart and swap the speedo gear for the later reluctor wheel, then install the magnetic VSS instead of the gear driven VSS, change the harness connector over and have it all work together seamlessly.

Other than that, I don't see an easy way to do it. There are easy ways to filter the higher frequency signal of the newer reluctor down for the Fiero instrument panel to use, but not the other way, that I know of, short of an active convertor box.

[This message has been edited by Will (edited 12-28-2008).]

Check http://www.dakotadigital.co...mode=prod/prd126.htm

Get a DRAC from an S10, Astro van or C/K truck. It will take the VSS in and let you select the divisors to drive the speedo via dip switches.

quote Originally posted by TK: Get a DRAC from an S10, Astro van or C/K truck. It will take the VSS in and let you select the divisors to drive the speedo via dip switches.

The cars speedometer works fine as he's using the stock Fiero VSS. The issue is that the PCM is not being fed the right input that indicates true engine speed. I believe that the GTP 3800SC powertrain puts out a 28,000 PPM signal so on my auto swap we just use the GTP VSS in the 4T65eHD and send it to the PCM. However, for a Fiero stick swap reprogramming the PCM might end up being the only alternative.

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

The fee should be $50 for the 2 GM credits needed to do his car with HP Tuners. If your going to buy the credits, might as well do some tuning...

quote Originally posted by Dracor: The fee should be $50 for the 2 GM credits needed to do his car with HP Tuners. If your going to buy the credits, might as well do some tuning...

If its only $50.00 then it probably pays to try reprogramming for the right inputs. I'll look at the HP Program tonight to reconfirm the price. .

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

Or, just don't feed the PCM any info. I never had a VSS signal going to my PCM when I had an a four speed in mine, with no ill effect. Ran 12.5x with a 3.0", 1.9 rockers, and some cleaned up stock manifolds. Can ran and drove fine, only problem I ever had was the engine sometimes wanted to stall coming to a stop, but I believe that is common on manual transmission swaps. I bumped up the idle speed with the throttle plate(over riding the IAC), and never had a problem again.

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11.425 @115.60
Best 60' 1.543 seconds
3800 S/C 4T65E

Adding in the VSS is easy to do, just change the ppm to 4000.

You will gain stall protection from it, as it keeps the desired idle very high most the time your moving.

You should be splicing it to the PCM and the speedo, so you wont need to convert anything.

quote Originally posted by darkhorizon: Adding in the VSS is easy to do, just change the ppm to 4000. You will gain stall protection from it, as it keeps the desired idle very high most the time your moving. You should be splicing it to the PCM and the speedo, so you wont need to convert anything.

I have to check the guys wiring harness over. He used a Dansflyer harness on his swap which appeared to be well made. The VSS might very well be already wired in but I'll ask Dan if he does them this way. I believe that the VSS may be wired in as the scans show vehicle speed of like 10 MPH while the car is going 50 or so. This would seem to indciate that some input in getting to the PCM and that a simple change of VSS values in the PCM speedometer table would cure the problem. if we go this route we would also have to run the calculations for new tire sizes and the revs per mile as the car runs 17's. Thanks again for the comment.

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

On my Fiero V6 engine, it's installed in a VW kit car - with full Fiero computer, sensors, etc. The VSS is hooked up to the speedo cable that comes out of the front wheel. I don't know what rpm the cable from the Fiero into the VSS was. Is there a source that has that information? That is, how many revolutions of the cable is equal to one mile.

I have been having problems with the car stalling and not holding idle, and the CEL indicated it was the VSS malfunctioning. However, someone had indicated that the VSS doesn't effect idle. When the car is stopped, the VSS is not giving any signal no matter how it's functioning. I further saw that a VSS CEL could be triggered by a bad IAC. I replaced that, which was definitely bad, but didn't cure anything. And, I see above that someone did attribute stalling to the VSS. So, maybe my computer was right all along!!!!

I do know that the VSS on the '87 Fiero V6 is a 4 pulse per revolution sensor, and some of the earlier Fieros had 2 pulses per revolution. But, I don't know the cut off. I think it matched a change to the injection system.

I would recommend reprogramming the ECM is that option is available. I have a similar situation as my transmission generates 60,000 ppm and my ECM is expecting 24,000 ppm.

The DRAC or Dakota Digital SGI-5 will work, but it's kind of a band-aid and they tend to be noisy and could cause idle (while moving) issues if it hiccups at just the right time.

[This message has been edited by jscott1 (edited 12-30-2008).]

quote Originally posted by Alpinej: On my Fiero V6 engine, it's installed in a VW kit car - with full Fiero computer, sensors, etc. The VSS is hooked up to the speedo cable that comes out of the front wheel. I don't know what rpm the cable from the Fiero into the VSS was. Is there a source that has that information? That is, how many revolutions of the cable is equal to one mile. I have been having problems with the car stalling and not holding idle, and the CEL indicated it was the VSS malfunctioning. However, someone had indicated that the VSS doesn't effect idle. When the car is stopped, the VSS is not giving any signal no matter how it's functioning. I further saw that a VSS CEL could be triggered by a bad IAC. I replaced that, which was definitely bad, but didn't cure anything. And, I see above that someone did attribute stalling to the VSS. So, maybe my computer was right all along!!!! I do know that the VSS on the '87 Fiero V6 is a 4 pulse per revolution sensor, and some of the earlier Fieros had 2 pulses per revolution. But, I don't know the cut off. I think it matched a change to the injection system.

GM speedometer cables made 1,000 revolutions in a mile. At 60 they turned 1000 RPM. When GM went to gear driven VSS's, they used a 2 pulse per rev sensor, and the 2,000 pulse per mile standard was born. My '88 Fiero shop manual indicates 4,000 PPM, but there are different systems around the car that use different PPM numbers. My '90 6000 manual says that PPM varies by options, so the transition was far from smooth.

Now I'm a little confused. Phil, who posted above, wrote in a separate thread of mine that the Fiero is a 2000 ppm. He said that with the 4 pulse VSS, 4000 ppm goes to the speedo then it is electronically split to 2000 ppm for the computer. That supports that the gear is spinning the VSS at 1000 ppm. It also supports what Will just said. But above people are talking about 28,000 ppm. That's a whole different ball game. Am I missing something?

I will note that I did learn that there are 2 pulse VSSs and 4 pulse VSSs for the Fiero - early and late. Perhaps for some reason they found the 2000 ppm from the 2 pulse VSS wasn't enough for the speedo so they went with the 4 pulse to double the signals, and then cut it so they didn't need to change the computer. Just a crazy guess.

[This message has been edited by Alpinej (edited 01-03-2009).]

28,000, 40,000 and other numbers are from OBDII applications. The 1K, 2K and 4K are all OBDI apps.

quote Originally posted by Alpinej: But above people are talking about 28,000 ppm. That's a whole different ball game. Am I missing something?

Yes, the 60,000 ppm is generated in my G6 transmission and my Deville ECM is looking for 24,000 ppm. The Fiero speedometer is 4,000 ppm and the Fiero ECM is 2,000 ppm.

Ok, in calculating my revolutions, my cable puts out about 780 rpm. If I have a 1 plus VSS, that's 780 ppm, if I have a 2 pulse VSS that's 1560 ppm. So, with the 780, the car will be going faster than the ECM thinks it is; with the 1560, the car will be going slower than the ECM thinks it is. Will that margin of error matter to the ECM?

My car uses a mechanical speedo. So, that's irrelevant. The thing I've been trying to cure is its inability to hold idle after it warms up, which someone mentioned a bad VSS can cause, and my CEL is saying the VSS is bad. Is there anything else that the VSS output is used for other than speedo, somehow idle, and the cruise control that I really need to get it converted correctly? Or, will it work ok with the error?

780 RPM at what speed?

My misstatement. 780 revolutions per mile of the cable going into the VSS. Although that's also "rpm" I meant miles not minutes.

So, if I have this right, at 60 miles an hour, on a 2 plus VSS my car would be putting out 93,600 pulses per hour, and the ECM expects 120,000 pulses per hour at that speed. So while the car is going 60 mph, the ECM will think the car is going 46 mph.

As I asked, will that margin of error matter to the ECM? The thing I've been trying to cure is its inability to hold idle after it warms up, which someone mentioned a bad VSS can cause, and my CEL is saying the VSS is bad. Is there anything else that the VSS output is used for other than speedo, somehow idle, and the cruise control that I really need to get it converted correctly? Or, will it work ok with the error?

The ECM uses the VSS to tell if the car is moving or not. The mismatch shouldn't matter for that. If you use ECM controlled cruise control, then the lower limit on speed for engaging cruise will not be correct. I don't think anything else suffers.

So speed has no impact on fuel mixture/ratios? I would figure it would not unless the ECM was pretty sophisticated and it balanced speed with rpm with load and who knows what else, for just the perfect mixturebv.

But, then how or why does a bad VSS cause idle issues? I mean even if the thing is bad, wouldn't it give no signal at stop no matter what? Or, perhaps its normal state at stop is to give a signal and when it spins the pulses are the break in the signal. So, a bad VSS would not be giving a continuous idle signal.

quote Originally posted by Will: The ECM uses the VSS to tell if the car is moving or not. The mismatch shouldn't matter for that. If you use ECM controlled cruise control, then the lower limit on speed for engaging cruise will not be correct. I don't think anything else suffers.

I don't know about the 3800SC ECM, but the Caddy ECM uses the speed and the throttle position sensor to adjust the Idle Speed Control motor to match the predicted throttle position when you lift off the accelerator.

If the VSS is not accurate this system doesn't work properly.

It's not just a one or a zero to tell it that the car is moving. You will have idle while moving issues of a too high or too low idle, which will either lead to stalling or a sail on condition.

[This message has been edited by jscott1 (edited 01-06-2009).]

That only applies to the 4.9 and OBDI Northstar cars which use the ISM idle control. Everything else uses an IAC which does not suffer from those difficulties.

The difference between the two systems is that the ISM functions as a "throttle kicker" and opens the throttle slightly for idle air, while the IAC is a stepper motor driven valve that controls airflow through a passage that bypasses the throttle entirely.

The IAC functions as a stall saver and does have some "throttle follower" functions when used with a stick shift so that the engine doesn't stall when you sink the clutch from a throttle closed coast-down. It is a LOT easier to tune than the ISM. Even Jon Lagler has had problems tuning ISM's correctly.

[This message has been edited by Will (edited 01-06-2009).]

quote Originally posted by jscott1: Yes, the 60,000 ppm is generated in my G6 transmission and my Deville ECM is looking for 24,000 ppm. The Fiero speedometer is 4,000 ppm and the Fiero ECM is 2,000 ppm.

Sorry to thread jack, but it's along the same topic. I'm doing a G6 six speed swap to a 3800 as well. I know the dakota digital SGI-5 is the best option to take care of the VSS, but I was wondering...

What voltage and current is the speedometer looking for?
Would it be possible (and dirt cheap) to use a 4 bit TTL ripple counter to divide the 60,000ppm by 16 (gives 3750 so the speedo would be off 6.25%, about 4mph at 65)? Perhaps use the 60,000ppm as clock, then bit 0 would give 30,000 and bit 1 15,000.... not exactly what the ecm is looking for, but closer than 60,000. Bit 3 would be 3750ppm which is close to what the speedo wants...

[This message has been edited by zi_gravedigger (edited 01-09-2009).]

Well the easiest way is to use the PCM as a signal converter, and run the speedo the same way us automatic guys do, using the signal converter. You can do any conversion you want in the pcm, and output it via the speedo driver.

quote Originally posted by darkhorizon: Well the easiest way is to use the PCM as a signal converter, and run the speedo the same way us automatic guys do, using the signal converter. You can do any conversion you want in the pcm, and output it via the speedo driver.

Unfortunately I don't have any tuning software. The money and any hope for it, as well as my carputer, totally went out the window when my 4 speed started acting up. I'm doing the 6 speed swap but can't do anything special with the PCM except trick it with the Dakota Digital or similar.

quote Originally posted by zi_gravedigger: Sorry to thread jack, but it's along the same topic. I'm doing a G6 six speed swap to a 3800 as well. I know the dakota digital SGI-5 is the best option to take care of the VSS, but I was wondering...

I don't know that the SGI-5 is the best option...

It is very noisy and temperamental. If someone had a more elegant solution I would be interested.