1987 911 Turbo Boost Gauge does not work

[1963SWC]

I am hoping that someone out has worked through this same issue. My boost gauge does not work on my turbo. I do not have a wiring diagram for this and cannot locate much about diagnosing this issue as well. Here is what I have going on.

Turn the key on, and gauge maxes out and stays there as long as the key is on and motor is running. I saw that the previous owner back in 2017 replaced the sending unit but I know that this does not mean anything. Ohmed out the wire, brown with red stripe, from the sending unit to the back of the gauge, no continuity, so there is a break somewhere along the way. Not sure if there is a fuse for this line somewhere or a relay to check.

Ran a jumper wire from the sending unit to the back of the gauge, and plugged into the “G” terminal, and then the gauge moved to below zero with the key off and once the key was turned on the needle moved to zero but when the motor was running, no boost measured, no gauge movement.

Not sure what the failure mode is for the sending unit, millivolts? Ohms? Look forward to some insight.

[sigchuck]

There is a resistance spec for the sending unit. If it’s not registering boost with a new jumper wire, you may want to check resistance of the sender.

Here’s an answer from a post I made with similar issue - FSM says that at 20C and atmospheric pressure, resistance of the boost sensor should be between 5-13 ohms (measured between the connector and the housing).

Boost Gauge and Sender Testing

[1963SWC]

Sigchuck, Thank you very much for your link. I will check my ground on my IC but with no continuity on the sensor wire, I might have a couple of issues simultaneously. The replacement back in 2017 of the sending unit, might not have fixed it and they just threw their hands in and ignored the non-function of the gauge.

[john walker's workshop]

Pegged = open circuit. Check continuity between sender and gauge. Could be that the sender is the culprit. Pull the wire off the sender and ground it and watch if the gauge goes the other direction.

[spuggy]

Originally Posted by 1963SWC

Turn the key on, and gauge maxes out

No continuity between gauge and sender. Same as when the gauge us entirely disconnected or you took the connector off the sender.

Originally Posted by 1963SWC

Ohmed out the wire, brown with red stripe, from the sending unit to the back of the gauge, no continuity, so there is a break somewhere along the way. Not sure if there is a fuse for this line somewhere or a relay to check.

Nope, no fuse or relay. Simply a broken wire/open circuit by the sound of it.

Originally Posted by 1963SWC

Ran a jumper wire from the sending unit to the back of the gauge, and plugged into the “G” terminal, and then the gauge moved to below zero with the key off and once the key was turned on the needle moved to zero

So, when the bad wire is replaced/eliminated, the gauge behaves as expected.

Originally Posted by 1963SWC

but when the motor was running, no boost measured, no gauge movement.

Uhhmm.. You know that old school turbos require both load and RPM to make boost right?

You can rev a 930 to the moon in neutral and it won't make boost (this ain't some WRC car with anti-lag ).

With no load, still vacuum in the intake manifold == zero movement on the factory boost gauge.

[1963SWC]

Thanks guys for the help. Tested the sending unit and it worked like a charm. Tested to make sure that I had a good ground, yep good ground on the sending unit.. Then started to trace wires and found the issues very fast. Broken wire at the gang plug at the rear fuse box. Re-soldered the wire and she is good to go now!