Boost Gauge Problem (not boost problem)

[oldass31]

I have a new-to-me '87 951. I've never seen the boost gauge needle move from the "off" position since I bought the car. Not a single blip. The car hauls after 3000 rpm so I know the car is actually boosting. I'm positive the problem is between the KLR and the gauge itself but I'm not quite sure how to go about diagnosing the problem. I've used my multimeter before for things like battery voltages and spark plug cable resistances, but never something close to circuit board/computer stuff. I'm terrified I'm going to fry to KLR or the instrument cluster with whatever idea I come up with.

I know the signal for the gauge is supposed to be between 0-5 volts.

My first idea is to confirm some amount of voltage on the boost gauge circuit by attaching my DMM to the KLR and ground. (short out the KLR circuit?)
My next idea is to supply a potential, maybe from a AAA battery, to the gauge circuit itself and see if I can get the gauge to blip.

Is there a reason I should be talked out of these choices? Should I be testing resistances first of each side of the circuit? Or will that supply too much amperage? Thanks for any help.

[carlege]

You really need an Aftermarket one to tell you

[oldass31]

Originally Posted by carlege

You really need an Aftermarket one to tell you

I'll be getting an aftermarket one soon enough but I still want to see the OEM do something. I actually ran into the previous owner's grandson at a car show over the weekend. Both he and his friend clearly remember the boost gauge working.

[Rogue_Ant]

First thing to check is the vacuum hard line that goes from the intake manifold to the KLR.


-Rogue

[wild man]

Try removing the line from the KLR and replacing it with one from a regulator hooked up to a supply of compressed air, and see if it registers anything (up to 15psi, with ignition on, of course).

[oldass31]

So I followed my instincts and disassembled everything in sight. Perhaps I'll upload all my steps... Anyways, I successfully got the gauge to blip by connecting the probes as close as I could to the boost gauge's D'Arsonval device. However, if I try to blip the gauge by connecting to the posts on the back of the tach/boost gauge assembly, nothing happens. That's when I discovered this...



If one of you gents could be so kind as to pull your own gauge and tell me the Ohm rating of the blown resistor....I'd be much obliged.

[kdjones2000]

I have a cluster halfway apart. Let me take a look tonight....

[wild man]

Replacing the cooked resistor may or may not fix it. It cooked because there was more power running through it than it was rated for (looks like a 1/4 watt size). The question is why?

Let me add that if replacing it does make does make the gauge work, you might want to consider going with a larger one (of same resistance value) that has a higher wattage rating (like a 1/2 watt) to prevent the same thing from happening again.

[kdjones2000]

I see 43 ohms....

[oldass31]

Thanks a bunch! I'll let everyone know if this fixes the gauge.

[oldass31]

Holy rusted metal Batman! Replacing the resistor has worked! The boost gauge now reads 1 bar with the ignition on. However, it wasn't that simple. I called all around town looking for a 43 Ohm resistor which is apparently something of a rarity. Fry's electronics and Radio Shack only carry 33 and 47 Ohm resistors. If you find yourself in a similar situation, do what I did and buy a bunch of 47 Ohm resistors with a 10% tolerance. Then take your multimeter and find one that happens to be close to 43 Ohms and use that resistor.

Note: You don't need to risk breaking the gauge needles in order to remove the gauge face to gain adequate access to the solder joints. Just unscrewing the face plate loose is all that's necessary.

[choinga]

...if you have any mods, you still should have something aftermarket that's going to be more accurate. They are easy to install...switched +12v and the run a direct line from the gauge to the intake manifold. Done.