I just installed an aftermarket cruise control on my 89 GTU. (Didn't come with one and I couldn't find a complete working OEM unit at any of the local junk yards.) Everything works fine except the cruise only holds for 2 to 3 seconds before it shuts off. (and that with a vacuum reservoir added.) I checked with a mazda tech at the local dealer and was told that rotaries don't develop as much vacuum pressure as piston engines cars. Anyone seen anything about, or dealt with this problem? My thought is to find a 12 volt vacuum pump such as were used to run cruise on some diesels and for brake power boosting in some GM cars, but I'm open to suggestions. Any thoughts? Anyone know what the vacuum draw in inches is in a piston engine so I can look for a vacuum pump that will supply sufficient? (No specs for the required vacuum draw in the cruise control docs.) If I can solve this problem, it will give rotary owners an alternative to the very expensive OEM unit parts. I'll post a step by step on one of my websites for everyone's reference.

Thanks in advance. You can post replies to the forum or email me directly at dallenmarket@***.net.

 

Hmm - the stock NA cruise control uses a vacuum servo.
Even idle vacuum is 12-15 inHg, which should be good.

Is your vacuum source off the boost sensor hose?
If so you need to "T" on the manifold side of the restrictor pill in that line.

Are you sure it's not something like a speed sense fault?

 

You could ask on the "parts for sale" section here... I got a full Turbo II cruise control unit from a friend who had done an engine swap to a 3rd gen engine, plopped it into my NA, and it's been wonderful. Plus, it's a full electric servo, so no need to mess around with more vacuum lines.

-=Russ=-

 

According to Mazda, the rotary cruise is all electronic. No vacuum used. I'm sure I could find an OEM unit with enough looking, but I've got this unit and I want to see if I can get it working properly. The tach and ECC signals are processed correctly. The only problem is that the vacuum isn't sufficient to keep it running. Once it drains the reservoir, the throttle linkage releases. (I checked the signals and they stay strong and correct.) I've used the AudioVox cruise on non rotary vehicles and it is a very precise and dependable unit. At $79, it is a great deal for upgrading a car. I "T" into the intake manifold directly, so no restrictors are in the way. The Turbo may have more intake vacuum due to the pressurization created by the turbo, but I'm running an 89 GTU non-turbo. This is the 160 hp, 8000 RPM engine that was made available in 89, vs the standard 142 hp, 7000 RPM engine. I don't have a vacuum gauge to check exactly how much vacuum I'm getting, but not very much pressure is felt at the hose from the manifold vs my also throttle body equipped V6 Ford Aerostar. (Actually a Mazda 3.0 liter power train in a Ford van body.)