260 Tach, rectifier

[Cervelo777]

I just got done rigging my Allison XB2003 and the first test run was, in general a success, Just a few bugs to work out. The tachometer worked intermittently. I have spent the last four days reading over 40 pages of posts related to the subject and I see the Promaxes seem to blow the rectifier/regulator often. One of the solutions for them is to connect the grey wire to the other regulator/rectifier. I think my 260 only has one rectifier? It is pictured below, with two yellow wires going to it. Question is will my tachometer work any different if I connect the grey wire to the other pole? I suspect that the other probable cause could be a bad ground somewhere. My plan of action would be to first connect the grey wire to the other pole, if that does not solve it, I will run a new ground from the battery to the gauge, if that solves it I will go on the hunt for a broken wire or inadvertant grounding somewhere. I sure would appreciate any thoughts. Does anyone know that part number for the rectifier on a 260?

The other issue is my fuel pressure gauge shows 38lbs at start up but then slowly drops to about 22-25lbs even when running, would this scare you? It seems to run fine, I am only brave enough to go 75 on my first run, it seems to have a whole bunch more!!

[vnemous]

8M0058226

[outasite]

If you switch to the other pole it would not effect the tach,you can use ether.The fuel pressure scares me though! A bad ground can also cause problems with the rectifier/charging system

[vnemous]

You dont want to go lean

[90 5.0]

I've been having issues with my tach too. It jumps erratically sometimes when accelerating hard.

About to run all new wires for it.

[Cervelo777]

Update: I changed the Tach wire to the other pole on the rectifier and went out and burned 20 gallons of fuel, and the tach worked perfect. The connectors on the wires and rectifier, looked a little oxidized, so it could be that just by taking the connections loose, scraping them a little and tightening them down good did the trick. I think I will replace the rectifier anyway, I guess I am to lazy to try and run tests on it to find out if its really faulty. On to other questions. My fuel pressure is still reading 22-23 lbs at 5000 rpm, and really every RPM. (I have only had it over 6000 RPM for 10 -20 seconds, still learning to drive, plus rough water.) What should my Pyrometer be reading at 5000 to 6000 RPM?

[outasite]

Seems like my pyros run around *1200 and I think that is rich,can't remember for sure it's been more than two days! Where and how are you reading fuel pressure?

[Dave Strong]

If those are accurate press readings, Stop driving it till you figure out the pressure issue or you Will squeak it.

Dave

[Cervelo777]

I installed the fuel pressure sender in a tee about two inches before the fuel rail. It is a speed hut digital pressure gauge, I installed in the dash. I will pull the plugs in the morning and see if they look lean. I pulled one after my first test run and it was very black, seems very rich. My pyrometer has never read more than 650 degrees. Which seems odd to me. Water temp starts at 60 cold and I can idle out and give a little gas and I try to wait till it reads 75 before I get on a plane. Then wait till I see 82 before I go to 5000 rpm. It gets to 89 at 5800 rpm. I have pushed it a little and then came off plane and watched it go to about 110. Thanks for the warning Dave. I was actually waiting for someone to warn me. I can't wait to drive this thing and would probably seize it if someone didn't side with the voice in my head telling me to get fuel pressure figured out before you burn a piston.

[JCSmile]

You should definitely be over 1000 degrees at 6000 rpm

[j_martin]

Yer right on the crusty connections messing up the tach. That rectifier is really a small 15 amp, 50V T05 can (read past era small transistor housing) rectifier punched into an aluminum plate. The slightest problem in the wiring or battery condition will usually lunch the rectifier. That's why they have a bad rap. They deserve it. I replace them with a real 25-35 amp 200V or better bridge rectifier, or better yet a 5 or 6 wire mercury voltage regulator.

Looks like the guys here don't trust your instruments. I'll join them.

[Markus]

Update: I changed the Tach wire to the other pole on the rectifier and went out and burned 20 gallons of fuel, and the tach worked perfect.

That is the easiest way to diagnose a bad rectifier.

Instead of buying a Mercury rectifier, your can buy a cheap aftermarket automotive rectifier for fraction of the price and with higher quality. I used a part for a European GM car (Opel).

[WillySteve]

I know this is a little late on this post but as far as the Pyro Temps....The melting point of Aluminum is 1221 degrees! For sure heat is power and 1000-1150 maybe, but I certainly wouldn't run an engine at 1200 degrees!!!
There is just too many variables in the avereage temps of the cylinders plus guages, wiring, etc. and all it takes is one hotspot! JMO

[Dave Strong]

I know this is a little late on this post but as far as the Pyro Temps....The melting point of Aluminum is 1221 degrees! For sure heat is power and 1000-1150 maybe, but I certainly wouldn't run an engine at 1200 degrees!!!
There is just too many variables in the avereage temps of the cylinders plus guages, wiring, etc. and all it takes is one hotspot! JMO

The biggest variable is probe placement, perfect example is my sled. Runs with perfect plug and piston wash at 1295-1330. Combustion temp and temp at the probe are not the same. EGT's are a great tool to use to monitor things After you tune by plug and piston wash. To install EGT's and tune with them alone Will cost you $$$.

Dave

[TEXAS20225]

one piece plate on the 260, no poppet, washesr, no thermostats is the normal water cooling style what do you have ive never had a 260with that much pressure even at 7500