compression test prep 1969 911e

whizbang · 09-24-2014, 10:12 AM

Hello all,
I'm getting ready to do a compression test on a 1969 911e. I read Wayne's article and am using that as my guide. I've prepped the spark plugs and have located the CDI box to pull the cable and disable the ignition. I also want to follow his suggestion to disable the fuel pump, but I can't find a relay or fuse to pull. Is there any way to disable the fuel pump without jacking up the car, removing the belly pan and pulling the wire on the fuel pump itself?
Thanks for any help you can give.

T77911S · 09-24-2014, 10:53 AM

according to the drawing for the 71, the fuel does goto the fuse block, but is not fused. you can pull the wire off the fuse block. it looks to be red/green.

Peter Zimmermann · 11:06 AM

Why not just do a leak down test?

This link is for an old thread, but has some good info...

Compression & Leakdown Results

whizbang · 09-25-2014, 11:53 AM

Thanks for the quick replies.

Peter, I'm doing a compression test because I don't have a leak down tester yet ;-).

T77911S, I like your idea of pulling the wire off the fuse block. I believe the 69 and 71's have different wiring diagrams. Looks like 71s have one block of 12 fuses while 69s have 2 blocks of 8 fuses each. On the SL33, it looks like a blue and yellow wire going to fuse 6 of block 1 powers the fuel pump. I'll pull that one and let you know what happens.

whizbang · 09-25-2014, 08:51 PM

So I pulled the blue and yellow wire and fired her up. Started up fine and ran until I shut it off. My conclusion is the fuel pump must have been running. Not sure what the blue and yellow wire is for, but clearly it's not the pump. Bummer, because all the wire colors for that and all the other fuses seemed to match the SL33 wiring diagram.
I dropped the belly pan and looked at the wires on the pump. Sure enough, blue and yellow just like the diagram shows. Any way, I figure I'll just pull the pump connector when I'm ready. It's just something of a mystery why pulling the wire off the fuse block didn't stop it.
On a related note, it looks like it will be a real test of patience to get the plug adapter into the spark plug holes. Anybody have any tips for doing a compression test?

T77911S · 09-26-2014, 02:48 AM

carbs or mechanical.

carbs can run for a while with the fuel in the bowls and no pump.

its not hard to get the hose in there. just think of others that have less room. line it up to the center of the hole and how you remember the angle of the plug.

ossiblue · 10:42 AM

You have two threads running on the same subject so it will be helpful to link one to the other.

When you "pulled the wire" for the fuel pump, did you actually disconnect it from terminal 6 in the fuse block or did you pull the fuse? If you pulled the fuse, that will not cut the power to the pump as the blue/yellow wire is fed from the common connector at the terminal, not through fuse 6.

If you actually disconnected the wire, then I suspect you have carbs as mentioned by T77911S. If so, simply disconnecting the wire at the connector of the fuse block will work or pulling the top fuse in the small fuse block in the engine compartment, or pulling the blue/yellow wire from the rpm transducer.

whizbang · 10-02-2014, 11:05 AM

My apologies, there are indeed two threads running on this same subject. The other one is in the comments section of Wayne Dempsey's excellent article here. Please advise if there is a better way to link the threads.
As I mentioned there, I did not pull the fuse but instead disconnected and taped off the blue/yellow wire from the right hand side of fuse #6 on block #1.
It's also definitely an MFI, not carbs. I thought it might run a bit on residual fuel, but it was a good 10 minutes before I finally shut it off.