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Troubleshooting Sequential Turbocharger problems:

Primary Turbocharger

You first need to get the Primary Turbocharger working before attempting to fix anything on the Secondary Turbocharger. The Secondary Turbocharger requires the Primary Turbocharger to generate more than 8 psi of boost to properly operate actuators that control the Secondary Turbocharger.

It is also recommended to perform a Boost Road Test to get your boost pattern before getting into all those hoses.

When you first start looking around the engine bay you may notice more than a few vacuum hoses, well most of them are under the intake manifold, check out the colored vacuum hose diagram. Do not be too alarmed about this, as the trouble shooting typically narrows down to a couple of actuators and or hoses.

Primary Turbocharger Leaks

In order for the Primary Turbocharger to operate and generate more than 8 psi of boost check the items indicated on the Primary Turbocharger Leak Diagram. The items below are on the diagram and are listed in most likely to fail order:

Y-Pipe connector hose, (coupling)

Check ~1" diameter hoses for leakage

Primary Turbo Inlet

To/From Intercooler

Air Bypass Valve

Test for leakage and operation. With nothing connected to port (A), verify that port (B) can hold 15 psi of pressure and/or that port (C) can hold 30 inHg. With a vacuum tester apply 3.9-6.7 inHg of vacuum to port (A), verify air will flow from port (B) to port (C). At 9.2 inHg of vacuum to port (A) the valve will be fully open. The Air Bypass Valve is essentially the factory blow-off valve. It is not computer controlled, but simply operates by the vacuum from the Intake Extension Manifold, (pressure = closed, vacuum = open). You will hear the Air Bypass Valve vent boost when you let off the gas and get a vacuum in the Intake Extension Manifold. Typically costs about $125 at dealer. Note that the operating points of the Air Bypass Valve are different from the Charge Relief Valve, but can be interchanged temporarily for testing.

 

Charge Control Solenoid / Actuator / Valve

The Charge Control Valve controls the transition from Primary to combined Primary and Secondary Turbocharger operation. When ever the pressure applied to both Chambers of the Charge Control Actuator are equal, the spring force of the Charge Control Actuator will open the Charge Control Valve.

Below 4500 RPM this actuator is ON, (actuator rod pulled in), this closes the valve between the Primary and Secondary Turbochargers. This valve seals the air passage the same way as a throttle butterfly valve.

A simple test for the Charge Control Actuator is to start the engine and let it idle, the actuator rod will be pulled in. Stop the engine and remove the hose from Chamber A and the actuator rod will be out.

Chamber B is always connected to the Primary Turbo Compressor and normally will have 0 to 12 psi of boost pressure applied. Chamber A is connected to the Charge Control Solenoid, from 0 to 4,500 RPM a vacuum, from the Vacuum Chamber is applied, above 4.500 RPM the Secondary Turbo Compressor is applied to chamber A.

From 0 to 4,500 RPM with a vacuum applied to Chamber A, the Charge Control Valve stays closed, as Primary boost is always greater than a vacuum. Note, that the vacuum supply is from the Vacuum Chamber, thus leakage of the one-way check valve for the Vacuum Chamber will result in Primary boost applied on both sides of the actuator resulting in this valve opening when it should not, dumping Primary boost through the Charge Relief , Secondary Turbo and out the Charge Relief Valve. Another symptom of a leaky Vacuum Chamber one-way check valve is a loss of boost that is restored if the throttle is let off, (re-charging the Vacuum Chamber from engine vacuum) then quickly re-applying throttle results in boost.

Primary Turbocharger Control, (see Turbocharger Overview for more details).

Turbo Pre-Control Solenoid / Actuator

Wastegate Solenoid / Actuator

Turbocharger Exhaust

Exhaust Leaks

Exhaust Restriction

Other Things

Double Throttle Control


Secondary Turbocharger

You first need to get the Primary Turbocharger working before attempting to fix anything on the Secondary Turbocharger. The Secondary Turbocharger requires the Primary Turbocharger to generate more than 8 psi, (preferably 10 psi) to operate actuators that control the Secondary Turbocharger.

If you are at this point in your troubleshooting then a HIGHLY recommended tool to get is a hand-pump that can provide a vacuum as well as pressure, check out http:\\www.mityvac.com. Just about any local auto-parts/tools place will have these, also don't forget to get some "T" pipe fittings and some vacuum hose to allow tapping into various hoses. Basically you can attach this to the various actuators and solenoids to see if the they operate properly, much nicer than having to run the car to test things out.

Secondary Turbocharger Leaks

Assuming you have checked and verified the Primary Turbocharger for leaks there are several leakage paths specific to when the Primary and Secondary Turbochargers are operating, see the Secondary Turbocharger Leak Diagram. The item(s) below are on the diagram and are listed in most likely to fail order:

Charge Relief Solenoid / Valve

Test for leakage and operation. With nothing connected to port (A), verify that port (B) can hold 15 psi of pressure and/or that port (C) can hold 30 inHg. With a vacuum tester apply 6.5-9.3 inHg of vacuum to port (A), verify air will flow from port (B) to port (C). The Charge Relief Valve vents boost during the pre-spin stage of the Secondary Turbocharger, (3,000 to 4,500 RPM). The Charge Control Valve isolates the Primary Turbocharger boost from the Secondary Turbocharger during the pre-spin stage. After 4,500 RPM the Charge Relief Valve is closed to allow Secondary boost to be added to Primary boost. Note that the operating points of the Charge Relief Valve are different from the Air Bypass Valve, but can be interchanged temporarily for testing.

 

Secondary Turbocharger Control, (see Turbocharger Overview for more details).

At this time you should have the Primary Turbocharger functions completely, and checked for Secondary Turbocharger boost leaks.

Turbo Pre-Control Solenoid / Actuator

Turbo Control Solenoid(s) / Actuator

The Turbo Control Solenoid / Actuator is one of the more complicated actuators as it requires both vacuum and pressure to operate properly. This actuator is controlled by two solenoids, (both are wired together to the one ECU output) one solenoid applies pressure to one side of the actuator and the other applies vacuum to the other side of the actuator. With pressure on one side and a vacuum on the other side of the actuator, the speed of the actuator is improved. A typical problem is loss of Secondary boost in 1st or 2nd gear at 4,500 RPM, but reliable operation in other gears. This points to one side of the Turbo Control Actuator not getting it's pressure/vacuum, so it will still operate but not quickly enough.

 

Charge Control Solenoid / Actuator

The Charge Control Valve controls the transition from Primary to combined Primary and Secondary Turbocharger operation. When ever the pressure applied to both Chambers of the Charge Control Actuator are equal, the spring force of the Charge Control Actuator will open the Charge Control Valve.

Below 4500 RPM this actuator is ON, (actuator rod pulled in), this closes the valve between the Primary and Secondary Turbochargers. This valve seals the air passage the same way as a throttle butterfly valve.

A simple test for the Charge Control Actuator is to start the engine and let it idle, the actuator rod will be pulled in. Stop the engine and remove the hose from Chamber A and the actuator rod will be out, see also Boost Test Step 5.

Chamber B is always connected to the Primary Turbo Compressor and normally will have 0 to 12 psi of boost pressure applied. Chamber A is connected to the Charge Control Solenoid, from 0 to 4,500 RPM a vacuum, from the Vacuum Chamber is applied, above 4.500 RPM the Secondary Turbo Compressor is applied to chamber A.

From above 4,500 RPM with Secondary Turbo Compressor applied to Chamber A, when the Secondary Turbo is producing the same or more Boost than the Primary Turbo, the Charge Control Actuator's spring will allow the Charge Control Valve to open, allowing the both the Primary and Secondary Turbo provide boost.

Wastegate Solenoid / Actuator

Other Stuff

Wastegate and Turbo Pre-Control Solenoids

One-way Check Valves, Chambers, and Tanks

  Air Supply
Turbo Pre-Control Solenoid and Turbo Pre-control Actuator Primary
Wastegate Solenoid and Wastegate Actuator Primary
Charge Relief Solenoid and Charge Relief Valve Vacuum
Charge Control Solenoid and Charge Control Actuator Vacuum
Turbo Control Solenoid and Turbo Control Actuator Vacuum and Pressure

Air Supply

Vacuum

Vacuum Chamber

Primary

Pressure

Pressure Tank


Rebuilding and/or replacing the Turbochargers

OK, you have found out that the Turbochargers do actually need to be replaced, (major bummer). I read somewhere that anyone with a turbocharged car should really look at the turbocharger(s) as a "consumable" part, much like clutch, brakes, etc... this may make you feel better but will not help with the cash-flow situation.

If you have a Mazda dealer than knows 3rd Gen RX-7s then they can do most of the work for you. Typically the Mazda dealer removes the turbos, then sends them out for re-build at a turbo re-builder, then the Mazda dealer puts the re-built turbos back in the car. Ask them how they go about doing this and if they have done this before and used the turbo re-builder before. These are very important questions to ask and get satisfactory answers to.

If you do not know of a good Turbocharger re-builder, then look in your local Yellow Pages directory under, hey you guessed it "Turbochargers". Phone everyone listed and ask them if they have worked on your type of Turbocharger for the 3rd Gen RX-7, (they are made by Hitachi, model HT12). There are basically two types of turbocharger re-builders, one will just work on the turbo cartridge and you get to remove this from the RX-7 specific exhaust and compressor housings. The other type of re-builder will accept the whole RX-7 turbocharger assembly. Both have their advantages and disadvantages.

Turbocharger cartridge re-builders will cost you less for the re-build of the turbos, but you will need some special tools to remove the housings and some set-up jigs to have everything re-align when putting everything back together. The other thing is that typically there will be cracks in the cast iron exhaust housings, typically around the wastegate, where the exhaust enters the housing from the engine, and around the exhaust wheel of each turbo. These cracks can be anywhere from benign to irreparable depending on how deep and where they are located. Any cracks that are where the exhaust wheel is is generally considered irreparable. For around the wastegate these can be welded up or some shops can machine and put an insert in. Note that any of this type of welding must be performed at a shop that does this kind of work, see Turbocharger assembly re-builders. So in the end, you may spend about the same amount of money, except that it will not be all in the same place. Another disadvantage, is that re-assembling the turbocharger into the RX-7 housings requires lining up oil in/out fittings and compressor outlets with respect to the exhaust manifold attachment plane, non-trivial and requires measuring or adding alignment marks of the various parts prior to disassembly.

Turbocharger assembly re-builders will cost you more but you get the complete assembly under warranty. It is very important that the place you use has experience with the 3rd Gen RX-7 turbochargers, because there are a bunch of non-obvious things that a first time re-builder will not realize about re-assembly of the turbos and the housings as mentioned in the previous paragraph, (i.e. alignment of oil in/out fittings and alignment of primary and secondary compressor outputs).

Note, it is VERY common to loose the inline pills that are between the Primary Compressor and the Wastegate and Turbo Pre-Control Actuators when having the Turbochargers replaced. Typically the re-builder will replace all hosing on the Turbocharger, and it is non-obvious that there are restrictor pills in the hoses.

Parts and Instructions for Turbocharger Remove and Replace


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