Altitude compensator solenoid stuck on ground.

[Grubby]

Thanks George. The piece I was missing was that there is a switch external to the pump that activates the alt. comp. solenoid. I will just wire it to be on all the time. I am going to post another question about glow plug wiring that you may also have some advice about. Thanks very much for your thorough response. I don't think I could have started this thing without you and others on this forum. Yes, Wren and I started it yesterday, and after we tightened up some injectors, it ran great!
Thanks,
Abe

Originally Posted by v8volvo

Based on what Jason said in the other thread (which I did not know before), it sounds like the roller cage has a limited range of movement... meaning that regardless of your starting point, e.g. whether you have the cold advance lever pulled forward, or the altitude compensation solenoid disconnected, etc, the resulting increase in advance only has effect at lower RPM, or until the pump's roller cage reaches the limit of its movement. The cage can be moved (resulting in timing advance from base static position) either mechanically by the CS lever, or hydraulically by the alt solenoid.... but in the case of the CS lever, the position that it bumps the cage up to is only so far, and above a certain RPM (I have heard something around 2500 from the 4cyl VW guys), the hydraulic dynamic advance takes over and moves the cage further than the CS lever had pushed it to anyway. Presumably when using the alt solenoid for advance instead, that shifts the entire advance curve rather than just altering the bottom half of it like the mechanical lever does (since the solenoid works by allowing some pressure to escape on one side of the advance piston in the bottom of the pump, and pump internal pressure is simply proportional to RPM). However, if I am understanding Jason's explanation correctly, even though the whole curve is shifted, at some point the advance still tops out at the same place regardless as the roller cages reaches the end of its range of motion. With the alt solenoid disconnected, it probably just reaches that max advance point at an earlier RPM and is more advanced than spec all the way there.

Of course, all this assumes that the pump's pressure regulator valve, advance piston, and front transfer pump are all working correctly -- if any or all are out of spec then that will throw off the whole dynamic advance curve and then all bets are off as to what happens when, and how.

The other thing I have wondered is just how much timing difference the altitude compensation solenoid difference makes. On other VE motors like the Cummins 6BT, the BMW M21 524td motor, and some VW industrial engines, the whole cold timing advance is accomplished using a similar solenoid. I have access to a diesel timing light that I have always wanted to play with... one of these days I will experiment and see what the solenoid does exactly. Since the Volvo cold start wax thermostats are getting harder to find now, it may someday come to the point where we will want to make manual (or thermoswitch-controlled automatic) cold advance systems electronically using the solenoid rather than the mechanical cold start setup.

Abe, if you just wire your 240 conversion so that the altitude compensation solenoid receives power at all times whenever the key is on, that will work fine... As Wren described, 700 series TD cars from 1985-on (and some very late 240 Diesels) have an altitude-sensitive switch mounted on the driver's inner fender that controls current flow to the solenoid, but presumably your gasser 240 body has no such switch and you probably didn't get one with your D24T engine ... and even if you did, you are better off eliminating it and powering the alt comp solenoid directly off the key. That's assuming your mechanical cold start system is functional and you don't want to use the alt comp solenoid to effect some kind of cold advance, as I was talking about above... but that is probably something you will not want to worry about until later on after you have gotten the car on the road.

George