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VW MKIII-A3/B4 TDIs This is a discussion about MKIII-A3/MkIII Jetta/Golf (<99.5) and B4 Passats (96,97) TDI's. Non TDI related postings will be moved or removed.

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Old February 7th, 2018, 13:02   #1
Jakub
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Default Ahu turbo boost control question

Hi
I converted a 93 suzuki sidekick gas engine to a 1997 VW Jetta TDI alh. I had a code jump up and noticed that when Iím driving my boost stays constantly high as if the actauator never kicks in to let it out when it reaches to high and puts me in limp mode . The actuator moves freely and has no obstructions and seems to be set properly. What can I adjust to allow the actuator to open to allow boost out? Or do I need to get a manual boost controller ?

Last edited by Jakub; February 7th, 2018 at 13:03. Reason: I corrected the title
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Old February 7th, 2018, 14:45   #2
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For some reason the code I get on vagcom is 000625 vehicle speed signal
27-00 implausible signal
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Old February 7th, 2018, 16:13   #3
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So it was working before right? Boost control is controlled by vacuum on the ALH and the n75 valve. Check your vac lines and pull a vacuum on the turbo actuator and check the n75 valve. It sounds like its the n75 or the controller within the ECU or some value its getting improperly and not letting it open to dump the exhaust waste gate open.
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Old February 8th, 2018, 08:36   #4
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Yea it was working before or it appeared to be bec I didn’t get the code.
I was thinking n75 valve but it appears to be clicking away working.
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Old February 8th, 2018, 09:31   #5
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get a vacuum tester tool and test the actuator for leaks and im asuming that the vtn ring and blades are freely moving from your description. You go into limp mode so i would say most likly you have a vacuum leak or boost leak and its not closeing or its getting stuck open
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Old February 8th, 2018, 13:42   #6
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I only go into limp mode on the highway going up a steep grade
I’ll check things
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Old February 8th, 2018, 14:11   #7
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are you sure your lines are good?
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Old February 8th, 2018, 14:25   #8
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Well when I installed them with the engine everyting was new but I will check
The car makes boost just something doesn’t let the boost off it just continues to climb
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Old February 9th, 2018, 05:45   #9
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what lines did you use? OEM lines like supplied from IDparets.com lasts a a while, the hoses you get a auto stores last about 4 months if your lucky, ESPECIALLY in diesels.
the silicone high quality hoses on amazon last for near close to forever.
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Old February 10th, 2018, 11:42   #10
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I used the braided black vacuum lines
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Old February 10th, 2018, 15:00   #11
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sounds like a bad actuator, ripped diaphragm, thats easily the 2nd likly issue. Have you checked the line going into the ECU? there is a small part of vac line inside the ECU.
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Old February 10th, 2018, 18:50   #12
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if your engine is from a 97 jetta it's an AHU not an ALH. ... AHU uses boost (not vacuum) through the N75 to control the wastegate, so if you've plumbed it to vacuum that's a big problem...
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Old February 10th, 2018, 18:55   #13
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OH good point lol, i missed that one. HAHA, well it was working before, i think he is confused and does have a AHU so yea boost lol

OP can you elaborate for us what turbo you have on it and what engine please.
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Old February 13th, 2018, 14:39   #14
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Right I acccidnetly wrote alh and tried to edit that but I guess it didn’t work.
Yeah it’s the Ahu.

The actuator seems to hold air
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Old February 13th, 2018, 18:43   #15
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yea, its all boost, 99% sure if your lines are good and your still having issue, take the ECU out, open it up and CAREFULLY replace the hose inside it.
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