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Old 05-08-2003, 06:29 PM
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Post 1991 Sonoma ABS Question

I put a later model Syclone rear end into a 1991 Sonoma. The original Sonoma setup was for a rear-wheel only ABS, the Syclone setup had a 4-wheel ABS. The Syclone rear looked like it monitored axel speed with something like an eluctor notched wheel on each side. The older Sonoma ABS had no electronic feedback system that I could see...just the "brain" next to the master cylinder. When I drive it the brakes seem to work, but I don't have the OBD system hooked up yet to see if I'm getting an error code.

Question: Will the original Sonoma ABS still work with the Syclone rear? If not, is there something I can do to make it work?

Thanks for your help.

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Old 05-09-2003, 08:41 AM
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Would it be possible to simply swap in all the Syclone stuff? Just a thought.....
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Old 05-09-2003, 11:16 AM
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The Sonoma rear wheel ABS only monitors the VSS (speedometer) and limits the pressure to the rear brake lines based on that. Later four-wheel ABS versions used sensors on all four wheels to monitor each wheel's speed.

You should just be able to hook up the brake lines normally and ignore the ABS sensors on the Sy rear end.

Does the Sy rear end have disk brakes? If so, you may have some trouble with this...
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Old 05-11-2003, 01:41 PM
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Thanks for the advice. Unfortunately, the rest of the Syclone was gone. Fortunately (I guess) it still has drum brakes.

Next question...if the Sonoma ABS only monitors the VSS, can I use the VSS from a '98 5.7 vortec? That's what in the Sonoma at this point. Should I take the speed directly from the eluctor on the 4L60E or the speedo output from the VCM?
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Old 05-12-2003, 07:04 AM
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Hmm. Good question, I don't know. The difference is, one will be adjusted for the rear end gear ratio. If you only swapped a new rear end into it, you shouldn't have to change anything - assuming the ABS was hooked up before and that the gear ratios are the same.

I think that the way to adjust for different gearing is by changing the piggyback module on the ECM. I haven't verified this, it's an assumption from casual visual inspection. I have to look into this more soon, since I just changed ratios as well. I'd be interested to hear if you find anything out.
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Old 05-13-2003, 07:34 PM
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The '98 VCM doesn't appear to use a piggyback, if you mean the DRAC (or whatever they called that thing that used to be between the tranny and the VCM/speedo). Now the eluctor signal feeds directly into the VCM and you reset the gear ratios by reprogramming the VCM with something like a hypertec.

Do I sound like I know what I'm doing? Want to buy some cheap bridges in New York?? I'm pretty much winging it here. I guess I'll give it a try and see what happens.
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